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.
Be Consistent
You should check the moisture level around the base of your plants. Determine the watering schedule you need to keep in order to maintain the right level of moisture. Watering every day or every two days doesn't matter nearly as much as consistent watering. Make a schedule and stick to it.
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.
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.
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.
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.
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.
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.
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.
Potted plants will probably want to be watered more than once a day during a heat wave. The next best time to water during hot weather is in the early evening. Trees and plants in or near irrigated grass yards will often want more frequent waterings as they can become spoiled and and often have shallower root systems.
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.
Most outdoor plants can go without water for about a week. Container plants are the thirstiest. Most need to be watered daily. If you're planning to be away, adding mulch around flower and vegetable beds helps keep moisture in longer — even if it doesn't rain during your absence.
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.
Watering Plants At Night Isn't Needed: Although the idea has been around for years, most plants don't need extra care by watering them at night. There are a few exceptions, but 99% of your plants, indoors and out should only be watered during the day.
Even in the warmest days of summer, a good soak every three or four days is best for plants because it forces their roots deeper into the soil where they are healthier and cooler.
If you are unsure how long to water new plants, aim for 30-60 seconds for small plants – longer for larger plants while moving the hose to a few locations around the plant. Avoid watering when the soil feels moist. The earth must be allowed to dry out between watering.
Overwatered Plants Symptoms
Some symptoms of overwatering to look for include: Slowed plant growth with yellowing leaves. Plants may also suffer from leaf scorch/burn. Water soaked spots and/or blisters (i.e., Oedema) may be visible on plant stems and leaves.
For vegetables in the summer, we recommend applying about 1 inch of water over the surface area of the garden bed per week. That is equivalent to 0.623 gallons per sq ft. Using that rate, a 32 sq ft bed requires 20 gallons of water per week.
DON'T water too frequently—or not frequently enough.
Instead, opt for a less frequent watering routine that thoroughly saturates the soil. This method encourages the plants' roots to reach deeply for residual water, even when the surface of the soil appears dry.
Apply mulch, preferably a reflective mulch such as dry grass clippings. The first line of defense against hot weather and the windy conditions that can dry surface soil is to apply a liberal layer of mulch around the plants. This protects the soil from direct sun exposure, keeping it moist at the surface.
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.
Signs Your Plants Are Too Hot
The leaves dry out. Excessively hot plants will dry out faster. The leaves turn yellow. One of the causes of yellow leaves is excessive heat, so this is one thing to consider!