The best time to fertilize a bluegrass lawn (and other cool-season species) is in the fall. Fertilizer applied at that time provides maximum benefits, such as good root growth, the grass retains good color longer in the winter and regains good color earlier in the spring without stimulation of excessive spring growth.

Fertilizer applied in the spring results in little or no new root growth but heavy top growth.The grass has to be cut more often and the overall effect is a lawn that is more susceptible to disease and invasion by weeds.

The best and only time to fertilize zoysia and bermuda grass is during late spring and summer.

Improved Kentucky bluegrasses such as Merion, Fylking, Pennstar, Adelphi, Baron and Sydsport require higher amounts of nitrogen fertilizer than the common Kentucky bluegrasses such as Kenblue and South Dakota certified.

Generally speaking, the improved varieties should be given 4 to 5 pounds of inorganic nitrogen per 1,000 square feet per year whereas the common varieties do much better if given 3 pounds of inorganic nitrogen per 1,000 square feet per year.

Give the improved varieties 8 pounds of 12-4-8 or 10 pounds of 10-6-4 or 5 pounds of 20-10-10 per 1,000 square feet in late September or early October. In late October and again in late November give them 12 pounds of 12-4-8 or 15 pounds of 10-6-4 or 8 pounds of 20-10-10.

If the November application is omitted, the same amount of fertilizer can be applied in later February or early March.

Give common Kentucky bluegrasses about 10 pounds of 10-6-4 (or the equivalent) in late September, in late October and again in late November.

Specialists say that these fertilization rates are maximum from a disease control standpoint.

"Anything above these rates will cause thatching at an excessively rapid rate and also predispose the leaves to infection by certain fungi," says Dr. Houston B. Couch, Virginia Tech professor plant pathology, specializing in turf and root diseases.

Apply fertilizer only when grass blades are dry. Otherwise the grass may be burned. After application, turn the sprinkler on for an hour or so to wash the fertilizer off the grass.

A clogged or poorly adjusted spreader that doesn't spread evenly can put down too much fertilizer in some places and not enough in others. Starting or stopping the spreader with the trap open can cause uneven distribution and possibly burn the grass.

Roots continue to develop during the winter when temperatures are above freezing. A strong root system permits the grass to resist the heat of summer and to make better recovery if infected by a disease.

Fertilizing the grass during the winter will not cause injury, the latest research has shown.

Zoysia and bermuda grass can be given 10 pounds of 10-6-4 per 1000 square feet in May, and again in June, July and August.

If the zoysia is good enough to prevent weeds from becoming established, grass color is good and there are no bare spots, zoysia need not be fertilized at all.