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.
In general, a heat-stressed plant will have leaves that are yellowed or exhibiting some brown spotting. This is usually accompanied by wilting. They may look burned in some places, particularly if they are also receiving too much light.
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.
Studies have shown that high temperatures can increase the plant's rate of reproductive development, which shortens the time for photosynthesis to contribute to fruit or seed production. Heat stress problems also make the plant more susceptible to pests and other environmental problems.
In general, foliage plants grow best between 70 degrees and 80 degrees F. during the day and between 60 degrees to 68 degrees F. at night. Most flowering plants prefer the same daytime temperature range, but grow best when nighttime temperatures range from 55 degrees to 60 degrees F.
The books are right – planting in the summer when it is hot and dry is not the best time to move new plants into or around the garden. During a particularly bad heat wave it can be hard enough to keep the soil hydrated, let alone provide ample moisture for plants that are establishing new roots.
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.
In hot weather you need to water in the mornings when it is cooler. This will allow more water to reach the root system before it evaporates in the heat. There may be times when you cannot schedule watering in the morning. In this case, water the plants in the late evening before bed.
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.
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.
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.
You can cut back the plant to remove the damaged parts and fertilize. Even if you prune it back hard, it should grow back better than ever and bloom in about 6 weeks or so.
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 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.
Move pots to cooler locations.
Place containers where they receive some shade during the hottest part of the day. This usually won't reduce their performance, but it will cool the plants and reduce water needs. Moving containers off or away from hot pavement will also help.
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.
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.
When To Stop Watering Plants in Fall. When the air and soil temperatures consistently fall below 40 F, it's time to stop watering. The ground can't absorb water once the top few inches freeze. Continue to water your plants up until this point so they're as well-hydrated as possible going into winter.
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.
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.