other hand, excessive nitrogen applications must be avoided as they will encourage shoot growth at the expense of root development. Turf managers who have
experimented with multiple applications of the growth regulator Primo Maxx (Trinexapac-ethyl) may have observed greater heat and drought resistance on treated areas. Trinexapac- ethyl increases levels of zeatin riboside, a cytokinin that alleviates leaf senescence. Sunny, hot days that immediately follow a heavy rain period can be particularly damaging to the turf. The combination of high temperatures and lack of soil oxygen can result in rapid root loss. For the same reason, excessive irrigation must be avoided. Deep, infrequent irrigation (termed wilt-irrigation) should be practiced to harden off the plants to drought and heat stress. Plants that have undergone moisture stress prior to high temperatures are better prepared to withstand heat stress. Pre-conditioning of grasses by inducing a degree of moisture stress enables them to accumulate higher concentrations of ion solutes. In turn, the plant is better able to take up water and maintain transpirational cooling.
Mowing heights should be raised prior to and for the duration of the heat stress period. The insulating effect of the additional foliage helps to maintain
Relative Resistance Very Good
Good Medium
cooler soil temperatures.
The higher height of cut also provides a larger photosynthetic area; enhances transpirational cooling; and assists carbohydrate production and subsequent rooting.
Avoiding and relieving heat stress
Monitoring of soil temperatures can offer an early warning of potential heat stress. Using relatively inexpensive soil thermometers, readings taken at 5cms depth at about mid-day will provide an approximate figure for average daily soil temperatures. During sunny, hot weather, the soil temperature will generally be a few degrees higher than the air temperature. For example, when air temperatures exceed 32ºC, temperatures in the upper 1.5cms of soil can exceed 38ºC. Turfgrass canopy temperatures can be moderated during hot periods by 'syringing'. The process involves the application of a small quantity of water to the plant foliage - without wetting the soil. By this technique, the turfgrass canopy temperature is lowered by a few degrees and encourages stomatal opening for transpiration cooling to continue. However, it is important that the water is applied as a fine mist of small water droplets so that it remains on the foliage. Large droplets tend to
Crested dog’s tail Sheep’s fescue
Chewing’s fescue Highland bent
Fair Hard fescue
Creeping bentgrass Browntop bent
Perennial ecotypes of Annual Hard fescue meadowgrass
Creeping red fescues Poor Very Poor
Annual ecotypes of Annual meadowgrass
Turfgrass Drought Resistance Tall fescueCrested hairgrass
Crested hairgrass
Turfgrass Heat Resistance Tall fescue
Smooth-stalked meadowgrass Smooth-stalked meadowgrass Perennial ryegrass
Chewing’s fescue Highland bent Browntop bent Velvet bent
Perennial ryegrass Creeping bentgrass
'irrigate' and wet the soil;
ultimately risking saturation of the soil surface and a
reduction in soil aeration. When average daily soil temperatures remain above 21ºC, syringing should be carried out. Cooling is best accomplished between 11.00a.m. and 2.00p.m., provided the atmospheric humidity is low enough to promote evaporation from the leaf surfaces. If extremely dry, hot, sunny, and still days are encountered, it may be necessary to syringe a number of times during the day. When combined with the use of fans, syringing can be even more effective at reducing air and soil temperatures. For greatest effect, fans should produce approximately 5.5kph air flow at the surface of the turf. Any ‘irrigation’ that is needed during the period of high temperatures should be scheduled for late night/early morning, and not during daytime. Localised dry spots should be syringed to lower the canopy temperature but when hand 'watering' it required, it should be conducted during the cooler part of the day. For the duration of the heat stress period, intense mechanical maintenance practices such as scarification, deep aeration, and heavy topdressing should be avoided. Shallow pricking of the surface, however, can enhance infiltration rates and improve aeration of fin-textured soils. Nevertheless, such treatments are best executed during the cooler part of the day.
Global ‘warming’ is the challenge Creeping red fescues
Sheep’s fescue Perennial ecotypes of Annual meadowgrass
Annual ecotypes of Annual
meadowgrass Crested dog’s tail
Table 1. Relative Drought and Heat Resistance of Cool-s
Rough-stalked meadowgrass Rough-stalked meadowgrass season turfgrasses
Water will increasingly become an issue. But water shortages are a consequence of global warming; of ever increasing demands from a swelling population; and of growing industrial consumption. Water issues should not camouflage or mask the primary problem - global warming. Water conservation is one of the responsibilities that turf managers will have to respect. But, it is only one element of global warming. And it is global ‘warming’ that may be the biggest challenge they have to contend with.
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