It’s obvious when plants have had too much cold. They freeze. They brown. They turn to mush. Too much heat can be nearly as destructive, but the damage is less obvious. Or the trouble gets blamed on drought instead of heat.
Wilting, for instance, isn’t always a sign that a plant isn’t getting enough water. It could be flat-out hot temperatures.
All kinds of heat trouble begins when daytime temperatures go above 86 degrees. Among the threats:
- Flower buds may wither.
- Chlorophyll production begins to shut down, robbing leaves of their healthy green color.
- Pollen becomes non-viable, preventing popular plants such as tomatoes from setting new fruit until the weather cools.
- Subtle chemical changes occur in plant leaves, making them more vulnerable to bug attack.
- Soil temperatures may rise to the point where root activity slows and plant growth is stunted.
- Most noticeable, moisture loss from plant leaves increases, making plants more susceptible to dry soil.
Heat is causing this hydrangea to wilt. The soil has plenty of moisture.
Some plants take the heat much better than others, as any Southern gardener will tell you. Unfortunately, gardeners slowly learn which plants can stand it on the hot seat by trial and error or word of mouth since heat-hardiness ratings aren’t nearly as common in the plant industry as cold-hardiness ones. However, the American Horticultural Society has a Heat Zone Map and heat-gardening observations.
Besides the genetic makeup of plant species, a second heat factor is plant location. Every yard has different microclimates. It might be hot and brutal in the middle of the back yard, or on the west side of a brick wall, but 10 degrees cooler along the eastern foundation or under a shade tree.
Matching a plant to its heat and sunlight tolerance can mean the difference between survival and a fried plant. Don’t be afraid to move a plant that’s struggling – just wait until spring or fall to do it.
A third anti-heat maneuver is keeping plants healthy with good soil and adequate water. Plants lovingly rooted in rich, loose, compost-enriched soil put out better roots than ones jammed into lousy clay or packed shale. That makes them better able to deal with any stress, including heat and drought.
Be careful not to overdo it with water. Even in hot, dry conditions, it’s possible to kill a plant by rotting its roots.
Use your finger or a watering gauge to make sure the soil really is dry a few inches down where the roots are. If it’s already damp or wet, more water isn’t the answer.
Shade from a tree creating a cooler microclimate.
Hydrangeas are a perfect example of a plant that wilts from excessive heat. The large leaves often wilt in daytime heat but recover at night when the temperatures cool. If a hydrangea is still wilted first thing in the morning, then it most likely is dry soil – or it’s in the process of dying from previous overwatering. Dead roots can’t deliver moisture, so the result looks like drying.
A 2 to 3 inch covering of mulch is a good way to both keep moisture in the soil and prevent sunlight from baking plant roots. Bare soil in the sun can be 20 or more degrees hotter than the air temperature, and on 95-degree days, that’s bad news for plant roots. Using a mulch product like Preen Mulch Plus is a great way to maintain moisture and help to prevent weeds from growing.
Shade trees and vine-covered arbors are other ways to spot-protect those heat-baked parts of the yard.