There are many environmental stresses that affect our lawns: drought, heat, insects, weed seeds blowing in from neighbors etc. Unfortunately we do not have much control over these stresses. But we do have control over one necessary stress: mowing. This is something that you should keep in mind, that mowing is actually stressing your lawn, it’s not helping it. Grasses are the best equipped plants on earth that can tolerate this type of defoliation, but there are certain guidelines we can follow to mow properly to reduce this stress to a minimum.
When grass is mowed at extremely low mowing heights, it is under considerable stress due to the constant tissue removal. Under these circumstances, your lawn is more susceptible to diseases. A low mowing height also encourages weeds. With less tissue cover, there is more sunlight to reach the soil to encourage weed seed germination. Also, when your lawn is under stress, it cannot out compete these germinating weed seeds. The shorter the lawn, the greater the weed population. When a lawn is mowed short, grasses compensate the loss of tissue by becoming more dense over time. The carbohydrates that are channeled into the formation of higher shoot density are not available for root growth. This results in less root mass and a shorter depth of rooting. You can imagine that this adds to the stress. Less roots equals less water and nutrient uptake.
Remember never to remove more than 1/3 of the leaf tissue at a time. Removal of more than this in one mowing can cause your roots to stop growing for a period of 6 days to 2 weeks. Lowering the height too quickly can also result in a condition called scalping. Scalping occurs when too much tissue has been removed at one time, causing the shaded portions of the grass to be exposed to sunlight. In the heat of the summer, this will cause your grass to burn. If the lawn has a yellowish or brownish cast immediately after mowing, that’s a sure sign that it has been scalped.
Below is a table of MINIMUM mowing heights for different types of grass species. Do you have a lawn with several different kinds of grass? Go by the species with the highest mowing height.
High-Temp Stress Period
|Kentucky Bluegrass||1.5 - 2.25"||2.25 - 3"|
|Perennial Ryegrass||1.5 - 2"||2 - 3"|
|Tall Fescue||1.75 - 3"||2.5 - 3"|
|Buffalograss||1" to unmowed||N/A|
|Fine Fescue||.5 - 2"||1.5 - 3"|
A lawn mowed at 2 inches will need to be mowed about once a week. Establishing proper mowing frequency is important. Mowing too frequently places extra stress on the turf from water loss and excess traffic. Infrequent mowing may result in scalping and a buildup of clippings on the surface that can contribute to diseases and other problems. Following the rule of not removing more than 1/3 of the clipping in a single mowing is the best guide.
Many believe that not collecting your clippings contributes to a thick thatch layer, which in turn can decrease water penetration, increase disease potential, and harbor insects. In fact, most of our thatch buildup comes from the natural death of roots, rhizomes, and old leaf sheaths. A thick thatch layer is caused by the excess growth of these structures cause by excessive nitrogen and excessive watering. Watering too much creates an anaerobic (low-oxygen) condition, which kills off the microbes that break down this thatch layer. Returning your clippings to the soil is actually very beneficial because it can add up to 2 pounds of nitrogen /1000ft2 /year. The only time that clipping removal would be suggested would be if there are weeds in flower or you’re planning to remove a lot of grass in a short amount of time.
Over all, grass is an amazing plant that can tolerate many environmental stresses, and even some cultural stresses such as mowing. If you follow these guidelines on mowing height, frequency and clipping removal, you can greatly increase your lawn’s ability to combat diseases, weeds and our hot dry summers.