Extension notes is written by K-State Extension of Harvey County extension agents Scott Eckert, Susan Jackson and Ryan Flaming. They focus on horticulture and agriculture.
Kansas is in what is known as the transition zone. Cool-season grasses such as tall fescue and Kentucky bluegrass are better adapted farther north than Kansas and warm-season grasses such as Bermuda grass and zoysia grass are better adapted south. It is difficult to grow a lawn that is attractive throughout the growing season in Kansas. In addition, there are differences among members of these two groups as well. Though tall fescue and Kentucky bluegrass are both cool-season grasses and both struggle during hot, dry summers, tall fescue is better adapted
Why is the summer so hard on cool-season grasses? As temperatures increase so does the photosynthetic rate, but only up to a point. Cool-season grasses usually are most efficient at between 70 and 80 degrees F. Above that point, photosynthetic efficiency drops.
Less efficient photosynthesis means less food (sugar) made for the plant to use. Also, every living cell in a plant respires. In other words, each cell breaks down the energy captured in photosynthesis and uses it to fuel the cell. Respiration also increases with temperature, but it never stops. It continues day and night. Hot days and hot nights give cool-season plants a double whammy. Less efficient photosynthesis results in less food production, and high rates of respiration results in food being used very quickly.
Eventually plants start to run out of energy. They weaken and possibly die. Both turfgrass species try to avoid this by going dormant. Tall fescue has better heat and drought tolerance than Kentucky bluegrass and is better able to withstand these stresses. Although tall fescue is often damaged by hot, dry summers, it usually bounces back more quickly than Kentucky bluegrass. Even though bluegrass has the ability to thicken up due to the presence of underground runners known as rhizomes, it often is slow to do so. Tall fescue is a bunch grass and must be thickened up by overseeding.
— Scott Eckert is a Kansas State Research and Extension Agent for Harvey County. Horticulture is his specialty.