Irrigation for the Garden

The primary objective of irrigation is to provide your plants with the right amount of water at the right times so that they can develop vigorous root systems, which are vital for plant health. A large root system allows a plant to utilize a greater volume of soil for extracting water and nutrients. It also provides a consistent supply of water, even during periods of drought. While too little water can seriously limit plant growth, or even kill the plant, too much water is unnecessarily wasteful, and can also impair growth. Successful watering requires knowledge of your soil type, of the temperature and rainfall patterns in your area, and of the specific needs of your plants throughout their life cycle.

The ideal soil for most plants has good structure, meaning that the soil particles and organic matter are arranged so that they retain a reasonable amount of moisture, yet are fairly well drained. The air spaces between soil particles provide oxygen, which is essential for soil microorganisms and healthy root growth. If a soil is water logged, such that all the spaces between soil particles are filled with water, most plants (with the exception of water-loving plants such as rice) will become weak and may die if the soil remains saturated. On the other hand, soil that dries out quickly needs to be irrigated frequently to maintain healthy plants (with the exception of drought-tolerant species that prefer dry soils).

Amending your soil with mature high-quality compost, and growing and turning under soil-building cover crops, help achieve this balance between water retention and drainage. These are the most effective activities that the gardener can do for improving soil structure. Otherwise, the water retention/drainage balance is largely pre-determined by the texture of your soil. A heavy, clay soil should be irrigated less frequently because it retains water, whereas a light, sandy soil requires more frequent watering because it rapidly drains water.

Plants growing under hot, dry conditions lose more moisture into the air than those in cooler, more humid regions. A good rule of thumb is that in temperate, humid regions most vegetables need about 1 inch of water per week over the course of a growing season (either from natural rainfall or supplemental irrigation). In arid regions they need about 2 inches per week. As plants become older the amount of water should be increased because of larger leaf surfaces and root systems, which use progressively more water.

Cool season annual vegetables such as brassicas (broccoli, cabbage, etc.) and leafy greens typically root at shallower depths than warm-season annuals such as tomatoes and vine crops (squash, cucumber, melon, etc.) and perennial crops. Therefore, the deeper-rooted crops are watered less frequently, but more water is applied at each irrigation.

Many native plants in arid climates use very small amounts of water, so be sure not to over water in these instances. It is wise to seek advice from your local plant experts.

Regardless of plant type, more frequent watering is necessary in the early stages of growth to insure plant establishment. Once the plants are established, longer but less frequent watering encourages deeper root growth.

There are several methods of irrigating, each with particular advantages and disadvantages. In all cases it is best to avoid watering in the middle of the day to help conserve this vital resource.

1. Watering Can: The well-designed watering can is a great way to water a small garden. It puts the water right where you want it, thereby discouraging weed growth in the paths, and conserving a precious commodity. It is also a nice combination with a rain barrel and allows us a more intimate relationship with our plants.
2. Garden Hose: A quality garden hose fitted with a water breaker is a good method when the garden is too big to be handled with watering cans but you still want the intimacy and control of hand watering. Care must be taken though to prevent the hose from trampling over the plants. Carefully placed pieces of pipe or stakes at the ends of the rows can help alleviate this.
3. Overhead Sprinklers: Garden sprinklers attached to a hose take almost no time to set up but must be moved around if the garden is larger than the sprinkler can handle. These are good for small- to medium-sized gardens. For more time and expense, micro-misters can be hooked up to plastic irrigation lines and automated with a timer in larger gardens or in commercial production fields. The water spray from these misters is very effective for cooling off leaf surfaces of lettuce and other greens, peas and brassicas, which is important in hot, dry climates. For large operations, Rainbird sprinklers (drop latch) attached to aluminum pipe are cheaper and less time-consuming than micro-misters, but the large droplets they emit (much larger than the mister droplets) can cause crusting in some soils.
4. Drip Irrigation: A drip system requires extra time in the initial setup, but it is the most water-conserving technique, especially when combined with deep mulching. It directs water only to the plants you are intentionally growing and keeps unwanted weed growth to a minimum. In extremely arid climates, drip irrigation and deep mulching may be the only economically and environmentally feasible watering technique.
5. Flood Irrigation: This ancient method of watering is extremely effective for deep watering, and is especially useful for tall crops such as corn and sunflowers. This method does not require much hardware, except for a pump and some pipe or, if you're lucky, gravity flow from a tank or a ditch. Flood irrigation does require a lot of water though, as well as labor to form the furrows and ridges. Flooding also can also promote the growth of noxious weeds such as wild bindweed.

The type of irrigation that you choose may, in part, be governed by the kinds of plants you are growing. Many plants prefer to be watered at the base of the plant (bottom watering), because watering their leaves encourages bacterial and fungal leaf diseases. This can be a problem particularly in humid climates. These crops include beans, cucumbers, eggplant, gourds, squash, melons, peppers, tomatoes, okra, onions, and potatoes. Corn is also best watered from below because it often gets too tall for most sprinklers to effectively water all the plants. As mentioned above, leafy greens, peas, and brassicas prefer overhead watering in arid, hot climates. However, in cooler, moister climates bottom watering may be better to minimize leaf diseases.

Most flowers, herbs, and vegetables (including carrots, beets, chard, radishes, turnips, leeks, rutabaga, and parsnips) can be overhead or bottom watered regardless of climatic conditions.

Finally, be sure not to wait until your plants are wilting before you irrigate. While they can recover, it certainly slows their overall growth and makes them more susceptible to insect and disease problems. The good-old time-honored method is to stick your finger down into the soil a few inches to see if there is adequate moisture. Paying attention is always the best medicine.

Steve Peters, Agricultural Planner at Seeds of Change