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.
Since the sun is harshest from midday to early afternoon, the best course of action is to provide shade for your plants during this period. There are several ways to accomplish this: Put heat-sensitive plants in garden beds that receive afternoon shade from structures like a house, a shed, or a tall fence.
Don't Fertilize:
A sick or stressed plant needs a break from dealing with fertilizer, even if it's just for a couple of weeks. Let it rest, chill, and deal only with completely innocuous clear water. Nitrogen is good for plants, but if a plant's system is not working well, it is stressful for the plant to deal with it.
Once leaf tissue is scorched, the damaged areas will not recover; however, minor damage, while unsightly, will not kill the plant.
We're in the thick of winter, which means your plants might be looking a little worse for wear. In fact, chances are those yellowing, brown, or wilting leaves you're noticing are early signs of plant stress. Plant stress describes a set of non-ideal growth conditions within which your plants are forced to survive.
As long as there is some green foliage or even if only parts of some leaves are green, your plant should eventually make a complete recovery. It is not a good idea to detach dead leaves from a burned plant because those leaves may be shading healthy foliage that could also burn when suddenly exposed to all-day sun.
So, how hot IS too hot for plants? 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.
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.
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.
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.
If possible, take a cool shower, soak in a cool bath or put towels soaked in cool water on your skin. If you're outdoors and not near shelter, soaking in a cool pond or stream can help bring your temperature down. Loosen clothing.
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 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.
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.
Symptom. Leaf scorch is a common symptom of transplant shock. Leaf scorch first appears as a yellowing or bronzing of tissue between the veins or along the margins of leaves of deciduous plants (those that lose their leaves in winter). Later, the discolored tissue dries out and turns brown.
When a plant doesn't get enough water, the tips and edges of leaves dry out and turn brown. Ultimately, entire leaves will brown and die. Slow growth. If you are chronically under watering a plant, but still giving it enough water to survive, growth will be slower than normal or expected.
The telltale signs of shock are yellowing or brown wilted leaves that droop drastically. Often a stressed plant becomes very delicate and the leaves easily fall off, if touched or bumped. There are two kinds of shock to be aware of when relocating or repotting your plants: plant shock and transplant shock.
Once leaf scorch has occurred, there is no cure. The dehydrated portions of the leaf will not turn green again, but with proper water management, the plant may recover. Prevention of scorch needs to begin with winter watering.
Scorch symptoms may differ between plant species, but it typically appears in July and August as a yellowing between leaf veins and along leaf margins, and a browning on the tips of leaves. Since these leaf parts are the last to be supplied with water from the roots, they are usually the first to be affected.
“If you find your plant has been burnt, it is best to cut off the damaged leaves or trim them if you can,” says Steinkopf. “They will not heal or turn green again.” Then move your plant into a less bright area—choosing filtered light over the direct sun—whisper your deepest apologies to it, and resume proper care.
Prognosis/Possible Complications. If you avoid heat stroke, recovering from heat exhaustion usually takes 24 to 48 hours.