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.
And most importantly, plants absorb water more readily when they're not under heat stress—a condition that's more likely to occur when the sun is at its peak. So while it's perfectly okay to water plants while they're in full mid-day sun, it's not the most efficient way to go about it.
The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt. Water evaporates into the atmosphere more quickly in high heat, draining a plant's reserves.
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.
It's important to apply water to the whole root zone, which reaches at least to the dripline of the plant. Use a soaker hose or sprinkler for 30-60 minutes at a time. During normal summer temperatures, this type of watering can happen every 2-3 weeks. In times of extreme heat, 1-2 times each week may be necessary.
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.
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.
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.
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.
Water deeply at least two to three times a week. When we say deeply, that means at least an inch of water every time you water.
But don't overwater
Plants sometimes wilt from the severe sun, even when they have sufficient water. Test the soil before you water, Young said. If a pot feels heavy or the soil feels damp an inch or so under the ground, your plant probably doesn't need more moisture.
Most plants shut down during extreme heat, so if it is over 100 degrees, it may be best to wait to water even if your plant is stressed.
Most of the time, when plants experience trouble due to relatively short heat waves, the problems go away quickly after the heat subsides. If you take the following measures to reduce heat stress on plants during a heat wave, they should recover once the temperature drops.
If the soil feels dry, it's time to water. In the spring and fall and in cooler climates, it might be sufficient to water container plants every two to three days. In the summer and in warm climates, container plants usually require daily watering, unless they are succulents or other drought-tolerant xeriscape 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.
Keep in mind that morning sun, which is cooler and less intense, is easier on shade loving plants then the hot afternoon sun. Plants that receive too much intense light will scorch if pushed into too much hot sun.
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.
In the morning, plants are ready to absorb ample of water that helps them get through the day's heat. Helps the water seep into the garden significantly. Leaves time to dry before nightfall, which helps prevent fungal diseases in the plants.
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.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
If it's 70-80 degrees outside try watering 2-3 times a week. During the spring your lawn will be fairly easy to manage so you don't have to water all the time. In the blazing summer, when temperatures are between 90-100+, try watering 3-4 times a week.
Watering late in the day in this heat and humidity increases opportunities for fungal diseases and visits by snails and slugs. Water in the early morning, period. Water plants deeply and not every day. Although it's labor-intensive, the best way to water during heat waves is by hand.