Food from Dryland Gardens - An Ecological, Nutritional, and Social Approach to Small Scale Household Food Production (CPFE, 1991) Part II - Garden management (introduction...) References 5. How plants live and grow (introduction...) 5.1 Summary 5.2 The vascular system in plants (introduction...) 5.2.1 Roots 5.3 Photosynthesis 5.4 Transpiration 5.5 Coping with heat and drought 5.6 Salt tolerance 5.7 Seasonal constraints to plant growth (introduction...) 5.7.1 Daylength Requirements 5.7.2 Temperature Requirements 5.8 Resources References 6. Growing plants from seeds (introduction...) 6.1 Summary 6.2 Sexual reproduction in plants (introduction...) 6.2.1 Life Cycles 6.2.2 Flowering 6.2.3 Pollination 6.2.4 Fertilization 6.3 Seed germination and dormancy 6.4 Suggestions for planting seeds under dryland conditions (introduction...) 6.4.1 Preparing the Seeds 6.4.2 Preparing the Planting Site 6.4.3 Planting the Seeds 6.4.4 Planting Density 6.4.5 Covering the Seeds 6.5 Caring for newly planted seeds and young seedlings (introduction...) 6.5.1 Watering 6.5.2 Mulching and Shading 6.6 Diagnosing seed planting problems (introduction...) 6.6.1 Testing Seed Germination 6.7 Thinning 6.8 Resources References 7. Vegetative propagation (introduction...) 7.1 Summary 7.2 Cuttings (introduction...) 7.2.1 Trees 7.2.2 Perennial Herbs 7.2.3 Cassava. 7.2.4 Sweet Potatoes 7.3 Tubers, tuberous roots, and bulbs 7.4 Offsets 7.5 Suckers 7.6 Grafting (introduction...) 7.6.1 Compatibility for Grafting 7.6.2 Effects of Stock and Scion on the Grafted Tree 7.6.3 Approach or Attached Scion 7.6.4 Budding 7.6.5 Apical Grafting 7.6.6 Topworking 7.7 Layering (introduction...) 7.7.1 Simple Layering 7.7.2 Air Layering 7.8 Resources References 8. Plant management (introduction...) 8.1 Summary 8.2 Nursery beds and container planting (introduction...) 8.2.1 Nursery Beds 8.2.2 Container Planting 8.2.3 When Direct Planting is Better 8.3 Planting sites and the sun 8.4 Transplanting (introduction...) 8.4.1 Timing 8.4.2 The Site 8.4.3 Water 8.4.4 The Transplant 8.5 Plant interactions (introduction...) 8.5.1 Mixed Planting 8.5.2 Allelopathic Plants 8.5.3 Crop Rotation 8.6 Weed management (introduction...) 8.6.1 Resource Use 8.6.2 Effects on Pest Populations 8.6.3 Timing 8.6.4 Methods of Weed Control 8.7 Pruning (introduction...) 8.7.1 Reasons to Prune 8.7.2 Guidelines for Pruning Trees 8.8 Trellising 8.9 Resources References 9. Soils in the garden (introduction...) 9.1 Summary 9.2 Soil and land-use classification (introduction...) 9.2.1 Indigenous Classification Systems 9.2.2 The USDA Classification of Soils in Drylands 9.3 Physical properties of soils (introduction...) 9.3.1 Soil Texture and Structure 9.3.2 Soil Porosity and Permeability 9.3.3 Soil Color 9.3.4 Soil Temperature 9.4 Soil profile and depth 9.5 Soils and plant nutrients (introduction...) 9.5.1 Soil pH and Plant Nutrition 9.5.2 Nitrogen 9.5.3 Phosphorus and Potassium 9.5.4 Other Nutrients 9.6 Organic matter (introduction...) 9.6.1 Animal Manures 9.6.2 Composting 9.7 Preventing soil erosion (introduction...) 9.7.1 Decreasing Runoff 9.7.2 Decreasing Raindrop Impact 9.7.3 Increasing Soil Resistance to Erosion 9.7.4 Reducing Wind Erosion 9.8 Building garden beds (introduction...) 9.8.1 Sunken Beds 9.8.2 Raised Beds 9.9 Resources References 10. Water, soils, and plants (introduction...) 10.1 Summary 10.2 Dryland garden water management 10.3 Water, soils, and plants (introduction...) 10.3.1 Water Storage in the Soil 10.3.2 Water Movement in the Soil 10.3.3 Evaporation 10.3.4 Water Uptake and Transport by Plants 10.4 Soil water and garden yield 10.5 How much water? 10.6 Measuring water applied to the garden 10.7 When to water 10.8 Mulches, shades, and windbreaks (introduction...) 10.8.1 Surface Mulches 10.8.2 Vertical Mulches 10.8.3 Windbreaks, Shades, and Cropping Patterns 10.9 Resources References 11. Sources of water for the garden (introduction...) 11.1 Summary 11.2 Water quality for plants 11.3 Water quality for people 11.4 Rain (introduction...) 11.4.1 Rainfall Records 11.4.2 Measuring Rainfall 11.5 Harvesting rainwater for dryland gardens (introduction...) 11.5.1 Patterns of Water Harvesting 11.5.2 Building on Local Knowledge 11.5.3 Catchments and Runoff 11.5.4 Estimating the Catchment to Garden Area Ratio 11.6 Harvesting stream flow and floodwater (introduction...) 11.6.1 Water Spreading 11.6.2 Flood Recession Gardening 11.7 Groundwater and wells (introduction...) 11.7.1 Groundwater 11.7.2 Locating a Well 11.7.3 Hand-Dug Wells 11.7.4 Small-Diameter Wells 11.8 Water storage 11.9 Resources References 12. Irrigation and water-lifting (introduction...) 12.1 Summary 12.2 Irrigation efficiency 12.3 Surface irrigation (introduction...) 12.3.1 Transporting Water to the Garden 12.3.2 Basin Irrigation 12.3.3 Furrow Irrigation 12.3.4 Trickle Irrigation 12.4 Root zone irrigation (introduction...) 12.4.1 Pitcher Irrigation 12.4.2 Water Table Irrigation 12.5 Sprinkler irrigation 12.6 Irrigation problems (introduction...) 12.6.1 Waterlogging 12.6.2 Salinity 12.7 Water-lifting (introduction...) 12.7.1 Lifting with Human and Animal Power 12.7.2 Lifting with Other Power Sources 12.8 Resources References 13. Pest and disease management (introduction...) 13.1 Summary 13.2 An ecological approach (introduction...) 13.2.1 Pest and Disease Management by the Crop Plant 13.2.2 Environmental and Mechanical Management of Pests and Diseases 13.2.3 Pest and Disease Management Using Other Organisms 13.2.4 Pest and Disease Management with Chemicals 13.3 Examples of pest and disease management (introduction...) 13.3.1 Insects 13.3.2 Nematodes 13.3.3 Large Animals as Pests 13.3.4 Diseases 13.4 Diagnosing pest and disease problems (introduction...) 13.4.1 Wilts (Table 13.1 and Figure 13.26) 13.4.2 Leaf Problems (Table 13.2 and Figure 13.27) 13.4.3 Abnormal Growth (Table 13.3 and Figure 13.28) 13.4.4 Fruit Problems (Table 13.4 and Figure 13.29) 13.5 Resources References

