The sight of a freshly mown lawn often includes an unwelcome dusting of yellow or brown on the tips of the grass blades. This immediate discoloration is a common observation for homeowners and is not a sign of disease, but a direct physical reaction by the plant to the trauma of mowing. Understanding the mechanics behind this color change helps diagnose whether the issue is related to the equipment or the technique used.
Mechanical Damage from Dull Blades
Mowing is a traumatic event for the grass plant, and the quality of the blade determines the severity of this trauma. A sharp mower blade slices through the leaf tissue cleanly, minimizing the wound. Conversely, a dull blade tears and shreds the delicate vascular tissue, creating a ragged and extensive injury site.
This tearing exposes the leaf’s internal cellular contents to the air, initiating a rapid drying and oxidation process. Healthy grass is green due to chlorophyll, but the inner layers contain other pigments, such as carotenoids and xanthophylls, which are naturally yellow or orange. When the chlorophyll-rich outer layer is shredded, these underlying pigments are exposed and quickly dry out, resulting in the yellow or brown appearance commonly termed “tip burn.”
The plant must expend considerable energy to seal and heal these large, jagged wounds, diverting resources away from root development and growth. Furthermore, the extensive open wounds created by dull blades provide easy entry points for lawn diseases and fungal pathogens, increasing the grass’s overall vulnerability to infection.
The Problem of Cutting Too Low (Scalping)
Widespread yellowing, beyond just the tips, often results from removing too much of the grass blade in a single session, a practice known as scalping. Most chlorophyll, the pigment responsible for green color and photosynthesis, is concentrated in the upper portion of the blade. Lower sections, closer to the crown, receive less light and develop less chlorophyll.
This lower, shaded part of the plant retains a higher concentration of non-photosynthetic pigments, giving it a lighter, yellower, or tan appearance. When the mower deck is set too low, it removes the majority of the green tissue, suddenly exposing this less-pigmented lower section to full sunlight. The lawn immediately takes on a widespread yellowish-brown cast because the primary green coloring has been stripped away.
To avoid this discoloration and minimize stress, turf managers adhere to the “one-third rule.” This rule dictates that you should never remove more than one-third of the total leaf blade height during any single cut. For example, if your ideal grass height is three inches, mow when the grass reaches a maximum of four and a half inches tall.
Strategies for Green, Healthy Mowing
Preventing post-mowing yellowing begins with attention to the mower’s cutting edge. Blades should be sharpened regularly, ideally at least once per season or after every eight to ten hours of use. Sharp blades provide a clean, surgical cut, minimizing the plant’s wound size and reducing the area exposed to oxidation and disease pathogens.
Proper mowing height is equally important for maintaining a vibrant green color. Set the mower deck as high as your turf variety allows while following the one-third rule to prevent scalping. Taller grass blades cast shade onto the soil, which cools the ground and reduces water evaporation, promoting a healthier turf environment.
A higher cut encourages a deeper, more robust root system, helping the grass better manage environmental stressors like heat and drought. Healthy, well-rooted grass recovers much faster from cutting trauma, maintaining its green appearance.
To further reduce stress, consider environmental conditions before mowing. Avoid cutting the lawn when the grass is wet, as moisture causes blades to stick together, resulting in an uneven cut and increased likelihood of tearing. If the grass is under severe drought stress, delaying the mowing will prevent further desiccation and discoloration.