Spurge infestation in relation to mowing height. Far left is mowed at 1 inch, middle at 1.75 inches, right at 2.5 inches. Photo: Dr. Pete Landschoot, Penn State
The one common maintenance practice that most turf has in common is mowing. A common overlooked fact is that mowing plays a large role in both the type and amount of weeds present.
You may have heard of the one-third rule, which states to never mow off more than one third of the plant. This is a good rule of thumb to follow, but there is more to keep in mind. Think of the turf canopy as a forest. When a forest is clear cut, hundreds of different plants grow rapidly, competing for sunlight and nutrients from the soil. As time passes and trees get large, less weeds and other plants plants grow, allowing the trees to proliferate. This same concept applies to turf.
As turf grows in height, and in density, it is shading out the sun from reaching the soil. As you know, weed seeds need proper temperatures and sunlight (along with water) to germinate. If sunlight is not penetrating the turf canopy, many weeds are unable to germinate, allowing for the turf to grow better. Studies have shown that increased mowing height alone reduces weed pressure significantly, even in the absence of pre-emergent herbicides.
As summer begins, mowing heights in lawns should be raised up to three inches or more. This will allow the turf to grow thicker and reduce weed pressure. Our cool season grasses grow best in spring and fall, while crabgrass prefers hot temperatures. Another thing to keep in mind is that there is a direct relationship between mowing height and root depth and density. As turf height increases, root depth increases as well. In Pennsylvania, turf can be subjected to hot and dry temperatures on a regular basis. By encouraging deeper root growth, turf plants can better mine the soil for water and nutrients, which will allow the turf to better withstand heat and drought, along with disease and insect pressure.
Finally, a few more things to keep in mind about mowing include blade sharpness and clipping management. It is very important to always keep mower blades sharp. Dull mower blades damage the turf, which further exacerbates any stresses that the turf may experience. Another good practice is to return clippings to the turf, and not collect them in bags or baskets. This can be accomplished by using a mulching mower blade. Turf clippings are a source of nitrogen and other nutrients that the turf can re-use throughout the year. Think about clippings as free fertilizer. Studies have shown that returning clippings to the turf can result in up to two pounds of nitrogen per year! That is about half of what turf managers typically provide to turf through fertilization in a given year.
In order for turf to out-compete nuisance weeds, promote root growth, and stay greener through the summer, educate your clients on the benefits of raising their mowing height and performing proper mowing practices.