COLUMNS
Vegetable Irrigation

by Vern Grubinger
Money well spent for grower insurance

Photos by Vern Grubinger Trickle irrigation is especially efficient when used to deliver water to small crops in small areas of the field.

In just about any area of the country, an irrigation system is a good investment for vegetable growers. When I started in extension 20 years ago, there were still quite a few vegetable farms in the Northeast without irrigation; now they are very few and far between. There are several reasons for this change: greater awareness of the benefits of irrigation; the development of lower-cost systems, especially among drip systems; and the growing need for irrigation as rainfall patterns have become more erratic with the changing climate.

The following information is taken from a fact sheet by Erik J. Sorensen of Washington State University Cooperative Extension. He points out that on the West Coast, too, vegetable crops, with few exceptions, are grown under irrigation. Even in areas where precipitation is relatively high, reliable yields generally require supplemental irrigation. In low-rainfall areas, irrigation assumes a special importance, as profitable vegetable production is only possible if irrigation is available.

Mulches that reduce evaporation of water from the soil surface can reduce the need for irrigation. Sprinkler irrigation is not as efficient as drip irrigation, but this solid set system has the advantage of being able to deliver a lot more water quickly and it can be moved from field to field as needed.

Reasons to irrigate

There are two main reasons to irrigate. First, most vegetables have shallow roots, rarely exceeding 24 inches in depth. Second, the marketable product of most vegetables is sold on the basis of fresh weight and appearance, which make vegetables particularly sensitive to water shortages.

Other reasons include allowing flexibility in planting time, establishing a uniform plant stand, influencing soil temperature, controlling soil erosion by wind and promoting the uptake of plant nutrients from fertilizers and residues.

Lack of water has a negative impact on crops in many ways, the severity of which depends on the duration and time of water stress in relation to the stage of growth of the crop.

Irrigation needs of crops

Vegetable crops require around 6 inches of water in a season for radishes to 24 inches for tomatoes and watermelons. There are two periods when an adequate supply of water is critical that are common to nearly all vegetable crops: during harvest and two to three weeks before harvest. However, different types of vegetables have unique irrigation needs.

Leaf vegetables such as cabbage, lettuce and spinach are generally planted at or near field capacity. Shallow-rooted, they benefit from frequent irrigations throughout the season. As leaf expansion relates closely to water availability, these crops, especially cabbage and lettuce, are particularly sensitive to water stress during the period of head formation through harvest. Overwatering can result in burst heads.

Broccoli and cauliflower, although not grown specifically for their leaves, respond to irrigation much as the leaf vegetables do. Cauliflower, in particular, is sensitive to water stress at all stages of growth, responding to drought with reduced growth and premature heading.

In root, tuber and bulb vegetables, like carrots, beets, radishes, potatoes and onions, yield depends on the production and translocation of carbohydrates from the leaf to the root or bulb. The most sensitive stage of growth generally occurs as these storage organs enlarge. Carrots require an even and abundant supply of water throughout the season. Stress causes small, woody and poorly flavored roots. Uneven irrigation can lead to misshapen or split roots in carrots and early bulbing in onions.

Fruit and seed vegetables such as cucumbers, melons, pumpkins and squashes, lima beans, snap beans and peas, peppers and tomatoes are most sensitive to water stress at flowering and as fruits and seeds develop. Fruit set on these crops can be seriously reduced if water becomes limited. Regular supplies of water during the period of fruit enlargement can reduce the incidence of fruit cracking and blossom-end rot in tomatoes. Irrigation is often reduced as fruit and seed crops mature.

The New England Vegetable Management Guide includes this table showing the critical periods for adequate water supply where growers should be prepared to irrigate different crops.

Making the most of irrigation

Water is a precious resource. In some areas it is a costly input. Here are irrigation practices and strategies for vegetable crops that can help make efficient use of water.

• Reduce the area planted if irrigation water is in short supply. It may be necessary to take some land out of production. If you have a choice, plant the most productive land rather than marginal land.
• Begin the season with adequate soil moisture. Preplant irrigation benefits many vegetable crops, as such irrigation builds subsurface soil moisture and promotes a deeper root system.
• Avoid over-irrigation, which wastes water and can leach nutrients and chemicals into groundwater.
• Promote rapid crop emergence and a uniform plant stand to make the most efficient use of soil moisture. Wet soil exposed to sunlight has greater evaporation loss than does soil shaded by a crop. Once a full canopy has developed, differences in evapotranspiration per area due to plant population are negligible. Reducing the plant population in vegetable crops saves little water.
• Consider using transplants. Proper germination and emergence in the field require careful water management. Less water and more precise control can often be obtained by using transplants.

Once in the field, however, transplanted crops generally develop shallower root systems than direct-seeded crops and may require more frequent irrigation.

• Use mulches. Plastic or organic mulches can save water by reducing surface evaporation.
• Consider drip irrigation, since it is an efficient system of delivering water to high-value crops like vegetables. Combine use of such systems with mulches for added efficiency.
• Optimize irrigation scheduling so as to apply the correct amount of water at the correct time. Irrigation scheduling requires careful attention to monitoring soil moisture, climate and crop growth.
• Maintain good soil structure and fertility. Good soil structure permits optimum infiltration and water holding. Proper soil fertility encourages the best plant growth and utilization of available soil moisture.
• Achieve good weed control, since weeds compete with crops for soil moisture and decrease yields. In particularly weedy fields, weeds can use more water than the crop. Good weed control reduces competition for soil moisture and increases water use efficiency.
• Maintain good plant health. Insect and disease damage restrict the growth and water use efficiency of vegetable crops, reducing both yields and quality. This is especially true with regard to diseases classified as wilts, which reduce the ability of the crop to absorb and translocate water.

Careful attention to irrigation is an essential part of vegetable production and will pay off with improved crop quality, more reliable yields and greater profit. As production costs rise, so does the need to safeguard investments in seed, fertilizer, labor and land against losses resulting from changes in the weather.

The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at vernon.grubinger@uvm.edu.