If extreme heat continues for weeks at a time, plants can actually die from a depletion of their food reserves. High temperatures can also cause severe water loss (desiccation) when transpiration (the process by which leaves release water vapor to the atmosphere) exceeds moisture absorption by the roots.
Leaves under heavy heat loading progress to senescence, brown-out and finally are shed. If unchecked, the heat stress syndrome results in progressively decreased photosynthesis, increased respiration, a major slow down in transpiration, and plant starvation through the rapid use of food reserves.
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.
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.
When temperatures are exceptionally high and prolonged, plants can lose water through evaporation much faster than the roots can absorb it. Wilted, drooping leaves, resulting from a combination of excess water loss and high internal temperatures, are often the first signs of heat stress.
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.
Intense heat and sunlight can give your plants a sunburn, just as they would to you. Leaves and fruit can get burned, crack and develop bleached spots. damage is usually worse for newly transplanted plants or young plants that are yet not fully established.
Managing heat damaged hair
As we've noted, damaged hair repair isn't possible because the protein bonds and cuticles have been permanently altered. Instead, the focus should be on maintaining hair to prevent further damage. If your hair is severely damaged, the most extreme solution is to cut it off and start fresh.
High temperatures mean a high potential for water to evaporate before it ever reaches the roots. Avoid sprinklers and instead use spot watering at the base of the plant, soaker hoses, or drip irrigation systems to provide water efficiently. Plants in containers and hanging baskets may need water twice a day.
You will want to trim the dead or scorched foliage after properly watering. Your plant is expending energy on attempting to keep the burnt parts alive. Clip these areas off so that the energy it produces will go to the areas of the plant that are still healthy.
If the damage is isolated to a portion of the plant, it can be revived and recover. The focus is on minimizing the damage.
Put the plant in a cool place with indirect light. Trim away any burned or dead parts and then be patient. It will almost certainly recover. But no more direct sunlight.
Different species can tolerate different temperature extremes, but, in general, once air temperatures rise above 90°F, most plants will start to experience heat-related stress.
Above 104°F, many plants will survive but will show different signs of heat stress depending on plant type, maturity of the plant and factors that often come with high temperatures, such as drought or wind. Extreme air and soil temperatures slow down chemical activity and growth in plants.
For heat-stressed plants, the road to recovery could take anywhere from a few days to a few months. The time needed to recover from stress is proportional to the severity of the damage.
Damaged hair looks rough, dry, dull, and frizzy. It loses its natural shine because the cuticles, which give hair its sheen, have been damaged. Heat damaged curly hair may appear limp, and the curls may not hold their shape, and straight hair may not appear as polished and sleek.
As the sun beats down, it increases the soil temperature, which also contributes to the heat stress in a plant. To lower the temperature, a layer of four-to-six inches of straw, pine needles, leaves or grass clippings will help. Also watch for vegetables dropping blossoms in times of high heat.
As with most plants, daytime temperatures above 85°F and below 55°F slows or stops the growth of plants.
Browning of leaf margins and/or yellowing or darkening of the areas between the main leaf veins are symptoms of leaf scorch. Due to environmental causes, leaves may dry, turn brown, and become brittle. Look for damage to trees and shrubs on the upper portion on the sunny, southern side, and on the windy side.
Adding a layer of mulch around ground covers, shrubs, and trees will keep the soil a couple of degrees cooler while stopping it from drying out. Apply your mulch about three inches thick around the plants, spreading it near the drip line or water source. Ensure to keep it about six inches away from tree trunks.
The general rule of thumb is that most plants freeze when temperatures remain at 28°F for five hours. Of course, there are exceptions to this rule. Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds.
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.