The notion that wet leaves on sunny days cause scorch in plants was disproved nearly ten years ago. But there is no doubt that watering in full sun is not water efficient – as much of it will evaporate before entering the soil. In the current hot breezy weather it is probably best to water in the early evening.
In times of extreme heat, 1-2 times each week may be necessary. Know your plants though! Some plants prefer almost zero water during summer (manzanita, Ceanothus, madrone and cistus) and others wilt even when they have ample water (hydrangea, rhododendron). So adjust watering accordingly.
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.
During really hot weather, water your vegetables at least two to three times a week. Watering the garden deeply is critical. The water must go down, down, down to encourage deep roots and get away from the hot soil surface.
During extremely hot weather (daytime temperatures above 90F and nighttime temperatures above 70F), try to water daily or every other day. In a 10x10-foot garden, this would mean giving your plants 8 to 9 gallons of water each day.
If you forget or are too busy to water your plants on some summer mornings, you can water them at night. However, this should not be your ordinary watering time. Morning is the proper time for watering plants, though you should keep in mind that not all plants need daily watering.
Water Regularly
Using a hose or watering can, water at the base of each plant and strive to avoid overspray on leaves. When the mercury rises, keep an extra close eye on your container plants. Potted plants are the first to suffer in a heat wave and will need more water than in-ground greenery.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
As the weather heats up and plants reach full size, you may need to water containers (and especially hanging baskets) twice a day.
High temperatures speed up the normal living process of plants to a maximum rate at and above 90 degrees F. This means that most plants can take temperatures up to 90 degrees F. fairly well. Anything above that—the hotter it gets, the more they suffer!
Higher temperatures cause the plant cells which control the openings (stoma) where water is released to the atmosphere to open, whereas colder temperatures cause the openings to close. Wind & air movement: Increased movement of the air around a plant will result in a higher transpiration rate.
Mulch, mulch, and mulch some more!
Cover your soil with a blanket of organic material such as straw, leaves, shredded paper or cardboard, or bark. This will moderate soil temperature, prevent runoff and evaporation, and hold moisture in the for longer periods between waterings.
In hot weather you need to water in the mornings when it is cooler. This will allow more water to reach the root system before it evaporates in the heat. There may be times when you cannot schedule watering in the morning. In this case, water the plants in the late evening before bed.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
Best Water for Houseplants
Most tap water should be fine for houseplants unless it is softened because softened water contains salts that can build up in the soil over time and cause problems. Chlorinated water is also safe for most houseplants, but if you have a filtration system, that's much better for your plants.
Overly dry soil is a sure sign that plants will develop heat stress. Water is essential for plants to live. During a heat wave, correct watering is even more important. Early-morning or early-evening watering is the best time to provide moisture to all your plants in the garden or landscape.
The best time to water outdoor plants is in the morning when temperatures are usually cooler. This gives the plants time to absorb the water so they can get through a long, hot day. The second-best time is late in the afternoon or early in the evening. Try not to water at night.
During spring and summer in Southern California, she suggests drip watering three times a week for 18 minutes, then adjusting from there if the soil is too wet or dry. Don't know how to tell? Stick your finger in the soil. If it's consistently moist 2 inches down, your plants are fine.
Answer: Early morning (5:00 to 9:00 am) is the best time to water the garden when using a sprinkler, garden hose, or any other device that wets the plant foliage. When watering is completed, the plant foliage dries quickly. The rapid drying of plant foliage helps guard against the development of fungal diseases.
General advice is to avoid watering in full sun, however, because water will evaporate before it can properly wet the soil. Potted plants will use up water very quickly, so moving them into shade would be advisable, or watering several times a day may be necessary.
Usually when the first inch (2.5 cm.) or so of soil is dry, it's a good indication that watering is needed. In summer, watering outdoor potted plants is necessary daily (and even twice a day) for most species, especially when temperatures reach over 85 degrees F. (29 C.).
Dehydration is a primary contributor to heat exhaustion. Your work performance may suffer when you are dehydrated, even if you don't notice. When working in the heat, drink 1 cup (8 ounces) of water every 15–20 minutes. This translates to ¾–1 quart (24–32 ounces) per hour.