Food from Dryland Gardens - An Ecological, Nutritional, and Social Approach to Small Scale Household Food Production (CPFE, 1991)

Part II - Garden management

The interaction of many factors influences the health and productivity of plants in the garden. Among these are the microclimate, soil, water, microorganisms, neighboring plants, and the genetic inheritance of the plant itself.

There are also many ways to propagate and care for plants, and more than one technique can produce the specific results desired. For example, a gardener wanting to keep her garden free of weeds could spray an herbicide, mulch heavily, or remove the weeds by hoeing. If she wants to start seedlings for transplanting she could purchase specially manufactured, imported seed boxes, use locally made containers, or ones she made herself from free materials. The different techniques in these two examples will all produce the desired short-term results: weeds can be controlled by spraying, mulching, or hoeing; seeds can be started in specialized boxes, local containers, or home-built ones. Because there are many ways to accomplish these and other garden management tasks, the choice of technique must be based on other criteria as well.

If the main goal of garden management is to maximize production and profit, and the required resources are available, then the industrial agriculture model may be effective (Part I). Large-scale, industrial agriculture has been very successful in increasing production and yields by greatly increasing the use of machinery and the fossil fuels to run them, chemical fertilizers and pesticides, and irrigation water. Compared with small-scale, indigenous agriculture it has much higher returns to labor, but much lower returns to energy (section 3.2.1). Industrial agriculture is based on increasing centralization of management and marketing, and on increasing control over nature, rather than working with nature.

Genetic diversity in crops, ecological diversity in fields and regions, and social diversity in management has been drastically decreased in the drive to increase production (section 14.2). In turn, this lack of diversity results in decreased sustainability, because industrial systems are less and less capable of maintaining their high levels of production when challenged by drought, shortages of irrigation water, a break-down in the fertilizer distribution network, increasing oil prices, or outbreaks of pests and diseases.¹ The typical response of industrial agriculture to such problems is to attempt to increase control over nature and to centralize the system even more.

Experience has shown that the industrial approach to food production often results in increasing inequity because the capital and resource requirements are beyond the means of many Third World households. This approach has also been found to be harmful to the environment and to human health.

The criteria for selecting garden management techniques which we use in this book are self-reliance and local control of the food system; equal distribution of food for improved human health and nutrition; preservation of biological and cultural diversity; and conservation and protection of natural resources. This is why the approach we take to garden management in the Chapters of Part II is quite different than the approach of industrial agriculture, and reflects a growing interest in sustainable agriculture.

The term sustainable agriculture is widely used today to describe agriculture that has the goals of conserving the environment for the future and providing nutritious food for all people equitably (section 1.2). There is increasing awareness in both industrial nations and the Third World of the need to conserve resources for the future. Decreasing profits resulting from environmental degradation such as groundwater depletion and soil erosion, and consumer pressure for a more healthy food supply, are pushing industrial agriculture toward sustainability (section 3.2). For example, the United States’ National Academy of Sciences has published a major book titled Alternative Agriculture which advocates moving that country’s agriculture away from high inputs of chemical pesticides and fertilizers.² Agriculture is increasingly being studied from an ecological perspective.³ However, because the concept of sustainability has become so popular, it is sometimes used in ways that distort its meaning and make it subservient to production economics.⁴

In the Third World there is also interest in reorienting agricultural development away from the industrial model of the green revolution and toward sustainability.⁵ Detailed descriptions of indigenous agricultural systems contribute to a growing appreciation of their ecological (environmental) and social sustainability.⁶ However, in many of the World’s poorest communities population pressure, social disruption and incorporation into the world economic system have made indigenous agriculture environmentally destructive, socially inequitable, or both. A redistribution of resources from the rich, industrial sector is essential for those poor communities to create sustainable agricultural systems. Ultimately, a sustainable agriculture must be one that supports an end to growth of the human population, to our increasing levels of consumption, and to cultural and environmental destruction.

