How To Protect Trees, Other Landscape Plants From Heat Stress

Healthy landscape plants need care and maintenance, especially when summer rainfall is inadequate. Plants can dissipate summer heat by transpiring water drawn from their roots to leaf tissues and finally releasing it into the air. Many landscape plants in the Upper Midwest transpire more water than their own body weight, even during normal summer temperature — daytime highs around 70-85 degrees Fahrenheit.

However, as air temperature rises above 85 degrees, water loss through transpiration doubles and the roots struggle to tap enough moisture to compensate the loss from leaf tissue. The resulting moisture stress condition leads plants to exhibit symptoms that include leaf scorch, curled leaves, early fall coloration, defoliation, twig dieback and temporary wilting.

Moisture stress condition can make the plant to become vulnerable to pest and disease attacks, as well as winter injuries. Certain landscapes, including those adjoining concrete driveways and roadsides, or facing the south/southwest sides of buildings, can be exposed to reflected radiated heat that could further intensify stress on plants.

Here are several ways to minimize the effects of summer heat and moisture stress on landscape plants:

1. Plants should be watered early in the morning to reduce evaporation loss.


2. Trees and shrubs need a good soaking of water once a week. It is crucial to water newly planted trees and shrubs (1-3 years old) for better establishment of their root systems. As a general rule of thumb, trees and shrubs in heavy soil should receive 1 inch of water per week or 1.5 inches per week in sandy soil. Covering a 1-inch inch depth for 100 square feet in heavy soil requires about 60 gallons of water. If trees and shrubs are mulched, a soaker hose can be placed underneath the mulch to ensure the soil root zone gets adequate water. An alternate option is to use watering bags on newly planted trees that release water slowly into the root ball area.


3. Before watering flowering annuals and vegetable crops, soil moisture should be checked by poking a finger an inch deep into the soil. Dry soil should receive a deep, soaking watering around the root zone. The soil on raised-bed gardens and container plantings should be checked every day as moisture in these settings dries out quickly.


4. Herbaceous perennials should be watered when the soil dries out moderately. A soaker hose can be placed directly into the soil to water to a depth of 1 inch (0.6 gallons of water is needed to cover a 1-inch depth for 1 square foot). Certain herbaceous perennials might need more than 1 inch of watering.


5. To prevent foliar diseases such as leaf blights, spots and rots, annuals, perennials and trees should not be watered from above.


6. Mulch helps in conserving soil moisture. Shredded wood mulch on trees, shrubs and herbaceous perennials to a depth of 2 to 3 inches is best, and should be kept at least 6 inches away from the main stem of the plant. Straw, rice hulls, leaf mold, peat moss and hay are good sources of mulch for vegetable crops.


7. Plants should not be fertilized during hot and dry periods, as it can dehydrate the plants by absorbing moisture from the root zone. Also, excess fertilizer can burn the feeder roots.


8. Pruning, transplanting or digging of plants during stress periods should be avoided.


9. Herbicides should not be sprayed when temperatures exceed 80-85 degrees, as these chemicals can easily volatilize, drift and injure desired plants.

University of Wisconsin-Extension Horticulture publishes detailed information about gardening and landscape plants, and local Extension offices can provide additional advice.

Vijai Pandian is a horticultural agent and educator for the University of Wisconsin-Extension Brown County. This article is adapted from an item originally published by the Green Bay Press Gazette.