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.
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 is so hot out there!
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. Throw a damp sheet over it, or mist it lightly to bring the temperature down. Mist leaves with a spray nozzle to cool down plant and reduce transpiration.
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.
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.
Some signs of heat related stress on plants include: leaf scorch, leaf rolling, wilting, blossom/fruit drop, sunscald, blossom end rot, and other variable conditions that could be brought on by secondary pathogens. Heat stress adversely affects plant metabolism, disrupting their vital biochemical processes.
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.
If you are dealing with a very hot day, misting in the morning and again in the afternoon is a great way to help control the temperature of your plants and keep them safe and looking great.
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.
Plants in containers and hanging baskets may need water twice a day - especially when weather is windy and hot, containers dry out very quickly. Check often, as most will need water at least once a day, and some may need water in the morning and the evening.
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.
One of the first signs will be wilting. As the heat begins to evaporate large amounts of moisture, your plants won't be able to actively replace all the lost water, and they'll end up with droopy, sad-looking foliage. Other plants will also “close” their leaves to reduce moisture loss.
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.
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.
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.
For most species, actively growing tissue is damaged by brief exposure to temperatures above 45oC, while prolonged exposure can result in fatal injury. Temperatures between 30-40oC can be termed moderately high temperatures and result in reversible inhibition of metabolism (moderate heat stress).
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
Look out for white spots or dry brown splotches in random areas of a leaf. This is a clear indicator of sunburn. Sadly, the leaf won't repair itself but you can trim the areas or the entire leaf off, and there's no reason why your plant should continue suffering or not recover from its sunburn.
This depends on the plant types, garden light conditions, and weather. Plants in shaded areas may last longer without water compared to those in full sun. Container and window box plants can dry out in a day, whereas many houseplants can manage with one watering per week or less.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
While misting is commonly believed to increase humidity levels for plants, in most homes, that effect lasts just a few minutes before the water evaporates. In a greenhouse, however, it could last much longer. Misting also removes dust on leaves and washes away bugs on the plant's surface.