IN THE YARD: Turf war: lawns can suffer heat stress

(Note: The following information was provided by Michael Goatley Jr., extension turf grass specialist, Virginia Cooperative Extension in Blacksburg)

Cool season turf grasses throughout Virginia are experiencing difficulties associated with summer heat stress.

Heat stress results in the grass becoming weakened to the extent it becomes much more vulnerable to both mechanical and biological stress. Summer decline is therefore a number of individual factors that can interact to cause more summer problems than each individual stress acting alone.

Persistent daytime temperatures of more than 90 degrees, nighttime temperatures above 70 degrees and high humidity have combined to place cool-season turf under significant summer stress.

The fact that these conditions have relentlessly persisted for many since mid-June — especially in central and eastern Virginia — has resulted in direct heat stress injury as well as making the turf more vulnerable to damage from other summer related stresses. Those stresses include excessively close mowing, traffic and wear, warm weather fungal diseases, poor water quality, poor soil drainage, poor air movement and insect damage.

It is often difficult to get a handle on specific causes of summer decline. Put quite plainly, during a period of mid-summer heat stress, the grass plants are more vulnerable to everything that can cause damage.

During cooler times of the year these stresses are either non-existent or the grass is healthier and more tolerant.

Besides air and soil temperatures, another extremely important environmental influence on the plants ability to cool itself through transpiration is the relative humidity. At high relative humidity the plant is much less able to effectively cool itself. As a result the grass is even more prone to heat buildup and direct heat injury.

An important point to remember in measuring heat stress is standard meteorological weather data records temperature at 5 feet above ground and that temperatures of the turf itself will likely greatly exceed the recorded high.

Air temperatures of 90 degrees will result in grass temperatures of at least 105 to 110 degrees.

» Cutting heights: Taller mowed turf is performing better than grasses being mowed at heights that are ideal only during periods of optimal growth. Cool-season lawns of 4 inches or greater cutting heights have survived the heat and drought much better than those mowed at 1 to 2 inches.

» Traffic Stress: Heavily-trafficked areas have suffered during periods of summer stress. Traffic damage will be slow to recover with reduced plant vigor. Higher mowing heights will help in this regard. Plan on reseeding these areas in the early fall.

» Irrigation: As a rule of thumb, continue to irrigate deeply and frequently (using the 1 inch of water per week guide) if you have been irrigating this summer. If not, apply no supplemental irrigation and wait for more optimal growing conditions this fall. Irrigate early in the morning in order to reduce leaf wetness periods and ensure your system is applying water efficiently.

» Summer Diseases and Insect Pests: Rhizoctonia Brown Patch is most active under warm day temperatures and warm nights under 70 degrees. Heat stress weakens plants and they can become more susceptible to disease activity at a time of year when the disease pressure is the greatest. Fungicides are often applied more frequently and at higher rates in an attempt to control the pathogens.

» Weed control: In general, refrain from chemical weed control treatments during summer stress periods. The typical size and growth rate of summer annual weeds (things like crabgrass) is so significant at this time chemical treatments will likely have little to no effect and/or damage the existing cool-season turfgrass.

» Soil Aeration: The soil must be well aerated for plants to function. Tight soils will not readily absorb water and it can make efforts at irrigation difficult. Plan on utilizing core aeration to improve physical soil conditions this fall, and tie the aeration event to seeding or liming (as indicated by soil testing) applications.

» Fertilization: Conduct a soil test if it has not been performed for the past three years. Fall is the ideal time to benefit from nitrogen applications on cool-season grasses, and after a stressful summer, it is an important part of the grass’ recovery program. During the shorter, cooler days of fall, cool-season turfgrasses devote much of the food they produce in photosynthesis to root systems and storage for later use.

Summer heat injury is complex. However, one difference is we notice the decline as it happens and try to respond in an appropriate manner.

The Turf and Garden Tips podcast blog is also full of a number of seasonal tips that will help to manage your lawn. The podcast can be found at www.anr.ext.vt.edu/lawnandgarden.turfandgardentips/

Sutphin is an extension agent with the Virginia Cooperative Extension, Danville Unit Office. Contact him at  (434) 799-6558, www.ext.vt.edu.