The Chapters in Part II discuss methods of dryland garden management based on a striving for ecological and social sustainability in its fullest sense. This means making the most of the indigenous knowledge, ecological and social diversity, and locally adapted biological resources which characterize many dryland food systems, while using the knowledge and techniques of Western science to enhance sustainability.

References

¹ Cleveland and Soleri n.d.c.

² NAS 1989a.

³ E.g. Carroll, et al. 1990; Cox and Atkins 1979; Gliessman 1990.

⁴ Cleveland 1991; Orr 1988.

⁵ E.g., AGRECOL/ILEIA 1988; Dupriez and De Leener 1983.

⁶ E.g., Lagemann 1977; Richards 1986: Westphal, et al. 1981, 1985.

Figure 5.1 Plant Anatomy - the Chili, a Dicot

5. How plants live and grow

Knowing how plants live and grow enables gardeners to adjust management practices according to specific local situations and helps them solve problems in the garden. For our discussion of how plants live and grow we use the terminology of Western science. However, many local systems exist which serve the same purpose, using terms and concepts developed through people’s experiences. These local systems are also valid; appreciating and attempting to understand the local system is essential for working with gardeners.

5.1 Summary

This chapter begins with an illustration of basic plant anatomy (Figure 5.1). The vascular system transports food, water, minerals, and other essential substances throughout the plant. Photosynthesis and transpiration provide the plant with the food energy necessary to live, grow, and produce a harvest. Under hot, dry conditions the rate of water loss from the plant increases and can lead to water stress that reduces yields. Plants have evolved a variety of responses to help them survive under these dryland conditions. Some plants also have a tolerance of salty soil, a common problem in drylands. Many crops or crop varieties also have daylength and temperature requirements that can limit their growing seasons.

5.2 The vascular system in plants

The vascular system is the network of plant cells responsible for the movement of water, minerals, food (sugars), hormones, and other vital substances inside plants.

Water in the soil is taken up by the roots through a combination of osmosis and cohesion. Osmosis is the pattern of water movement across a water-permeable membrane such as the cell membrane. If two liquids are separated by such a membrane, water will move out of the more dilute solution, the one with a lower concentration of solutes like salt, and into the more concentrated solution (Figure 5.2). This movement will continue until both solutions have the same concentration of solutes per volume of water. If the concentration of solutes is greater in the root cells than in the soil, water will move into the roots. Water loss from transpiration increases solute concentration in the leaves and so water continues to be pulled up through the plant by osmosis.

Figure 5.2 Osmosis

Cohesion is the tendency of like substances to stick together. The cohesion of water molecules, together with transpiration and osmosis, causes a continuous flow of water to move up the plant. Once the soil moisture is depleted to the wilting point (section 10.3.1) osmosis and cohesion will no longer be strong enough to move water out of the soil and into the plant.

Dicots and monocots are the two major groups of garden plants. Their vascular systems are arranged differently. Dicots are those plants such as beans, cucurbits, amaranths, and many fruit trees which have two cotyledons, or seed leaves, in their seeds, and branching leaf veins. Monocots have only one cotyledon and usually the veins in their leaves are parallel to each other, running the length of the leaf as in maize, onions, date palms, and most cereals. In larger seeds the difference between a monocot and a dicot is obvious. For example, a bean seed can be easily split into two halves, the cotyledons. A maize seed, however, does not split because it has only one small cotyledon.

The xylem is the part of the vascular system that carries water and nutrients from the roots to the leaves. In monocots the xylem tissues are scattered in bundles that run the length of the plant, throughout the leaves, stems, and roots. In dicots the xylem tissues occur in a discrete layer, which in the stem surrounds the pithy center. In dicot roots the xylem is the tissue at the core (Figure 5.3).

The sugars made by photosynthesis (section 5.3) and many growth-regulating hormones produced by plants’ growing tips flow through the phloem. Osmosis is also thought to be the source of movement for substances in the phloem. As the concentration of sugars produced by photosynthesis increases in the phloem, water from the xylem enters these cells, building up pressure within them. This forces movement of the solution to cells with lower concentrations and pressure until it reaches a place where the sugars are needed or can be stored for later use. Because most photosynthesis occurs on the outer and upper layers of the plant, those leaf areas exposed to sunlight, the movement of solutions in the phloem is primarily inward toward the main stem and downward to the roots where there is little or no photosynthesis. Sometimes the fluids in the xylem and phloem are called sap.

In monocots the phloem and xylem tissues are grouped together in vascular bundles running vertically through the plant. In dicots the phloem is a distinct layer separated from the xylem by a thin layer of cambial tissue (Figure 5.3). These continuous layers of phloem and cambial tissue make grafting and layering of dicots possible (sections 7.6 and 7.7), whereas with monocots these techniques are not possible.

The outer surface of green plant parts is the epidermis. Underneath the epidermis in green shoots and stems lies the cortex, tissue that surrounds the vascular system. In dicot trees the outer layer of the trunk and branches is called bark, a term that refers to all of the tissue from the cambium and phloem to the outer surface. In bark the cortex and epidermis are replaced by a more rigid, woody tissue called the cork, which includes a layer of dead cells on the outer surface.

5.2.1 Roots

Even though they are not usually visible, the roots are one of the most important parts of a plant. Roots provide structural support by anchoring plants in the soil, and they absorb water and nutrients in the soil and transport them to the shoot system, the above-ground portion of the plant. Root hairs are fine “hairs” that grow out of the root’s epidermis, just above the actively growing part of the root and root tip. The root hairs provide much of the root’s surface area and so they are very important for the absorption of water and nutrients. Some plants have large, fleshy roots that store energy and water for the plant. A number of these large roots are commonly eaten such as sweet potatoes, carrots, beets, and cassava.

