The simplest way to keep your plants cool is to manage the sunlight that hits them. You can do this through strategic planting under trees, decks, or whatever else may create shade in your garden. You can also create your own shade in your garden with sheets, tarps, or patio umbrellas.
To keep them cool, use old screens, lattice and sheets to shade vulnerable mature plants from the effects of mid-summer sun. Cover new seedlings with opaque covers to protect them from the sun until their roots are established. Commercial growers often use sprinkling to cool plants down.
Sun-scorched plants can be cut back and fertilized and should recover with time (planting in the ground would be even better!). However, root rot is usually fatal. It is a delicate balance to keep plants well hydrated, but allow the roots to dry out enough to function properly.
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.
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.
It can take anywhere from a couple of days to a couple of months for stressed plants to recover. Recovery time depends on the amount of damage that the heat caused to the plant. If a stressed plant is completely neglected during a heatwave, it can take several months or even be impossible to revive it.
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.
Use shade cloth or other fabric such as old bed sheets pinned onto frames or hoops to provide shade for plants. Cool-season vegetables, for instance cabbage and lettuce, and fruits such as strawberries, will particularly enjoy shade from hot afternoon sun.
Plants also cool the landscape directly through the process known as transpiration. When the surrounding atmosphere heats up, plants will often release excess water into the air from their leaves. By releasing evaporated water, plants cool themselves and the surrounding environment.
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.
Whether your plants are annuals or perennials, try watering them in the morning during a heatwave. Morning watering gives the roots a good soaking and offers your plants enough moisture to get through the day without added stress. But during summer scorchers, you may need to water annuals and perennials more often.
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.
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!
There is a high likelihood that plants are experiencing heat stress if they wilt during the day's hottest period and then revive in the evening or morning. Dried Leaf Margins. Some types of crops dry off the leaf's outer edges to ensure survival.
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.
As the weather heats up and plants reach full size, you may need to water containers (and especially hanging baskets) twice a day.
Most plants can recover when removed from the heat as long as they are generally healthy and given plenty of water and shade to help them recover.
Most plants grow best in temperatures ranging from 59°–86°F. When temperatures above 90°F are sustained for long periods, plant growth is slowed, and some plants begin to show signs of stress.
By regularly spraying your houseplants with water, you raise the humidity around their leaves, keeping delicate species from the tropical regions of the world happy in distinctly untropical homes.
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 heat from the sun evaporates a significant amount of the water you apply to the plants, which makes it an inefficient time to do it. The ideal time to water is first thing in the morning before it's too hot and the plants have time to dry out.