Plants don't like heat

Published 12:00 am Friday, August 10, 2007

I stopped at a gas station on my way to Pinehurst earlier this week while attending a Cooperative Extension conference. Large shade trees at the station were shedding so many leaves that landscape maintenance crews were blowing leaves into piles for removal.
Prolonged excessive heat has created many problems for both growers and homeowners.
Plant materials already stressed by the late freeze and summer drought have to endure yet another test with excessive heat. With the temperature extremes, many plants, especially bedding plants and those in containers, often require daily irrigation. In some instances, container plants need irrigation twice daily. Extra water used to compensate for the heat and drought most likely leached fertilizers from the soil and potting mixtures.
Plant materials may need a water-soluble fertilizer to compensate for the rapid loss to maintain strength going into the fall months.
Another major problem associated with the extreme heat and drought is over-watering. Homeowners tend to panic when plants wilt and over-water during periods of extreme heat and drought.
A good rule of thumb is to observe plants early in the morning. If the plants are still wilted, then they are suffering from lack of water. Hydrangeas are perfect examples of plant materials wilting during excessive heat, but still having ample soil moisture. There is really no way to tell a person how much to water a plant each week. Plant species, soil types, exposures are only a few of the variables which confuse us. Newly planted shrubs and trees may need more water than well-established trees and shrubs.
Dogwoods, rhododendron, azaleas and camellias have shallow root systems, easily drying out in this type of weather, and they will need extra water.
Many fescue and fescue bluegrass blends have all gone dormant and will remain this way until appreciable rain and cooler temperatures return. Well established fescue lawns will often survive three weeks without any rain. If possible, irrigate an inch every three or four weeks just to keep the roots alive. The tops will still be brown but the roots will survive
Excessive heat also brings out other problems. Spider mites explode on shrubs and bedding plants during excessively hot conditions. Normally, water on the foliage is not recommended, but splashing water on the foliage washes away of many of these pests.
Other insect pests such as fall webworms have already arrived. Millipedes invade homes during periods of excessive rain or for some reason, during periods of heat and extended droughts.
The biggest threat from excessive heat now is the additional stress on already weakened trees and shrubs. These plants are now more vulnerable to disease and insects, creating more complex problems.
Unfortunately, the detrimental effect of this summer’s heat may not manifest itself for years and be forgotten by many.
Darrell Blackwelder is an agricultural agent in charge of horticulture with the North Carolina Cooperative Extension Service in Rowan County. For archived garden columns or other information, visit the Rowan County Master Gardener Web site at, e-mail or call 704-216-8970.