Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Irrigation is a critical management tool for producing high yielding and high-quality vegetable crops. Direct seeded vegetables such as peas, lima beans, sweet corn, spinach, cucumbers, and snap beans require adequate soil moisture and certain soil temperature optimums to germinate and emerge. If soils are dry at planting, irrigation will be required to assure rapid and even emergence. This is particularly critical for processing vegetables where delays in emergence can cause lengthened times to maturity, affecting harvest timing. Irregular emergence in dry soils can also lead to difficulties in processing crop harvest scheduling due to variable maturities in the field.
Sandy loam soils need about a half inch of irrigation to wet the soil down to 6 inches to insure germination until the next rain. Heavier loam soils may need 0.7 inches to 0.9 inches of water to wet the top 6 inches of soil.
In extremely dry soils, such as planting no-till into a burn-down rye cover crop, irrigation water should be applied prior to planting to improve planter performance and seed germination. Fields with heavy cover crop also may require irrigation prior to burndown and planting
Having the irrigation system ready to run when you plant can make the difference between a good stand with maximum yield potential or having a poor or variable stand with lower yield potential.
Scheduling irrigation for different vegetables grown under center pivot, travelling gun, or solid set overhead systems involves knowledge of the soil water holding capacity, the effective rooting depth of the crop (how deep water can be drawn by the crop), how efficiently water is being delivered (water losses to evaporation before it reaches the crop and how much water is lost to runoff), how much water is being used by the crop (transpiration) and how much water is being lost from the soil and wetted surfaces directly (evaporation). The combination of transpiration and evaporation losses is termed evapotranspiration.
To schedule irrigation, the goal is to replace water lost through evapotranspiration without excessive runoff or excessive loss through percolation out of the root zone. Another factor to consider is the permissible water depletion; how much will you allow the soil to dry down between irrigations. For most crops we set this at 50% of the water holding capacity of the soil. However, for some shallow rooted crops you may want to keep that value lower (only allow for 30% depletion between irrigations). By knowing how much water is being lost and how much is left in the soil, you can determine when to irrigate and how much to irrigate.