There are two easy-to-identify patterns of root growth: fibrous and tap roots (Figure 5.4). Fibrous roots spread out and downward in a mass of fine roots, none of which dominate. Fibrous root systems include many secondary and tertiary roots, or lateral roots, those that grow out of an older root and therefore do not tend to grow straight down (refer to Figure 5.1 in section 5.1). Monocots like maize and sorghum commonly have fibrous root systems. Garden crops that are dicots, for example, carrots, okra, chilis, sweet peppers, and amaranth, have a tap root, a dominant vertical root with other smaller roots growing out from it. These tap roots can make use of water deep below the soil surface. Many dryland fruit trees such as carob and olive also have a tap root. When the tap roots of mature plants are cut off, for example, in transplanting, the plants may die. Some of these plants can recover by developing alternative roots in a pattern similar to a fibrous root system. However, this will only occur if the plant is young, vigorous and its shoot system is relatively small.

Plants’ root systems also vary depending on a number of factors including the soil, irrigation patterns, distribution of nutrients, plant density, and neighboring plants. Root systems have a great capacity for compensatory growth. That is, in areas of soil where the conditions are favorable the roots will proliferate, compensating for areas of the root zone that are less favorable. This is important to consider when irrigating young plants, because the root system will develop most strongly where there is consistent moisture. If irrigations are frequent and shallow, for example 10-15 cm (4-6 in), then the plant will develop a shallow root system. Under hot, dry conditions moisture in this surface layer is lost quickly by evaporation. Shallow-rooted plants will require more water applied in more frequent irrigations than plants that have received deeper and less frequent irrigations, encouraging them to develop a deep root system.

Figure 5.3 Stem and Root Structures of Monocots and Dicots

Figure 5.4 Root Types

Poor drainage and overwatering also cause shallow rootedness as the roots avoid waterlogged soil. Watering patterns that encourage shallow rootedness can lead to other problems such as salinity (section 12.6.2) or roots growing primarily in upper soil layers where temperatures are high, both of which can inhibit growth and kill the plant in severe cases. For these reasons, when watering established seedlings and older plants it is important to wet the soil down to at least 15-40 cm (6-16 in), and below this for trees, in order to encourage deep root growth. However, because compensatory growth is a gradual process, one should not switch abruptly from frequent shallow irrigations to less frequent deep irrigations without a transition phase of deep but less and less frequent waterings.

Root growth is also affected by soil texture and structure (section 9.3.1). Roots will grow where soil conditions are best, for example, where compost and manure have been added and where the soil structure allows easy penetration of roots, air, and water. Extremely heavy, clayey soils with little structure make it difficult for roots to grow and they can become thick and deformed from trying to push through the soil.

From the soil roots obtain nutrients such as nitrogen and phosphorus which are essential for healthy plant growth. In some cases this is made possible through mutually beneficial or symbiotic relationships between plant roots and soil microorganisms. Mycorrhizae (Box 9.5 in section 9.5) symbioses enable plants to use more of the phosphorus, zinc, or copper in the soil.¹ Symbiosis between Rhizobium bacteria and roots of legumes makes nitrogen in the air available to the plant while also enriching the soil (section 9.5.2).

5.3 Photosynthesis

Photosynthesis is the process by which green plants change the energy in sunlight into energy stored in carbohydrates (CBHs), the food used for growth and reproduction. Chloroplasts are the structures in plant cells where photosynthesis occurs. They contain a green pigment called chlorophyll which uses sunlight to fuel a reaction with carbon dioxide (CO₂) gas in the air, and water (H₂O) in the plant. The products of this reaction are oxygen (O₂), water, and carbohydrates, such as starches and sugars (Figure 5.5). Any plant part containing chlorophyll can conduct photosynthesis, but the leaves are the main areas of photosynthesis in most green plants.

The carbohydrates produced by photosynthesis are broken down into the simple sugar glucose, which then combines with oxygen to produce CO₂, water, and energy. This process, called respiration, provides the energy necessary for the plant to live and grow.

Figure 5.5 Photosynthesis

5.4 Transpiration

For photosynthesis to occur carbon dioxide (CO₂) must enter the chloroplasts, most of which are found in the cells under the plant’s epidermis. Most CO₂ enters the plant through the stomata (singular is stoma), tiny holes in the epidermis which can close (Figure 5.6). When the stomata are open not only can CO₂ reach the chloroplasts, but moisture from the inside of the leaf is able to evaporate into the environment. This movement of water vapor through the plant’s stomata is called transpiration. As water evaporates from the leaves during transpiration, the concentration of nutrients in the surface cells increases compared with that in adjacent cells, from which water then moves by osmosis. The same process is repeated all the way down to the roots. Because of the great cohesiveness of water and its adhesion (the attraction between dissimilar substances) to the cells of the passages along which it moves to the leaves, the water is pulled upward from the roots to the leaves. The energy that keeps this water moving upward is supplied by the sun which causes evaporation of water from the plant during transpiration.

Transpiration is important for two reasons: as just described, it provides the “pull” that keeps water and nutrients moving up through the plant from the roots (Figure 5.7), and, under hot, dry conditions transpiration cools the plant the same way evaporation cools our skin when we sweat. About 90% of all water absorbed by plant roots is released in transpiration. Under stressful (hot, dry) conditions, the amount of water needed by the plant, and thus the amount released in transpiration, increases.

Transpiration rates vary depending on plant types and environmental conditions. Photosynthesis increases with available sunlight, so under sunny conditions the stomata are open longer to supply the necessary CO₂ thus increasing transpiration. Conditions that increase evaporation, such as low air humidity, heat, and wind, also increase transpiration.

