Mowing the lawn seems to be a rather routine job. In fact, the way the grass is cut is one of the most important factors in maintaining an attractive lawn. More lawns are badly damaged by low mowing than by any other factor, according to specialists.

Blue grass, fescue and perennial rye do much better if cut at about two inches in sunny locations and at three inches in the shade. In many lawns they are cut at one to 1 1/2 inches.

The grass blades (leaves) produce the food of the plant (photosynthesis), and the more leaf the plant has, the more food it produces, which results in stronger leaves, deeper root penetration and richer, fuller growth.

Low mowing is to the lawn what overgrazing is to the range, according to Robert W. Schery, director of the Lawn Institute. Grass consistently deprived of green leaf weakens, thins and becomes supplanted by prostate weeds such as spurge and knotweed.

Some newer varieties of bluegrass, including Fylking and Pennstar, tolerate a lower mowing height. The best mowing height for a lawn that is predominantly red fescue is two to three inches; for Kentucky 31 fescue it's two to four inches. Bentgrasses are usually at 1/4 to 1/2 inch, Burmuda at 1/2 to one inch, and zoysia at 3/4 to one inch.

Grass mowed at the proper height has a better root system and the ability to crowd out weeds and keep them from becoming established. The grass can stand the heat and drought of summer and is less likely to be damaged by turf diseases and insects.

The rule of thumb in mowing is not to remove more than one-third of the green leaf at a time. If you miss one or two mowings and the grass gets quite tall, raise the mowing height for two or three mowings and then gradually drop it back to the original height.

The frequency of mowing should vary with the season. In spring, when grasses are growing rapidly, they may need to be mowed twice a week. In summer, when they're growing slowly, mowing every two weeks is often enough.

Keep the blade of the mower sharp, particularly that of a rotary mower. A dull blade will tear or shred the grass instead of cutting it cleanly. Many gardeners have two blades for their rotary, one to be used while the other is being sharpened. The blades may need is several times during the season. A newly sharpened blade will cut cleanly at first but soon may become dull. Q: How do I get rid of crabgrass in my Kentucky 31 fescue lawn? A: Once crabgrass comes up in a fescue lawn it's difficult to get it out. It will be killed by the first frost this fall. Next spring, keep the crabgrass from getting started.Cut the fescue at 2 1/2 to three inches and keep it thick so no sunlight will reach the soil surface. You can also use a preemergence chemical such as Dacthal about three weeks before crabgrass is due to germinate. It will prevent germination. Q: We have been away, and returned to find a lot of bare spots in our bluegrass lawns. Is it too late to seed them this spring? A: It may be done successfully with perennial rye such as Pennfine, Manhattan, NK-100 or NK-200.