Rippling heat waves
The middle of July saw an intensive heat wave grip much of the Central and Eastern United States from Oklahoma to New England. That heat wave that moved across the country seems to have taken up residency in the form of a heat dome across Europe. Last Friday, July 26, there was the hottest weather ever recorded in London, Paris, Belgium, and Germany. The system responsible for the heat will shift north across the top of the globe.
Figure 1. Under summertime conditions moisture stress can occur in areas where shallow rooted trees outcompete the turfgrass for water, resulting in drought areas that appear like islands among green turf.
The continued heat and dry conditions through the United States and Europe raises concerns about the “health” of cool-season turfgrasses. In some areas, turf has gone dormant. Brown, dead leaves characterize dormancy. Visually, the brownish turf may look dead, but it is the condition of the buds in the crowns, stolons and rhizomes that determines plant survival. Buds, which give rise to new shoots, are extremely drought hardy because they have small vacuoles (cavity in the cell where water, waste and food are stored).
Figure 2. In the first of two pictures, this lawn was mowed while it was under heat and drought stress. Notice the mower patterns and the homeowner steps while mowing.
The cool season turfgrasses that can undergo dormancy for relatively prolonged periods include kentucky bluegrass (Poa pratensis), perennial ryegrass (Lolium perenne), and tall fescue (Festuca arundinacea). The fine fescues (Festuca rubra) growing on sand or un-compacted soils can tolerate periods of drought in dormancy. The bentgrasses (Agrostis stolonifera) and Poa annua do not survive dormancy well.
Figure 3. This is the same lawn as in Fig. 2 but taken several weeks later after sufficient rainfall. Note the damaged turf, where the wear of the mower and homeowner, has not recovered. Wear can cause injury on semi-dormant or dormant turf.
How long can a turfgrass plant like kentucky bluegrass remain dormant? It is contingent on a number of factors, but with regard to kentucky bluegrass, it is probably indefinite. However, if temperatures remain well above normal for growth, injury could occur. Generally speaking, kentucky bluegrass, perennial ryegrass, and fine fescues (depending on species and soil type) could survive four to 10 weeks. If heat and lack of moisture persist, light applications of water to moisten the crown could be applied.
Figure 4. Wear injury is evident in areas where this athletic field had undergone heat and drought stress.
Culturally, minimize wear and traffic to the turf. The degree of injury however varies depending on the turfgrass species. A dormant Kentucky bluegrass turf can most likely sustain more traffic due to the fact it produces rhizomes, which are protected by the soil. In Europe, the creeping red fescue (Festuca rubra spp.rubra ) growing on well-drained acid soils should also survive drought well in dormancy. Turfgrasses that are more bunch-type in are potentially more susceptible to injury because of the lack of rhizomes or stolons. Mowing on drought stressed turf can be injurious and should be minimized, as well as treating for weeds. Fertilization of dormant turf is possible, but a slow release fertilizer should be used.
About the author
Dr. Karl Danneberger is a professor of Turfgrass Science at The Ohio State University. Dr. Danneberger's contact information can be found here. You may also follow Dr. Danneberger on Twitter: