'Summer Stress' on Cool-Season Turf
August 5, 2010
Virginia’s Turfgrass Managers:
Cool season turfgrasses throughout Virginia are experiencing difficulties
associated with summer heat stress. Heat stress results in the grass
becoming weakened to the extent that it becomes much more vulnerable to both
mechanical and biological stress. Summer decline is therefore a complex of
individual factors that can interact to cause more summer problems than each
individual stress acting alone.
Extremely high day time temperatures of over 90 degrees, night time
temperatures above 70 degrees, and high humidity, have combined to place
cool-season turf under significant heat stress. The fact that these
conditions have relentlessly persisted for many since mid-May (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 (e.g.; excessively close mowing, traffic and wear, warm weather
fungal diseases, poor water quality, poor soil drainage, poor air movement,
insect damage etc.). As a result, heat related injury and the associated
decline in turf quality is often a result of a complex of individual factors
acting together. For this reason 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 like we had during the past month, 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 and as a result is
even more prone to heat buildup and direct heat injury.
An important point to remember in measuring heat stress is that standard
meteorological weather data records temperature at 5 ft. above the ground
and that temperatures at the turf level may exceed the recorded high. In
southside Va recently, when air temperatures were 93o F, the actual
temperature at the surface canopy of a bentgrass/Poa putting green was 106o
F. Note that concerns with the heat are not only limited to natural grass
systems – many artificial turf fields have (or should have been) rendered
unsuited for play because field surface temperatures have reached the 140 to
150o F levels during the day, temperature levels that present safety
concerns to field users. In addition, high air temperatures are only part of
the cause of reduced plant vigor. Prolonged heat stress significantly
increases soil temperatures and soil temperatures greatly influence root
growth, root health and function. Spring high temperatures are less damaging
since the root is in a cooler soil environment. As summer arrives the night
temperatures are less cooling on the soil and soil temperatures rise. High
soil temperatures result in less root production, rapid root maturation and
die back, and little production of new roots. Above ground the turf thins
and individual plants become more spindly
A profile of each site and its isolated microenvironments will involve a
number of “inventories” as they relate to susceptibility to prolonged heat
stress. Conditions and brief explanations of how they might influence summer
decline of cool-season turf follows:
Cutting heights: For the summer of 2010, taller mowed turf is logically
performing better than grasses being mowed at heights that are ideal only
during periods of optimal growth. Cool-season lawns of 4” cutting heights
have survived the heat and drought much better than those cut at 1-2 inches.
Although 4” cutting heights are not suitable for sports turf use, again,
taller mowed fields have performed significantly better during the summer.
Some recovery in cool-season turf is already being noted following spotty
rain events. However, many cool-season turfs have been invaded by heavy
populations of summer annual grasses such as crabgrass, goosegrass, and
foxtail. These grasses will die at first frost and will then lead to
invasion by annual bluegrass and other winter annual weeds. Make plans now
to initiate reseeding of these damaged areas in late August/early September
in advance of the biggest window of annual bluegrass germination (after
considering possible limitations from weed control programs detailed below).
Choose the best grass possible by consulting the Turfgrass Variety
Recommendations List from Virginia Cooperative Extension.
Traffic Stress: Heavily trafficked areas have logically 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 as described above, and where traffic continues (i.e.
sports fields) continue to apply small amounts of seed throughout the
playing season of the fall.
Public Perception and Maintaining Maximum Aesthetics/Playability: The
public ‘expects’ green grass 12 months out of the year. That obviously is
not possible even in an ‘ideal’ growing season in a transition zone state
such as Virginia. Seasonal environmental conditions will often dictate what
should and should not be done to the grass to maintain its ‘aesthetics’ and
its ‘playability’. On average, much of the climate of central and eastern
Virginia is better suited to warm-season grasses than cool-season turf, so
where persistent problems arise on an annual basis, consider if there is not
a better species of turfgrass available for your particular area and turf
uses. The fall season will provide the opportunity for significant
cool-season turf recovery and the memories associated with the summer of
2010 will slowly fade. However, the problems will return during the next
‘difficult’ summer in our future. A turf management program should be
flexible to “ebb and flow” with the vagaries of weather, especially in the
mid-Atlantic transition zone environment.
Irrigation: As a rule of thumb, continue to irrigate deeply and
frequently (using the 1” of water per week guide) IF you have been
irrigating this summer. If not, apply no supplemental irrigation (i.e.
hopefully allow the turf to remain dormant) and wait for more optimal
growing conditions this fall. Continue to irrigate early in the morning in
order to reduce leaf wetness periods (i.e. reduce disease infection periods)
and ensure your system is applying water efficiently (check heads and
nozzles for application pattern, ensure water is not running of the soil,
etc.).
Summer Diseases and Insect Pests: Rhizoctonia Brown Patch is most active
under warm day temperatures and warm nights (>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. Anticipate grub problems to be exacerbated on weakened turf and
be prepared to address significant activity of fall armyworms and cutworms
during late summer. Recommendations on disease and insect control are
available in the Pest Management Guide through Virginia Cooperative
Extension.
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 that
chemical treatments will likely have little to no effect and/or damage the
existing cool-season turfgrass. Applications of standard preemergent
herbicides can be made this fall for the control of winter annual weeds, BUT
REMEMBER THAT THESE PRODUCTS WILL ALSO CONTROL ANY GRASS APPLIED DURING
RENOVATION EVENTS. If your turf is heavily infested with perennial broadleaf
weeds (things like clover, dandelions, plantains, etc.), fall is an ideal
time to control these pests. Choose appropriate chemicals from the Pest
Management Guide and make plans to treat accordingly.
Soil Aeration: The soil must be well aerated for plants to function.
Soils that become sealed off at the surface, from compaction, will impair
root growth and function and the ability of the turf to cool itself. Tight
soils will not readily absorb water and it can make efforts at irrigation
difficult. Tight soils, once wet, can often stay wet too long. 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. Also, topdressing 1-2x per year with ¼ inch depth of a quality
compost will improve physical and chemical soil conditions, and its
application also is enhanced when done in conjunction with core aeration.
Fertilization: Conduct a soil test if it has not been performed for the
past 3 years. There are numerous private labs that offer this service or you
can work with your local VCE agent in having the Virginia Tech lab conduct
the test. 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. Depending on the
grass and the expectations/uses of the turf, up to 3 lbs N/1000 sq ft can be
applied during the fall months of September, October, and November; apply no
more than 1 lb of N/1000 sq ft per month. Apply no more than 1 pound of
water soluble N per 1000 sq ft in a single application, and when possible,
split the application of water soluble fertilizer into two 0.5 lb N
increments. Use phosphate-free fertilizers (example 32-0-10) if soil tests
indicate no phosphorus is needed. And be sure to keep all fertilizer off
hardscapes in order to protect water quality.
In conclusion, summer heat injury is complex. However, one difference is
that we notice the decline as it happens and try to respond in an
appropriate manner. I hope information helps regarding the complexities of
heat stress on cool-season turfs and the potential interactions involved.
Feel free to share this information with anyone that might benefit in better
understanding the challenges you are facing in cool-season turfgrass
management in the
summer of 2010.
Sincerely,
Mike Goatley, Jr.
Professor, Extension Turfgrass Specialist
Crop and Soil Environmental Sciences
Virginia Polytechnic Institute and State University
420 Smyth Hall
Blacksburg, Virginia 24061-0403
540/231-2951 FAX: 540/231-3075 goatley@vt.edu
