Heat, Drought, Summer Dormancy and Watering
Generally, summer dormancy is an important way for cool-season grasses to survive temporarily hot, dry periods during the summer. During dormancy the plants stop growth of leaves and shoots, causing both leaves and shoots to turn brown. In most situations, when adequate water and favorable growing conditions return, new growth will occur with little to no long-term damage.
The fine-leaved fescue grasses do not go dormant in the same way as the older Kentucky bluegrass varieties but are quite drought tolerant and just grow very slowly under dry conditions, thus using less water. Not all grass plants are sufficiently tolerant of dry periods to go without irrigation. Perennial ryegrass does not go into dormancy but survives temporary hot, dry conditions by growing more slowly. Some of the newer, improved varieties of Kentucky bluegrass are not as drought tolerant as the older, common Kentucky bluegrass types. Perennial ryegrasses and most of the improved Kentucky bluegrasses will require some irrigation to keep them alive through hot, dry periods.
There are exceptions when it is important to keep the grass growing throughout the summer by supplemental irrigation. Any turfgrass which is affected by or is recovering from damage due to diseases, insects, or other types of injury associated with excessive wear and tear should receive ample water to aid recovery. Any area which has been recently seeded should be watered frequently enough to keep the soil just damp. Newly sodded areas should be kept moist, but not soggy while rooting is established; usually about 3 to 4 weeks. Areas subjected to wear, such as ball fields or play areas, should be watered to maintain tolerance to traffic and aid recovery.
Life in the Turfgrass Crown When Things get Hot and Dry
When lawn grasses go into summer dormancy, life does continue in each plant's crown. Roots grow from the lower surface of the crown, and foliage from the upper surface. Healthy, vigorous crowns can withstand dormancy with ease. Weak, diseased, or insect-damaged crowns are less likely to survive dormancy.
Letting the Lawn Go Dormant
As the soil dries out from the surface downward turfgrass roots have an increasingly difficult time acquiring enough water to support topgrowth. When soil moisture supplies are too low for absorption to occur, lawn grasses eventually become dormant or they significantly slow their growth rates as a means of avoiding or tolerating those conditions. The onset of these survival mechanisms will be accelerated by high temperatures coupled by lack of sufficient rainfall.
Onset of Turfgrass Dormancy
Dormancy starts with the wilting of lawn grass foliage and shoots. First, a blue-green cast to the foliage develops. At this time, footprints will show as a person walks across the lawn. As the process continues, foliage shrivels, dries and turns brown (Fig. 9.6).
Figure 9.6. Dormant turfgrass.
High nighttime temperatures (in the 70s and 80s) cause northern, cool season turfgrasses to deplete their carbohydrate energy reserves rapidly. Extended periods of these conditions can cause serious, permanent turfgrass loss. Also, lawns growing on compacted soils are in a very vulnerable condition when weather conditions turn hot and dry.
Compacted soil prevents the development of a deeper root system and may encourage greater development of thatch. A shallow root system accompanied by an excessive (greater than a 1/2 -3/4 inch) thatch layer, provides little protection from potentially serious turf injury and/or loss during extended periods of hot, dry conditions.
A watering program will need to be continued to prevent serious turfgrass loss to lawns growing under these conditions. Aerifying the lawn, or better, properly preparing the soil before a lawn is installed, will help encourage a deeper root system and consequently improve the lawn's overall stress tolerance during hot, dry weather.