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Cutting blades with blades: the right way to mow your lawn. How you treat grass above can affect the roots below

The lawn with consistent good looks has one thing in common with the head of impeccably styled hair: It is trimmed frequently, and very little is taken off at any one time. That's the consistent advice of lawn keepers everywhere, to which they often add: ``Cut high.''

The question is, How high and how often?

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Before answering, Eliot Roberts, director of the Lawn Institute, says that homeowners must first understand that they can never cut the grass on top without affecting the roots below.

If the grass is kept closely mown, it will have a comparably shallow root system that is drought prone and vulnerable to weeds.

Also, if too much grass is removed at any one time, the balance between roots and top growth is so suddenly and drastically altered that many roots die.

So one rule of thumb is never to trim more than a quarter of the grass height at any one time.

In other words, if your grass is four inches high, don't cut it lower than three inches in one mowing. Generally speaking, a once-a-week mowing accomplishes this.

``We want to trim away enough so that the lawn looks neat,'' says Dr. Roberts, ``yet leave enough foliage to maintain a root system that is six to eight inches deep.''

To maintain that root depth, standard grasses in the cooler North should be maintained at between two and three inches.

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On the other hand, on some of the newer varieties of grasses (check with your garden center), the turf can be trimmed to as low as 1 inches without adversely affecting the roots.

In the South, the common Bermuda grass can be trimmed to between three-quarters and one inch. Zoysia prefers one inch, and St. Augustine and bahia grass, 1 inches.

There is one more major plus to frequent mowing:

The clippings are small enough to be left on the lawn, where they will rot away and return many of the nutrients to the turf. The amount of nutrients saved this way yearly amounts to about two pounds of pure nitrogen per thousand square feet.

If regular mowing helps maintain a good-looking lawn, so will sharp blades on the mower. Take a file to the blades at least once a month.

A still better approach: Lightly file before or after every mowing.

Meanwhile, the Outdoor Power Equipment Institute, noting that almost 60 million American households own outdoor power equipment, principally mowers, offers these standard procedures for safe operating:

Make sure your equipment has been tested and meets safety standards. To this end, members of the institute have a triangular seal on power equipment that has been independently tested and certified to meet voluntary safety standards.

Be sure you know how to operate the equipment. Read the instruction manual, and be sure you know where the engine ``kill switch'' is.

Dress for the job. Wear substantial shoes, long pants, and close-fitting clothes. Loose clothing, dangling jewelry, even unrestrained hair can get caught in moving machinery.

Handle gasoline carefully. Fill up before starting, while the engine is cold. Wipe up any spills immediately.

Clean out the area to be mowed before you start. Toys, stones, and sticks can be turned into projectiles by the spinning blades of the mower. Be sure children and pets stay out of the way while you are mowing.

Operate with care. Always turn off the engine and disconnect the spark plug wire before attempting to unclog the mower.

Keep hands and feet away from the moving parts of the mower.

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