Mowing a lawn is not exactly like giving a haircut. Cutting living tissue is stressful under the best conditions, but using a dull blade adds insult to injury, especially on a rotary mower (which we all use) that cuts by brute force, whacking instead of scissoring neatly like the old real mowers.
There is a problem with direct ability of disease organisms to enter the wound. Also, even with a sharp blade, the loss of leaf surface means reduced photosynthesis and reduced ability to take up water. These stresses make the plant weaker and less able to fight off diseases, and they slow down root growth. When the cut is ragged and torn, the increased surface area open to the air means a 10 to 15 percent additional water loss through evaporation, and a longer time to heal. Diseases find it easier to enter a host with weakened defenses.
It is also important to avoid mowing when the grass is wet with dew or rain. Not only are fungal populations higher when it is damp or humid, but the water on the grass impedes the lawn-mower blade, leading to a poorer-quality cut. Again, rotary mowers are bigger offenders than reel mowers under these circumstances.
Cutting grass right also means cutting it the right height. While grass may prefer never to be cut and we may prefer that golf-course greens look, the best compromise for a healthy, good-looking lawn is to cut it 2 1/2 to 3 inches high year-round. And at each mowing, you should only be removing about the top 1/3 of the grass blade. Consequently, a good time to mow lawns is when your grass is about 3 2/3 inches high.
Note: for anyone wanting the “ballpark” effect, take advice from the groundskeepers for the Yankees. A metal bar is attached to the front of the mower to bend the grass as it is cut, leaving it at a slight angle. After being mowed in alternating directions, the grass strips reflect light differently, giving the effect of lighter and darker stripes. But, to see the stripes, you’ll have to put a video camera up a tree and watch your lawn on television. The effect is really just an optical illusion. The small amount of contrast between stripes is enhanced by the angle of view and the camera.