The stomata in some plants such as grapes will shut under extreme water stress. However, this may not save the plant. When stomata close to prevent water loss, the cooling effect of evaporation also stops, which can cause problems with high leaf temperatures.

Under sunny, dry, hot conditions transpiration rates are extremely high. If the soil is unable to provide enough water to keep up with the rate of transpiration the plant will wilt. If this water loss is not replaced soon, the plant will die. By shading, mulching, and providing garden plants with protection from drying winds the gardener reduces the need for water and so the amount of water that will be lost to transpiration (section 10.8). Directing water down to the roots, for example, with vertical mulch (section 10.8.2), minimizes the amount lost by evaporation from the soil surface, and makes more water available to meet the plant’s needs. Gardeners also use plants with lower rates of transpiration and other characteristics that make them better able to survive and produce under dryland conditions.

5.5 Coping with heat and drought

Dryland garden plants must often produce food and other products under hot, dry conditions. For plants, drought is a condition in which there is insufficient water available in the soil to meet the plant’s needs. A consequence of drought can be water stress, or water deficit - that is, insufficient water inside the plant for it to maintain itself and grow. Distinguishing between drought, a condition in the environment (especially the soil), and water stress, a condition in the plant, makes it easier to understand how plants respond to dryland conditions.

Figure 5.6 Stomata

Drought-adapted plants either escape drought or resist it in some way (Figure 5.8). Drought-escaping plants have short, rapid life cycles, allowing them to take advantage of brief periods of adequate moisture and decreasing their chance of experiencing drought. Some “famine crops” like short-season millet varieties and tepary beans follow this strategy, maturing before late season drought sets in.

Drought-resistant plants use one of two strategies, either they avoid drought or they tolerate it. Drought avoidance means more efficient use of water so that the plant will not experience water stress. For example, during periods of drought cowpeas avoid water stress by changing the orientation and movement of their leaves in relation to the sun, minimizing the amount of sunlight and heat they receive. This, in turn, reduces the amount of water lost from the leaves due to excess transpiration.²

Physiological differences enable some plants to lower transpiration rates in other ways. C₄ plants, named for the four-carbon molecule they produce and use, are able to use CO₂ more efficiently in a special form of photosynthesis. Because of this these plants do not need as much CO₂ and therefore their stomata need not be open as long as in other plants. Shorter periods with stomata open mean decreased transpiration. Some C₄ dryland garden plants are maize, sorghum, sugarcane, and amaranths.

Figure 5.7 Transpiration

Another modification of photosynthesis is found in Crassulacean acid metabolism (CAM) plants. In these plants photosynthesis happens in two stages, one during the day and one at night. The stomata are open only at night when they receive CO₂ and transpiration occurs. Due to the cooler, moister, dark nighttime conditions the rate of transpiration is much lower than it would be during the day. The CO₂ is then stored for use during the day when light energy from the sun is available. CAM plants found in some dryland gardens are pineapple, prickly pear cactus, and agave. CAM and C₄ plants are not necessarily the best ones for dryland conditions. For example, maize and sugarcane, both C₄ crops, are high water users.

Figure 5.8 Plant Adaptations to Drought

Other plants may significantly reduce transpiration rates in different ways. Some other physical characteristics that cut down rates of transpiration, making plants better able to cope with drought conditions include:

· Small leaf surface area.

· Small number of stomata per unit of surface area.

· Majority of stomata on the more protected, underside of leaves.

· Thick, waxy or resinous layer or cuticle on the leaf surface.

· Light-colored leaves that reflect light, resulting in lower leaf temperatures and therefore less need for cooling by transpiration.

· Hairs on leaves also reflect light and provide additional surface area for cooling the plant and reducing air movement, leading to reduced evaporation.

· Self-shading canopy.

· Deep rootedness.

· Drought deciduousness (section 6.2.1).

Plants that can survive water stress are called drought tolerant. The ability to tolerate drought depends on the stage in the life cycle during which drought occurs. For example, if cowpeas experience a water deficit while they are flowering and forming seeds the yield will be significantly reduced. However, if the same water deficit occurs while the cowpeas are forming leaves, before the flowering stage, then the reduction in yield will be much less³ (section 10.4).

Heat tolerance refers to a plant’s ability to survive and produce under hot conditions. Plants commonly respond to hot air temperatures with increased transpiration to cool the leaf surfaces (section 5.4). Cooling through increased transpiration is an example of why heat tolerance and drought adaptation do not always occur together. A plant may be capable of withstanding high temperatures but if it does so only by greatly increasing transpiration it is not very drought adapted. However, a few of the physical characteristics listed above such as leaf orientation to the sun, hairs on leaves, and light leaf color reduce leaf temperature in ways that do not increase transpiration.

Distinguishing between heat tolerance and drought adaptation is useful. In most drylands hot daytime temperatures are very common and so heat tolerance is a desirable characteristic. However, in gardens that receive a regular supply of water, drought adaptation may not be necessary. This is especially true if other varieties or different crops will give a bigger and better harvest with the same amount of water and other inputs.

Gardeners in drylands recognize that heat-adapted crops may differ widely in drought adaptation. We saw an example of this at a new rural settlement in arid Sonora State in northern Mexico where villagers were planting fruit trees. The only source of water for the 30 households in the village was a well 2 km (1 mi) away, and each household had only a few small containers for carrying the water on foot. Orange, pomegranate, papaya, mango, guava, and lime trees were planted. All of these trees were growing vigorously in a nearby town which has a reliable piped water supply. But the harsh, dry conditions in the new settlement were killing all except the lime trees, which were growing slowly. According to the gardeners, lime trees are the best for coping with heat and drought. These villagers also have avocado seedlings in containers that they keep in the shade near their houses. They said they will not plant the seedlings out into their gardens until a more secure water supply is found, because they know that avocados would not survive the heat and sun exposure with the little water they could provide.

