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
August 4, 2011
Dear Virginia Golf Course Superintendents:
While the Virginia Tech Turf grass Team is in general agreement that the summer of 2010 resulted in a far greater number of calls regarding issues with the health of golf turf, the heat wave of mid-June through August in 2011 has finally started to play havoc with our cool-season grasses on many courses statewide. As usual, creeping bent grass and/or Poa annua (annual blue grass) putting greens throughout Virginia are experiencing difficulties associated with summer heat stress. However, many cool-season fairways are also in less than desirable quality and in many cases the reason is as much ‘financial’ as it is ‘environmental’. During this economic downturn, many courses simply do not have the budget to apply fungicides on a regularly scheduled interval to such large acreages as they likely have done in the past. Whether it is greens, tees, or fairways, the effects of the extreme heat are basically the same. 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.
The challenges of high relative humidity combined with persistent daily highs in excess of 90o F and night temperatures that do not dip below 70o F present a powerful combination for a superintendent to try to manage, much less overcome. Close-cut putting green turf can tolerate brief periods of heat, especially if the soil temperature remains cool. However, the fact that such high temperature extremes have relentlessly persisted throughout July has resulted in direct heat stress injury, as well as increasing the greens’ vulnerability 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, algae, nematodes and for the first time on a significant scale in Virginia, reports of bacteria associated with creeping bentgrass). As a result, heat related injury and the associated decline in turf quality results from a complex of individual factors acting together. Stated quite plainly, during a period of mid-summer heat stress, the grass plants are more vulnerable to anything that can cause damage. During cooler times of the year these stresses are most often negligible. And remember that besides air and soil temperatures, another extremely important environmental influence on the plant’s ability to cool itself is high relative humidity. High humidity reduces evaporatranspiration rates and plants are 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 feet above the ground and that temperatures at the turf canopy level may exceed the recorded high. For example, with air temperatures of 90o F, the actual temperature at the surface canopy of most putting greens will likely be in the 105-110o F range. Prolonged heat stress significantly increases soil temperatures and this directly impacts root growth, root health and function. High air temperatures in the spring are less damaging since the soil temperatures remain relatively cool (i.e. the soil is much more buffered against changes in temperature than the air). However, that high buffering capacity of the soil means that once it does warm significantly, it resists rapid cooling. Root production declines significantly under these conditions. The soil’s high buffering capacity to resist temperature change is not inherently bad. This is something we consider beneficial in the fall when we can optimize fertilization, recovery, and establishment of our cool-season grasses.
A profile of each golf course and 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 grasses are as follows:
Creeping Bentgrass vs. Annual Bluegrass Greens: Annual bluegrass is not nearly as heat, drought, or traffic tolerant as creeping bentgrass. The heat may have killed some annual bluegrass but its departure is likely only temporary. Tremendous amounts of annual bluegrass seeds remain in the soil where it has been growing. These seeds will germinate this fall and reinfest the green. For greens that have been severely thinned, measures should be taken to reseed the greens with creeping bentgrass in late August/early September in advance of the annual bluegrass germination. Seeding at 1 to 2 lb. per 1000 square feet is appropriate. Higher mowing heights and traffic control (maybe even temporary greens) may be required for a good stand of creeping bentgrass to get established.
Drainage and Water: Greens with good internal drainage (USGA-type sand construction), and good surface drainage have the benefit of being able to avoid excessive soil water buildup. Frequent syringing and/or hand watering is most appropriate in severe heat stress. However, even well drained greens can be excessively moist if “Mother Nature” produces frequent rain showers (a constant complaint by many in 2011) or the greens are over watered. For courses closer to the coast, don’t forget that salt build-up in the soil from brackish irrigation water imparts an additional physiological drought stress to the turf.
Air Movement: Trees and turf are at no more greater odds with each other than in periods of heat stress. Any trees that block air and wind movement should be removed or greatly thinned to provide adequate ventilation for putting greens. Putting greens, that by hole design are in low areas or surrounded by mounds, will also suffer from lack of air movement. Oscillating electric fans are part of the solution but they do not make up for poor design or natural air movement that is blocked by trees or other vegetation.
Mowing Heights and Frequencies: Heat hardiness is reduced at short mowing heights. Close and frequent mowing during heat stress will result in greater turf decline than if mowing heights are raised and frequency reduced. Increases in soil temperatures are more likely under shorter mowing heights. Golf courses, whether public or private strive try to deliver the highest quality putting greens year-round, and this usually involves close and frequent mowing. To continue to mow in this manner invites summer decline, especially with how severe the heat stress has been in mid-summer 2011.
Traffic Stress: Golf courses with high amounts of play are likely to be most affected during periods of summer stress. Traffic damage will be slow to recover with reduced plant vigor. Higher mowing heights will help.
Public Perception and Maintaining Maximum Playability: Seasonal environmental conditions will often dictate what should and should not be done to the grass to maintain “playability”. Often club officials that determine the standards of maintenance strive for maximum playability all year! This is fine when the grass is not overly stressed. However, a turf management program should be flexible to “ebb and flow” with the vagaries of weather, especially in the mid-Atlantic transition zone environment.
Summer Diseases and Other Pests: 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. Summer fungal diseases are often the most difficult to control. Damage from Pythium (both Foliar Blight and Root Pythium), Rhizoctonia Brown Patch, Summer Patch, Fusarium Blight, Take-All Patch, and nematodes is often most severe during periods of extreme day and night temperatures. There is a great deal of talk this year regarding bacteria-incited diseases on bentgrass. The number of reports of bacteria associated with bentgrass around the state and region are unprecedented in 2011. However, pathologists are debating the overall relevance and significance of this problem. There are no clear-cut chemical alternatives to bacterial-related problems at this time and there is no doubt that this topic is going to be closely scrutinized in the coming months. In general, disease control programs will increase their frequency and rates on bentgrass/Poa greens during summer stress periods, but superintendents must still use caution and follow label directions. It is highly possible that excessive chemical management approaches can actually exacerbate problems rather than correct them. If nematodes are suspected, have the soil analyzed to confirm if problems really do exist. Chemical strategies for managing nematodes are quite limited.
Soil Aeration: The soil must be well aerated for plants to function. Soils that become sealed off at the surface, from compaction or from algae formation, 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. The result is what is called ‘wet wilt”, when a near saturated soil condition exists which also reduces plant vigor and function. Venting the greens with pencil tines etc. continues to be a very sound philosophy to keep the rootzone oxygenated, even during the stressful environmental periods. However, it is always prudent to perform venting during the least stressful periods of the day.
Fertilization: Although it seems somewhat counter-intuitive, it is appropriate to continue to feed heat-stressed greens small amounts of Nitrogen. Note the emphasis on ‘small’! Anywhere from 0.1 to 0.15 lb N/1000 sq ft every 2-3 weeks will benefit overall plant health and recovery potential, and then initiate more aggressive N fertilization programs when the consistently cool temperatures of fall arrive.
In conclusion, summer heat injury is highly complex and it is frustrating with how quickly it can and does happen even with our best preventative efforts. I hope this information helps in your understanding and your ability to communicate the complexities of heat stress on cool-season grasses to your clientele. Please let me know if I can assist you with any of these challenges.
Mike Goatley, Jr.
Professor and Extension Turfgrass Specialist
CSES Department, Virginia Tech