What is the best time to water plants?

This summer's relentless heat has posed challenges to gardeners and their plants. Everything dries out faster when it's hot: flowers and vegetables, whether in a garden or in containers, young trees and shrubs, and lawns from small urban yards to large suburban spreads.

Repeated wilting and re-hydrating is detrimental to most plants, so one of the biggest challenges is trying to maintain a relatively steady supply of moisture to the soil. This is not just to keep plants looking their best (though it does that) but also to promote continued healthy growth and help plants cope with the stress of summer diseases and insects.

While some herbs (marjoram, oregano and rosemary, for instance) thrive and develop their most intense flavor when grown under hot, dry conditions, many vegetables are negatively affected by heat coupled with wide moisture swings. Cucumbers may be misshapen and bitter-tasting. Potatoes may be knobby, and can develop green areas that are mildly toxic when sunlight penetrates through the cracks of hard, dry soil. (Peel off those green areas before you cook the potatoes.)

Tomatoes frequently develop blossom end rot, a dark, leathery splotch that begins where the flower was attached, and enlarges as the fruit grows. (You can slice off that part of the tomato and use the rest, but you'll lose a sizable chunk of each fruit, and you'll need to use it right away.) The dark area is caused by a lack of calcium available to the developing fruit, primarily from an interruption in moisture uptake.

Adding calcium to the soil usually doesn't solve the problem. Instead, focus on maintaining constant moisture levels. Mulch around tomato plants in the garden so you don't have to hoe weeds, potentially severing tomato roots in the process. Water deeply, but don't wait until the soil is bone dry to water again. If your tomato is in a large container, you may have to water more than once a day during the worst heat.

Any plants -- flowers, herbs or vegetables -- growing in containers will dry more rapidly than if they were in the ground. How often you water will depend both on the size of the container and what you are growing. But frequent watering poses a problem, too: It leaches nutrients through the soil.

If you incorporated timed-release fertilizer pellets into the soil, the plants might be fine -- at least for most of the summer -- because every time you water, a bit of fertilizer is released. Otherwise, make a point of fertilizing more frequently than you normally would -- every week or two, using fertilizer dissolved in water. Don't fertilize when the soil is dry, though. That's a recipe for burning the plant's roots.

Spread several inches of mulch in the garden and around young trees and shrubs to help insulate the soil, keeping roots from getting too hot and dry. This also cuts down on surface evaporation of moisture from the soil. Set your hose to trickle slowly at the base of young trees and shrubs every few days during hot dry weather, to thoroughly soak their roots.

Water gardens early in the day when it's cooler and less windy. Watering in the heat of the day shouldn't hurt the plants -- it actually cools them off -- but it's a far less efficient use of water as much of it will evaporate before reaching the roots. Avoid getting plants wet late in the day unless it's the only possible time you can water them. If they don't dry before sundown they'll stay wet all night and be more prone to fungal and bacterial diseases.

Water lawns thoroughly once a week or perhaps a little more frequently in extreme heat, to yield deeper roots and healthier grass. Sometimes irrigation systems are set to water a relatively short period daily or every other day. Light watering, however, encourages shallow root growth, which leaves the grass more vulnerable to heat and drought damage, particularly if you're unable to faithfully maintain a frequent watering schedule. (This is a real possibility as municipalities often declare water restrictions in late summer.)

Don't fertilize the lawn or spray herbicides when temperatures are high. The potential for damaging the lawn is too great. Hold off fertilizing until September, when nights are longer and temperatures moderate. If you must get rid of weeds this time of year, dig them out manually. Unfortunately, the heavy rains we had earlier interfered with the efficacy of pre-emergent herbicides and as a result, we have a bumper crop of crabgrass. These plants are annuals and will die this winter, but their seeds will fall to the ground and sprout next spring.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota.