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.
When. Hands down, the best time of day to water is early in the morning. Watering during the day can increase risk of disease. If you can't water early in the morning, water late in the evening, especially before days forecasted to have dangerously high temps.
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. Throw a damp sheet over it, or mist it lightly to bring the temperature down.
What is generally agreed is that plants should not be watered while in full sun. 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.
The best time to water plants is in the morning or evening.
More importantly, watering at these times actually helps the plant retain water. If you water in the afternoon, especially during summer, the heat and sun are at their peak and the plant's water will evaporate instead of absorbing into the soil and 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.
As the weather heats up and plants reach full size, you may need to water containers (and especially hanging baskets) twice a day.
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.
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.
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.
So, how hot IS too hot for plants? The general answer is around 90 degrees F, with some exceptions to the rule.
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.
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.
These studies show that root growth slows down or stops when soil temperatures get above 86 degrees Fahrenheit and roots will die when soil temperatures get above 96 degrees Fahrenheit. When temperatures outside are well over a hundred degrees, it's easy for the soil in pots to exceed these temperatures.
Check Moisture Levels
Before watering a container, be sure that the plants need water. Over-watering is just as harmful as under-watering. The soil at surface of the container might look and feel dry to the touch, but the soil might be moist just an inch or two below the surface.
Place containers where they receive some shade during the hottest part of the day. This usually won't reduce their performance, but it will cool the plants and reduce water needs. Moving containers off or away from hot pavement will also help. Place liners or catch trays underneath.
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.
If you forgot to water in the morning, there's still that window in the late afternoon, between 3-5pm. That time, when the sun is almost setting, is perfect because the temperature has already started dropping. In addition, there is also enough sun to aid in water evaporation.
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.
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!
In times of extreme heat, it is vital to water your lawn for about 30 to 45 minutes daily. Once the temperatures drop below ninety, you can cut back to watering three to four times a week, until that blissful thunderstorm comes and quenches your lawn's thirst!
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.