Although all vegetables benefit from irrigation, each crop responds differently at various stages of growth, says Joe Kemble, horticulturist with the Alabama Cooperative Extension System.

“Critical periods of water needs can best be defined at that time when soil moisture stress can most reduce yield in an otherwise healthy crop,” says Kemble.

“This is not to say that it's the only time in the life of the crop when moisture stress reduces yield. It is, however, the time when moisture stress will exert its greatest effect,” he adds.

Most vegetable crops, he says, are sensitive to drought during two periods — two to three weeks prior to harvest and harvest. “More than 30 vegetable crops are grown commercially in the Southeast, and each class responds differently to irrigation,” says Kemble.

Leaf vegetables

Cabbage, lettuce and spinach generally are planted at or near field capacity. Field capacity is the maximum amount of water a field can hold without water runoff or loss due to gravity. Being shallow rooted, these crops benefit from frequent irrigation throughout the season. Since leaf expansion relates closely to water availability, these crops — especially cabbage and lettuce — are particularly sensitive to drought stress during the period between head formation and harvest. Over-watering or irregular watering can result in burst heads.

Broccoli and cauliflower, although not grown specifically for their leaves, respond to irrigation much like the leafy vegetables. Broccoli and cauliflower are sensitive to drought stress at all stages of growth, responding with reduced growth and premature heading.

Root, tuber and bulb vegetables

In sweet potatoes, Irish 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 become larger. Carrots require an even and abundant supply of water throughout the season. Moisture stress causes the formation of small, woody and poorly flavored roots. Uneven irrigation can lead to misshapen or split roots in carrots, secondary growth in Irish potatoes and early bulbing in onions.

Fruit and seed vegetables

Cucumbers, melons, pumpkins and squashes, lima beans, snap beans, peas, peppers, sweet corn and tomatoes are most sensitive to drought stress at flowering and during fruit and seed development. Fruit set on these crops can be reduced dramatically if water is limited. An adequate supply of water during the period of fruit enlargement can reduce the incidence of fruit cracking and blossom-end rot in tomatoes. For example, after fruit enlargement, irrigation often is reduced as fruit and seed crops mature.

Plant growth stage, says Kemble, also influences the susceptibility of crops to drought stress. Irrigation is especially useful, he says, when establishing newly seeded or transplanted crops.

“Irrigation after transplanting can increase significantly the plant survival rate, especially when soil moisture is marginal and the evapotranspiration rate is high.

“Irrigation also can increase the uniformity of emergence and the final stand of seeded crops. For seeded crops, reduce the rate of application and the total amount of water applied to avoid crusting. If crusting is present, use low application rates and small amounts of irrigation water to soften the crust while seedlings are emerging.”