How to Watch for Water Stress
When plants are chronically underwatered, they experience water stress—a more serious situation than simply wilting a bit during the heat of the day. A plant that gets a little too hot usually recovers when the day cools off, but plants still drooping in the morning or late evening need water fast! (Some waxy-leaved plants like cabbage, onions, and garlic don't show water stress as readily as plants like peas, lettuce, and spinach do, so water them very carefully for signs of water stress.)
Water-stressed plants can have leaves that are smaller than normal. The edges of those leaves may turn brown, and the flowers and fruits will be delayed or drop from the plant. Water stress also reduces the quality of the produce—for example, your cucumbers may be small and misshapen; tomatoes may develop blossom-end rot; and salad crops and celery may have tough fibers. (Note: Water-logged soil causes plants to exhibit many of the same symptoms.)
When Water Is Critical
Moist soil is essential for seed germination and seedling growth. Try to set your young transplants in the garden on cloudy days or in the evening. And always water your seedlings when you set them out.
Once plants pass the seedling stage, essential watering times vary from crop to crop. Sweet corn needs an abundant supply when the silks and tassels are forming. Tomatoes, cucumbers, and squash are especially thirsty when they're flowering and when their fruit starts developing.
On the other hand, there are times when it's necessary to cut back on watering. Most muskmelons will taste better and have better quality if you start to cut back on water 7 to 10 days before harvest. And onions cure faster and store better if you hold off the water after they reach maximum bulb size and about half the leaves have fallen over. For pumpkins and winter squash, you want to dry the vine up before harvesting.
One thing to remember if you have to ration water: Certain plants can take drought better than others. Deeply rooted melons are somewhat tolerant to such stress, as are asparagus and beets. Tomatoes and brassicas are semitolerant. But don't deny a drink to sensitive celery, strawberries, lettuce (especially head lettuce), cucumbers, squash, and peppers.