Maintaining Gardens in Summer Heat & Droughts

By Jerry A. Chenault, Urban Regional Extension Agent, Lawrence County

 

One of the best and most effective things a homeowner can do to help protect plants, trees, and shrubs from summer's heat and drought is to mulch around them. That might not seem like a real news flash; but what most people don't realize is that high temperatures kill plant roots! We often think of mulching plants as a way to prevent weeds and to hold in moisture; but moderating soil temperatures just seems to drift right by us--and it may very well be the most important fact!

Contrary to popular belief, the bulk of tree roots are in the top 12­18 inches of the soil, while soil temperatures at or above 95 degrees F. kills plant roots. Without mulch, summer soil temperatures can sometimes reach 110 degrees F. at 2" deep and 100 degrees F. at 4" deep. That alone can spell disaster for a tree or other plant. A 3-4 inch layer of organic mulch can go a long way toward protecting your plants from summer's heat and drought.

Dr. Bob Nuss, a horticulture specialist at Penn State's College of Agricultural Sciences, offers the following advice about mulches:

"Whatever material you use depends on where it will be used, appearance and cost." "Homeowners can use two types of mulch: organic or inorganic. Make sure no mulch touches the plant stem or trunk."

Organic mulches range from shredded newspaper to shredded wood or wood chips. Inorganic mulches such as black plastic will be just as effective although appearance may be an issue. "You can top-dress plastic mulches with gravel or wood chips," Nuss suggests. "But don't use gravel alone because it takes a layer 6 to 8 inches deep to slow moisture loss and control weeds." Gravel may increase heat.

Wilt is the most obvious sign of extreme heat; but wilt isn't always caused by a lack of water as many people believe. In fact, too much water can manifest itself in the same visual symptoms as too little water! They both cause wilt! And sometimes a plant will wilt (especially a young plant) even though the soil appears to be wonderfully moist. What causes that?

The explanation is that due to the considerable surface area of the leaves, the plant losses moisture (especially in high temperatures as it attempts to cool itself) faster than the plant's roots can take it up. Containers can sometimes make this situation worse (if they aren't big enough) whereas a planted landscape plant can develop a large enough root system to help maintain moisture levels. If we don't have a drought, that is.

So, how often should we water plants in a drought situation? Many a plant has been killed by the desire to provide it an oasis in time of drought. Instead of watering daily, rather, provide your plants with one good soaking per week. Watering lightly means the water won't penetrate very far and roots will grow only near the soil surface. Therefore, it's important to water deeply to get the moisture to quench the thirst of those roots deep within the subsoil. This, in turn, encourages deeper root growth and makes your plants better able to withstand drought conditions.

What else can we do to help in drought situations?

  • Maintain a lawn height of 2_-3 (2.5 to 3) inches to help protect roots from heat stress and to reduce the loss of moisture to evaporation.
  • Water before 8:00 a.m. or after 6:00 p.m. Avoid watering in high winds or in the heat of the day.
  • Install a drip irrigation system for watering young trees, gardens, and shrubs.
  • Consider using "gray water" (water left over from bathing and clothes washers) on ornamentals; but make sure soaps and detergents have been diluted. Don't use water with chlorine bleach, boron, or liquid fabric softeners (sodium from the salt used to soften water and in fabric softeners can be harmful to plants). Gray water can also be used on the soil around fruiting plants such as tomato, pepper, beans, and broccoli.

References

Costello, L. R., Perry, E. J., Matheny, N. P., Henry, J. M., and Geisel, P. M. (2003). Abiotic disorders of landscape plants: A diagnostic guide. Oakland, CA: University of California, Division of Agriculture and Natural Resources.

Montana State University Extension. (February 19, 2002). Heat and drought. Retrieved June 4, 2006.

Nuss, Bob. (July 27, 1999). Garden under stress from drought?: Relief is on the way. Penn State College of Agricultural Sciences News Release Archive. Retrieved June 26, 2007.

Hammond, Sheldon C. (2007). Urban tree specialty training program manual. Griffin, GA: Georgia Cooperative Extension Service.

HGTV. (2007). Heat and drought tolerance q&a. Gardening. Retrieved June 26, 2007.


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