5.6 Salt tolerance

The accumulation of salts can be a serious problem in dryland gardens and agriculture. Discussion of salty soils and water, and related management techniques can be found in section 9.3.1, Box 11.1 in section 11.2, and section 12.6.2. Whatever the source of salts, when they become concentrated in the soil they have an osmotic effect on plants. This results in a slower uptake of water and changes in hormone production leading to lower rates of transpiration and photosynthesis and increased respiration. The browning of leaf edges described in section 13.4.2 is a sign of this (Figure 5.9). Eventually under saline conditions, insufficient energy is available for the plant to grow or even maintain itself, and it will die.

Figure 5.9 Salt Burn

Some plants are less sensitive to salt accumulations than others and are referred to as being salt tolerant. Halophytic (salt-loving) plants actually like salty growing conditions, producing more as salinity increases to low levels. Some salt-tolerant dryland garden crops are beets, asparagus, cowpeas, spinach, date palms, and some tomatoes.⁴ There are also many salt-tolerant indigenous crops and wild plants, and new salt-tolerant varieties of widely grown crops are also being developed.⁵ Plants that are particularly sensitive to salinity such as the stone fruit and citrus trees are called halophobic.

5.7 Seasonal constraints to plant growth

No matter how carefully the garden environment is improved and managed, there are times when certain plants will not grow. This may be due to their needs for particular daylengths or temperatures that do not occur during some seasons. Local gardeners know the appropriate growing seasons for their crops, but they may be unfamiliar with the needs of newly introduced crops. Understanding seasonal constraints to plant growth improves the chances for healthy, vigorous garden plants.

5.7.1 Daylength Requirements

Some plants have a photoperiod requirement for a certain number of hours of darkness before they will grow, flower, and produce fruit. Without this they will not complete their life cycle and will not produce fruit and seeds for gardeners to eat and to plant in the future.

Closer to the equator there is less difference between hours of darkness and hours of daylight, both daily and seasonally. It is not unusual to find crop varieties from the tropics and subtropics with longer darkness requirements than varieties from higher latitude areas. For example, when grown in the northern Sonoran Desert where we live, some beans from central and southern Mexico will not flower until September, even if they are planted in March. This is because we live farther north where the longer nights the beans need to flower do not occur until September, the beginning of the cool season (Figure 5.10). Because beans are warm-season crops they cannot be sown early enough in the year to take advantage of the long nights of late spring. Therefore the only time for us to plant these varieties is in the late warm season, late July or early August, and this does not give some beans enough time to mature before the freezing weather in November.

Onions and sesame are other garden crops whose production is controlled by photoperiod sensitivity, although precise requirements differ by variety. For example, long-night (more than 12 hours, sunset to sunrise) varieties of onions and sesame are required for semitropical savanna West Africa. If short-night (less than 12 hours) onion varieties adapted to higher latitude temperate regions are planted in this area of West Africa they will not form bulbs.⁶ Tomatoes are an example of a photoperiod-neutral garden crop.

Figure 5.10 Daylength Sensitivity

5.7.2 Temperature Requirements

Like daylength, plants’ temperature requirements vary greatly both between and within species. Most egg-plants, cucurbits, some pulses and peppers require soil temperatures above 15°C (60°F) for normal germination and seedling development.⁷ Some varieties of deciduous fruit trees require a minimum number of days at temperatures below 0°C (32°F) for dormancy in order to produce fruit (section 14.4.1).

A uniform problem among all plants of one variety that cannot be traced to any other cause - such as failure to produce flowers, fruit, or bulbs, or bolting (premature flowering) - may be a sign that the plant’s daylength or temperature requirements are not being met (Figure 5.11).

Figure 5.11 A Uniform Problem Such as Bolting may mean Growth Requirements are not Being Met

5.8 Resources

Through careful observation and long experience many farmers and gardeners understand a great deal about their crops. They are the best resource for learning about how local crops live and grow. For a Western science approach, basic principles of botany can be found in many school textbooks. The perspective of a botanist or ecologist is often different than that of an agronomist. While the first two approach the subject with a broad environmental outlook, the agronomist often tends to emphasize production economics. This results in different priorities and concerns, and most importantly, in asking different kinds of questions. Frequently the information in botany or ecology books is more relevant to small-scale, low-input food production, such as household gardens. This is especially true for drylands because agronomy often assumes a modified, optimal environment for crop production, instead of considering how best to cope in a marginal environment with limited resources.

On the other hand, there are many valuable agronomy texts and a growing number of agronomists whose approach to food production is appropriate for small-scale, marginal systems. Lessons 23-27 and 31-32 in Agriculture Tropicale en Milieu Paysan Africain (Dupriez and De Leener 1983) describe the needs of plants for resources such as water, air, and light. Part I of Crops of the Drier Regions of the Tropics (Gibbon and Pain 1985) includes a section on “Crop Factors” with a good discussion of drought and water use in crops. The Better Farming series of pamphlets from the FAO (1976-1977) contain simple discussions of botany and other topics relating to agriculture.

References

¹ Feldman 1988.

² Hall, Foster, and Waines 1979.

³ Hall, Foster, and Waines 1979:156-157.

⁴ Ayers and Wescot 1985:31-35; Cox and Atkins 1979:300-304.

⁵ NAS 1990:17-39.

⁶ Kassam 1976:79, 82,104.

