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| Irrigating the Home Garden |
Author: Diane Relf, Extension Specialist, Environmental Horticulture
Publication Number 426-322, August 1996
Table of Contents
Introduction
Basic Techniques and Principles for Watering
Introduction
Adequate soil moisture is essential for good crop growth. A
healthy plant is composed of 75 to 90% water, which is used for
the plant's vital functions, including photosynthesis, support
(rigidity), and transportation of nutrients and sugars to various
parts of the plant. During the first few weeks of growth, plants
are becoming established and must have water to build their root
systems.
While growing, vegetable crops need about 1 to 2 inches of water per week in the form of rainwater or irrigation water depending on the type of soil. One thorough watering each week is usually enough for most soils. The soil should be wetted to a depth of 5 to 6 inches each time you water and not watered again until the top few inches begin to dry out. An average garden soil will store 2 to 4 inches of water per foot of depth. Keep a rain gauge near your garden, then supplement rainfall with irrigation water if needed. In addition, there are ways to reduce the amount of water you have to add.
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All of the water added to the garden may not be available to plants, particularly if the soil is a heavy clay. Clay particles hold soil moisture tightly; if, for example, there are 4 1/2 inches of water per foot of this type of soil, there may be as little as 1 1/2 inches of this water available for plants. A higher level of humus in the soil, brought about by the addition and breakdown of organic matter, can increase the amount of water available. By causing clay particles to aggregate (stick together), humus also adds air spaces to tight clays, allowing moisture to drain to lower levels as a reserve, instead of puddling and running off the top of the soil.
The moisture-holding capacity of sandy soils is also improved by additions of organic matter. Though most soil water in sandy soil is available to plants, it drains so quickly that roots are unable to reach water only a few days after a rain. Humus in sandy soil gives the water something to cling to until it is needed by plants.
Addition of organic matter, then, is the first step in improving the moisture conditions in your garden.
Mulching is another a cultural practice which can significantly decrease the amount of water that must be added to the soil. A 6 to 8 inch organic mulch can cut water needs in half by smothering weeds (which take up and transpire moisture) and by reducing evaporation of moisture directly from the soil. Organic mulches themselves hold some water and increase the humidity level around the plant. Black plastic mulch also conserves moisture but may dramatically increase soil temperatures during the summer if not covered by other mulch materials or foliage.
Shading and the use of windbreaks are other moisture-conserving techniques. Plants that wilt in very sunny areas can benefit from partial shade during the afternoon in summer. Young plants, especially, need protection. Air moving across a plant carries away the moisture on the leaf surfaces, causing the plant to need more water. In very windy areas, the roots often cannot keep up with leaf demands, and plants wilt. Temporary or permanent windbreaks significantly reduce this stress.
Despite the use of these cultural practices, your garden may need a lot of water. When rainfall is sparse and the sun is hot, watering can benefit your garden with increased yields. It may save the garden altogether in a severe drought.
Irrigation practices, when properly used, can:
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The home gardener has several options for applying water to plants including a sprinkler can; a garden hose with a fan nozzle or spray attachment; a portable lawn sprinkler; a perforated, plastic soaker hose; a drip or trickle irrigation system; or a semi-automatic drip system. Quality equipment will last for a number of years when properly cared for. When making a decision as to which type of watering equipment you will use, there are a number of things to consider.
The purchase of a specific type of watering equipment depends upon available water facilities, water supply, climate, and garden practices. If there is no outdoor spigot near the garden, the expense of having one installed may be greater than the benefits gained, except in very drought-prone areas or in the case of a gardener who is fully dependent on the season's produce. Where rainfall is adequate except for a few periods in the summer, it is wise to keep watering equipment simple. A rain barrel or a garden hose with a fan-type sprinkler may suffice; a water breaker for small seedlings is useful. In areas where there are extended periods of hot weather without precipitation, the local water supply is likely to be short. Since overhead sprinklers waste water, a drip irrigation system may be in order. Drip irrigation puts water right at the roots and doesn't wet plant leaves, helping to control some diseases. Timers are available that allow automatic watering with drip irrigation systems.
Several types of drip or trickle equipment are available. The soaker hose is probably the easiest to use as no installation is required. It is a fibrous hose that allows water to slowly seep out all along its length. It is simply laid at the base of the plants and moved around the garden. There are also complete kits containing attachments and PVC hose with holes to allow gradual water release. These are intended for semi-permanent installation and usually last 2 to 5 years. With this type, a flow regulator usually has to be included with the system so that the water can reach the end of the hose without being sprayed out at full force. A special double-walled type of irrigation hose has been developed which helps to maintain a more even flow. Finally, there is the emitter-type system, best used for small raised beds or container gardens, in which short tubes, or emitters, come off a main water supply hose and go right to the roots of the individual plants. This is generally the most expensive form of irrigation and the most complex to set up, but it has advantages. The weeds in the area are not watered and evaporation from the soil is minimized. This type of system is best used in combination with a coarse mulch or black plastic. Drip systems can have problems with clogging from soil particles and/or mineral salts suspended in water taken from springs or wells. New designs take this problem into consideration; some include filters and self-flushing emitters. It is wise to make a complete investigation and comparison before purchasing a drip irrigation system.
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For overhead or sprinkler watering, adjust the rate of water application to about 1/2 inch per hour. A faster rate will cause run-off unless your soil has exceptionally good drainage. To determine the rate for a sprinkler, place small cans at various places within the sprinkler's reach, and check the level of water in the cans at 15-minute intervals.
When using the oscillating type of lawn sprinkler, place the sprinkler on a platform higher than the crop (to prevent water from being diverted by plant leaves), and try to keep the watering pattern even by frequently moving the sprinkler and overlapping about half of each pattern.
Wet foliage overnight can encourage diseases, so do not use sprinkle irrigation in the evening. Morning watering is preferred as there will be less water lost to evaporation than in the heat of the day. Add enough water to soak the soil to a depth of 5 to 6 inches. The amount required varies with the nature of your soil. Frequent, light waterings will encourage shallow rooting which will cause plants to suffer more during drought periods, especially if mulches are not used. On the other hand, too much water, especially in poorly drained soils, can be as damaging to plant growth as too little water; it deprives the roots of oxygen needed to grow.
By knowing the critical watering periods for selected vegetables or vegetable types, you can reduce the amount of supplemental water you add. This can be important, especially where water supplies are limited. In general, water is most needed:
Specifically, the critical watering periods for selected vegetables are:
| Asparagus | Spear production, fern development |
| Broccoli, Cabbage, Cauliflower | Head development |
| Beans, peas | Pod filling |
| Carrot | Seed emergence, root development |
| Corn | Silking, tasseling, ear development |
| Cucumber, Melon | Flowering, fruit development |
| Eggplant, Tomato | Flowering, fruiting |
| Lettuce | Head development; moisture should be constant |
In areas prone to repeated drought, look for drought-resistant varieties when buying seed or plants.
If water supplies are short in your area and you wish to use "grey water" (water from household uses) on your vegetable garden, a few rules should be observed:
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