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.
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.
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.
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.
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.
So, how hot IS too hot for plants? The general answer is around 90 degrees F, with some exceptions to the rule.
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.
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. Drinking at shorter intervals is more effective than drinking large amounts infrequently. Do not drink more than 48 oz (1½ quarts) per hour!
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.
Avoid full sun
In the current hot breezy weather it is probably best to water in the early evening. This gives the plants enough time to dry out, but there is still the chance for overnight water uptake by the roots. And if you want to water in the morning then start very early – before the sun is shining.
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.
Containers dry out more quickly than garden beds, so it's best to increase watering in the summer. During heatwaves, water container plants twice a day, once in the morning to give plants a boost before the sun begins to bake down, and once in the evening to replenish what the day took away.
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.
Even though a water temperature of 110° F is 'relatively-safe', exposure can be painful; the human pain threshold is around 106-108° F. As the chart reproduced below reveals, the severity of a burn is a function of the temperature of the water and the duration of the exposure and the condition of the skin.
As water boils at this temperature, it changes from a liquid to a gas. That gas, or water vapor can continue to rise in temperature. Liquid water however becomes unstable at 212 degrees Fahrenheit or 100 Celsius and cannot “boil” in as it will have already transformed into water vapor.
Sometimes, it can take up to 10 to 15 minutes for the water in the kettle to cool after it has boiled. Pouring fully boiled water into an open pot allows it to cool faster, typically reaching 80 degrees in minutes.
Water daily for the first week to keep the soil moist, but not soggy. After seven days, you can cut back to watering the flowers just a few times per week to encourage deep root growth. While roses and other flowers look pretty with water droplets glistening in the sun, wet foliage does nothing good for plants.
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.
Q. How much water do plants need a day? Plants don't need daily watering. Instead, water deeply but less frequently.
Some plants died and others were fried by the record-breaking temperatures. Even some that look extremely bad, like rhododendrons and hydrangeas, can bounce back. Burned leaves don't necessarily equal a dead plant, so patience and observation is needed.
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.