⁷ Hartmann and Kester 1983:147.

6. Growing plants from seeds

Most annual garden crops are grown from seed, and so are some perennials. It is easy to grow crops from seeds, and seeds can be traded, transported, or stored. An important reason for using seeds from open-pollinated garden crops is to maintain genetic diversity. The variability that exists for many traits between individual, open-pollinated plants allows gardeners to continually select plants best adapted to changing needs and conditions. In Chapter 14 we discuss genetic diversity and what it means for the gardener, her garden, and for all of us. In this chapter we discuss how seeds are produced and present ideas for planting them in drylands.

6.1 Summary

Many garden crops reproduce sexually when male reproductive cells (contained in pollen) and female reproductive cells (contained in ovules) are joined together during fertilization, producing an embryo that will be contained within the seed. After the seed matures, it will germinate and grow if environmental conditions are right. Appropriate techniques for preparing and planting seeds and for watering, mulching, and shading seedlings conserve water and protect the seedling from the harsh environment. Diagnosis and remedy of seed planting problems may include a germination test to check the health of seeds. Once the seedlings have emerged, thinning them can improve vigor and production.

6.2 Sexual reproduction in plants

Seeds and the plants that grow from them are the products of sexual reproduction. Some garden crops can be propagated vegetatively through asexual reproduction, which is discussed in Chapter 7.

Sexual reproduction is the combination of genetic material from the reproductive cells or gametes: sperm contained in pollen from the male combines with the ovule in the female (Box 14.1). The result is a seed that carries characteristics of both parents. Flowers are the specialized plant parts where the gametes are produced, and those flowers with female parts are the site of pollination, fertilization, and seed production.

6.2.1 Life Cycles

Plants that produce seeds have two distinct phases of growth. During vegetative growth roots, stems, and leaves grow, and during reproductive growth the plant’s resources are focused on developing flowers, seeds, and fruit. A plant’s life cycle is defined as the time it takes to produce seeds. How long an individual plant lives is its life span. In some plants, life cycle and life span are the same length of time; in others they are not.

Annual plants are those that take 1 year or less to go through their entire life cycle: germination of the seed, vegetative growth, reproductive growth, and seed production, after which they die. That is, their life cycle and life span are equal. This is also true of plants that spend their first year in the vegetative growth stage, and enter and complete their reproductive growth stage and die in their second year. These plants whose life cycle and life span are both about 2 years long are called biennials. Perennials are those plants that live longer than 2 years, usually going through vegetative and reproductive stages each year after an initial period (1 or more years) of only vegetative growth. That is, their life cycle may be 1 year long, but their life span is much longer as in the case of olive trees, which can live for hundreds of years. On the other hand many agaves, whose swollen leaf bases, roots, and flower stalks are eaten, and leaf fibers used for weaving, have a life span of about 20 years. During this time they go through only one life cyle, producing a flower stalk once and then dying.

Most annuals and many biennials are herbaceous, that is their aboveground growth is green, pliable, and tender. Many perennials such as bananas and yams are herbaceous as well. However, some are woody in that their stems, trunks, or branches become hard, rigid, and covered with bark, as with olive and peach trees.

Some dryland perennials such as pomegranates, figs, the stone fruits, and jujubes are deciduous. That is they have a repeating seasonal cycle of losing their leaves, and becoming dormant, followed by a period of growth, leafing out, and flower and fruit production (Figure 6.1). Nondeciduous perennials are sometimes referred to as being evergreen. Both deciduous (e.g., fig) and evergreen (e.g., carob) trees may lose their leaves to reduce transpiration during extreme drought.¹ Because they avoid drought in this way such plants are said to be drought deciduous. Cassava is a drought-deciduous, short-lived perennial that loses all but a few leaves on the ends of its stems during drought.²

6.2.2 Flowering

In plants male gametes, contained in pollen grains, are produced in the anthers, and female gametes, contained in ovules, are produced in the ovary. Some plants like okra have perfect flowers which contain both male and female structures. Monoecious plants have separate male and female flowers on the same plant as in most squashes and maize. Dioecious plants such as the pistachio and date palm bear female flowers on one plant and male flowers on another (Figure 6.2). The papaya is an interesting example of a tree that can be perfect, monoecious, or dioecious. Dioecious papaya plants may even change sex, and in savanna West Africa dioecious male papaya plants are cutback to the ground to encourage female shoot production.³

The flowers of many herbaceous garden plants last only a very short time. Squash blossoms, for example, wither and drop off after only 1 day. Stressful conditions such as high temperatures and drought may shorten the flower’s life as well, making hand pollination useful (Box 6.1 in section 6.2.3).

Figure 6.1 Yearly Cycle of a Deciduous Tree - the Pomegranate

Figure 6.2 Perfect, Monoecious, and Dioecious Flowers (1)

Figure 6.2 Perfect, Monoecious, and Dioecious Flowers (2)

6.2.3 Pollination

Pollination happens when a pollen grain lands on the stigma, the receptive surface of the female flower part where the pollen grain germinates, and grows down the style to reach the ovary. Flowers can be cross-pollinated or self-pollinated. Cross-pollination occurs when pollen from one plant pollinates the flower of another plant in the same species that is genetically different. When the pollen from a male date palm is blown onto the flowers of a female tree, cross-pollination has occurred (Figure 6.3). An example of cross-pollination of a monoecious plant is the pollination of maize when pollen from one plant is blown to the silks of other plants (Figure 6.9 in section 6.2.4).

