Mowing, the partial removal of grass leaves is one of the most important cultural practices in lawn maintenance. Perhaps its most important function is to keep the grass plants in a vegetative stage by preventing seed formation. The routine partial removal of grass leaves also serves as a trigger for the plant to produce tillers and rhizomes, resulting in the establishment of dense turf. Although, lawn grasses are well adapted to mowing having been evolved under millions of years of grazing pressure, improper mowing can have negative effects on the health of the grass plants. It has been demonstrated that higher mowing heights promote the development of deeper root systems compared to the lower mowing heights, leading to greater drought tolerance in grass plants.

Our new research indicates that improper mowing practices can have a profound negative effect on the resistance of lawn grasses to insects. We have found that tall fescue and perennial ryegrass plants mowed at 3 inch height had higher amounts of insect defensive compounds than those mowed at 1 or 2 inch heights. We measured a group of compounds called alkaloids that plants produce to defend themselves against insect herbivores. Although, all lawn grasses may produce some type of alkaloids, the endophyte containing cultivars of both perennial ryegrass and tall fescue produce a large variety of alkaloids. In fact, the symbiosis between endophytic fungi and grass plants results in the production of alkaloids sufficient to provide effective defense against insect attack.

But why does the grass plants mowed at lower height produce less alkaloids than those mowed at higher heights? Lawn grasses, like other plants manufacture their food (carbohydrates) in the leaves through the process of photosynthesis. The food manufactured by the plants is used for growth, tiller production and root development, but it is also used for synthesizing defensive compounds such as alkaloids. However, plants perform a remarkable balancing act when allocating their limited food resources towards continued growth or defense. Our research indicates that improper mowing heights can tip the balance, rendering the grass plants more vulnerable to attack by insects and plant pathogens by depriving their resources that will be normally used for the production of defensive compounds. Grass plants mowed at lower heights have smaller leaf area and will produce less food than those mowed at higher heights. Therefore, the plants at low heights will use their limited food resources towards growth and not for producing defensive compounds. Therefore, grass plants mowed at higher mowing heights have more resources to allocate towards defense than those mowed at lower heights.

High mowing heights also have several other benefits on the health and appearance of lawns. In our studies, lawns mowed at 3 inch height had lower air and soil temperature, thus reduced temperature stress on the plants. This effect on air temperature also contributes to the mitigation of the heat-island effect in urbanized areas. The shading effect also reduces weed seed germination preventing the sunlight to reach the soil. In fact, the lawns mowed at 3 inch height had fewer dandelions, healall, clover, and other weeds than the lawns mowed at 1 or 2 inch heights. Research conducted at the Michigan State University demonstrates that lawns mowed at higher mowing heights harbor more natural enemies of insects than the lawns mowed at lower heights.

Mowing frequency may also have similar influence on the health of the grass plants. We are currently conducting a study to evaluate the effect of mowing frequency on the production of alkaloids in the lawn grasses. Until then we recommend the “one-third” rule that states to remove no more than one-third of the grass plant at any one time.

The mowing heights that we have tested in our study are applicable to home lawns. However, golf course fairways and athletic fields may also benefit from higher mowing heights when practical. It is obvious from our studies that the benefits of endophytes are dependent upon the mowing height and these benefits can only be realized when plants have sufficient leaf surface area to manufacture enough food to spare some for the synthesis of alkaloids. Therefore, we recommend that you raise the mowing deck to the highest possible on your mowers.