More lawns are ruined by improper mowing than by any other single cause, according to researchers at agricultural experiment stations. Grass with a good root system can crowd out weeds and keep them from becoming established. Healthy grass can withstand heat and drought of summer and is less likely to be seriously damaged by diseases and insects.

Mowing the grass has an important effect on root growth. The green blades produce food, and cutting them too close reduces food production. If most of the green part is removed, root growth stops for several days or even weeks.

"Lawn mowing of the lawn is the one major mistake of home owners in North America," says Paul N. Voykin, nationally known turf expert and author of Ask The Lawn Expert (Macmillian, $9.95).

"My recommendation is a minimum of two and a half inches for sunny yards -- new dwarf varieties are exceptions -- and three inches for shaded lawns, especially as you move southward.

"Remember that the more leaf the plant has, the better the process of photosynthesis works, which gives you more and stronger leaves, deeper root penetration and richer, fuller growth.

"Mowing too low is asking for trouble, such as your power mower blade hitting every root, bump and low spot."

Low mowing is to the lawn what overgrazing is to the range, says Robert W. Schery, director of the Lawn Institute.

Schery is the author of Lawn Keeping -- How to Create a Beautiful Lawn That's Easy and Economical to Maintain, published by Prentice-Hall.

He says that when grass is deprived of green leaf it weakens, giving way to prostrate weeds like spurge and knotweed. For plants that can stand mowing to 1 1/2 inches, he recommends the new lower-growing bluegrasses, fine fescues and perennial rye grasses.

For a really short-clipped lawn, no higher than 3/4 inch, he suggests colonial bent-grasses such as Exeter, Holfior and Highland, or creeping and velvet bentgrasses such as Emerald and Kingstown.