Figure 6.3 Wind Cross Pollinates a Dioecious Plant

Self-pollination refers to the pollination of a flower on a plant that is genetically identical to the pollen donor. Two thyme plants started by cuttings from the same original plant may pollinate each other, because they are genetically identical this is self-pollination, not cross-pollination. Other examples of self-pollination are when a monoecious plant such as a squash or a plant with perfect flowers, like sesame, okra, or tomatoes, pollinates its own flowers, often with help from insects (Figures 6.4 and 6.5).

Knowing how plants are naturally pollinated improves the gardener’s understanding of how different genetic combinations occur. It also helps her control pollination for seed production, selecting parent plants with the most desirable traits (Box 6.1).

When pollen is carried by the wind to female flowers, as in Figure 6.3, they are said to be wind-pollinated. Examples of wind-pollinated crops are maize, dates, pistachios, olives, and the amaranths. Insect-pollination occurs when insects carry the pollen to the female flower parts, as in Figure 6.4. Some insect-pollinated crops grown in dryland gardens are the cucurbits, pulses, tomatoes, garlic and onions, the stone fruits, and mangoes. Box 6.1 discusses how wind- and insect-pollination can be controlled.

Figure 6.4 Ants Assist the Self-Pollination of a Monoecious Plant

There is no absolute rule for distinguishing wind-and insect-pollinated plants, however, the flower is often a good clue. Plants with many inconspicuous, small flowers lacking color or fragrance are often wind-pollinated. Their pollen is relatively dry, light, and easily blown by the wind.

Showy, fragrant, white, or brightly colored flowers usually rely on insect-pollination. Their appearance or fragrance attracts insects such as wasps, bees, ants, flies, and butterflies. Bats, rodents, and some birds also act as pollinators. Because the pollen in these flowers is frequently heavy, moist, and sticky it adheres to the insects or other animals which carry the pollen to another flower, pollinating it when they rub against the stigma.

Figure 6.5 A Perfect Flower Self Pollinated with Help from a Bee

Box 6.1 Controlling Pollination If successful pollination of a crop by wind, insects, or other natural means is uncertain, then hand-pollination can be done. Examples are when the number of female flowers is limited (as in squash), when pollen supply is limited (as in date gardens), or to take advantage of environmental conditions most favorable for fertilization (as with cool mornings for maize). When controlling pollination, the first step is to identify the plant’s life cycle and flowering characteristics. Flowers that are just about to open are best for hand-pollination. With flowers such as squash, which are usually pollinated by insects, the pollen can be rubbed on the sticky surface of the stigma (Figure 6.6). For wind-pollinated ones, the male blossoms can be shaken over the female flowers, dusting them with pollen (Figure 6.7). Maize, for example, is a wind-pollinated crop that should be hand-pollinated in the cool of early morning because hot, dry conditions will kill maize pollen. The male flowers or tassels are shaken so that the pollen falls on the silks, which are the stigmas and styles of the female flowers. Maize should always be planted in clusters or blocks, not in single rows or as isolated plants, since it often needs cross-pollination between plants for good seed production (section 6.2.4). Pollination may also be controlled to maintain the purity of a specific variety. In these cases steps have to be taken to prevent unwanted pollen from fertilizing the ovaries. In wind-pollinated plants, female flowers can be closed or covered with cloth or paper before and after being hand-pollinated. Wind-pollinated varieties can also be separated from each other in space (e.g., planting different maize varieties at least 0.5-1.6 km or 0.3-1.0 mi apart), and in time (staggering planting times so that different varieties will not be flowering at the same time). The Hopi Native Americans of southwestern North America have maintained a large number of very distinct varieties of maize for hundreds of years by planting the varieties in fields separated from each other. Surrounding insect-pollinated plants with a frame of sticks, bamboo, or wire covered with a finely woven screen or netting may be enough to control pollination by large flying insects. If ants are pollinators, the female blossoms can be covered and tied shut. If the flowers are perfect, their anthers must be removed so that they will not self-pollinate. In monoecious plants the male flowers on the plant should be removed for the same reason. Pollination can also be controlled when gardeners want to improve the drought resistance, taste, yield, or other qualities of their crops. This is done by selecting the male and female plants with the desired characteristics and crossing them to produce seeds. Figure 6.6 Hand-Pollinating a Squash Blossom

Figure 6.7 Hand-Pollinating a Date Palm in Iraq

6.2.4 Fertilization

After pollination the pollen grain germinates and a pollen tube grows from it, down the style, into the ovary and finally the ovule (Figure 6.8). When the male gamete from the pollen grain joins with the female gamete in the ovule, fertilization has occurred. The fertilized ovule will develop into a seed, and in some plants the ovary will thicken around the seed or seeds. This thickened membrane is the fleshy part of a fruit.

Fertilization is important for two reasons: a) fruit and seed foods such as olives, jujubes, okra, tomatoes, and sesame will only be produced if fertilization occurs, and b) seeds are needed for growing many garden plants, especially annuals.

Fertilization will fail if either the pollen or the ovules are no longer viable. A cell, flower, seed, graft, or cutting is viable if it is capable of living. Maize pollen is only released for several hours around sunrise. The pollen is usually viable for about 24 hours but under hot, dry conditions this period is significantly shortened. This is why hand-pollinating maize in the cool of early morning improves chances of fertilization.⁴ Similarly, the pollen from tomato flowers may pollinate the stigma but hot, dry weather can kill the pollen during the approximately 50 hours it takes for fertilization to occur.⁵ This is why some tomato varieties stop bearing fruit under very hot conditions, and why shading can help increase production.

A few crops, such as some maize varieties are self-sterile or self-incompatible. This means that even though they are monoecious, flowers on the same plant cannot fertilize each other. While self-sterile plants are incapable of fertilization by self-pollination,