It's just too hot
There is no magic temperature that determines when it’s too hot for plants. In general though, plants will usually show signs of stress in the mid to upper 90s. If they produce fruit, that growth will slow down or stall as well. For some plants, this can happen when temperatures hit the mid 80s.
What's actually going on
Hot weather and evaporation go hand in hand. Evaporation reduces the amount of water that's available for the plant to use. In fact, during hot weather, a large portion of the water from your garden hose is evaporating before or shortly after it hits the ground.
There's also this: transpiration. When it gets really hot, transpiration helps move water from roots to shoot tips.
When plants lose too much water, and their environment is dry, it's stressful. As water leaves the plant, evaporation helps cool the area around the leaf. But when there’s no more water to evaporate, leaves get warmer and the plant starts wilting.
As if that isn’t enough, the rate of photosynthesis (the plant’s conversion of light energy into chemical energy) goes down. The ideal range for photosynthesis is between 65 and 85 degrees. But while photosynthesis declines, respiration (the process in which the plant takes sugars produced during photosynthesis and turns them into energy) does not slow.
"When you get really hot, you still have this relatively high rate of respiration, so it's using a lot of food, but you have less food through photosynthesis," says Aaron Steil, a consumer horticulture specialist at Iowa State University.
The end result: the plant can use up all of its reserves and - especially in less well established plants - die.
What you can do
There are two main things you can do to help your plants in the heat.
The first: Water, water, water. Container plants and hanging baskets especially can lose a lot of moisture through the soil. When it's really hot, don't be surprised if you need to water a hanging basket twice a day.
But that's not always the case.
"If you were doing a really good job of protecting the soil from evaporation by putting down a nice layer of mulch, and keeping it shaded, you may find that you may not have to water quite as much as you initially thought," said Steil.
The second: Make sure your plants have the shade they need. Pull hanging baskets and containers into a shady location during a hot spell. If you don't have access to adequate shade, try using shade cloth, an umbrella or even a bed sheet. Be careful not to lay the material directly on the plant. Elevate it with stakes - you can even use patio furniture. It might look goofy, but remember, it's only temporary.
And don't forget to take care of yourself - stay hydrated, wear a hat and limit how long you’re in the heat if you can!
