It is sometimes hard to believe that we are thinking about fall already, but it is a reality for those of us in this business as well as for homeowners with an interest in the long-term well-being of their lawns.
We have experienced unusually high temperatures in June, and there has not been much rain. The cool season grasses have reacted as one might expect. The University of Nebraska-Lincoln is nice enough to provide periodic updates on climate and its impact on turf. Dr. Zac Reicher, Professsor, Turfgrass Science, recently sent us the following:
High temperatures cause problems both above and below ground. Above ground, photosynthesis or energy production of cool-season turfgrasses starts to decline once daytime air temperatures exceed 70-75F. At the same time, respiration (the energy-consuming process to maintain the plant) is increasing with higher temperatures. At air temperatures greater than 80-85F, cool-season turfgrass plants are in an energy debit where energy demand is higher than energy production. Short periods of an energy debit are normal and the plant uses stored energy to meet its needs. However,energy stores are depleted with extended heat, and the plant weakens. This energy depletion is further exaggerated during warm nights when the plant uses up far more stored energy for respiration than during cool nights. Since the energy reserves of cool-season plants are fairly high early in the summer (June), they are usually capable of withstanding early summer heat like we are expecting. However, this early summer heat depletes energy reserves that may be needed later in the summer.
Below ground, root growth of cool-season turfgrass is optimal between 50 and 65F and declines quickly above 70F. At the same time, root death increases at elevated temperatures, especially in wet soils with limited ability to hold oxygen. The end result is that root systems become shallow and spindly with prolonged heat and thus have limited ability to take up water and nutrients.
Following are management suggestions to help the cool-season grasses cope with the heat:
The almost immediate response to high temperatures is to increase irrigation, which makes sense and is needed since most water is used for cooling the plant. However, saturated soils from over irrigation limits oxygen leading to root death and thus severely compromises the grass plant over the rest of the season. Therefore, we still want to keep turf on the dry side to maximize soil oxygen, but at the same time enough water should be applied to allow the plant to cool itself.
As you can see, the processes are complex. Dr. Reicher goes on to talk about mowing strategies as well.
So, how do we build that healthy lawn that is best able to withstand the rigors of Nebraska weather, turf disease, and insects? We cannot stress enough the importance of aerating lawns twice a year. We believe that periodic overseeding, following a double-plug aeration, is a way to introduce newer varieties of turf cultivars in addition to thickening up the lawn. In the case of lawns that have significant damage, we would add power-raking to remove excess damaged turf material. Fall fertilization is extremely important as well. It is not the time to skimp on applications.
By mid-July, we will have completed our applications for grub control. Healthy turf (there is that word healthy again) can withstand the presence of grubs at below treatment level thresholds. Some grubs may always be present at some stage of development. We do have products for insect break-through later in the season.
We try to minimize our use of post-emergent weed control products when the temperatures are high. These are products we spray, and we like to avoid damaging healthy turf surrounding the weeds. Additionally, the products simply don’t work as well in extreme heat. Optimal control for most broad-leaf weeds occurs in the fall.