December is typically the coldest month of the year in the low desert elevations. Freezing temperatures often occur and the grass can show typical symptoms for about a month.

In general, overseeded lawns have much reduced growth and show some loss of color, and the leaves appear thin. The "thin" condition is (in part) from the fact that cold temperatures slow the emergence of the youngest leaves and when they do emerge they do not unfold readily.

Frost damage on leaf tips if different. Annual ryegrass is more susceptible to this event than is perennial ryegrass. On annual ryegrass, the leaf tip will be twisted and appear yellow or white in color. After a few days, the leaf tip will dry and look straw in color (if it did not get mowed off yet). Tall fescue also can get frost damage, especially in Tucson, Sierra Vista, Benson, Payson and the like. The tall fescue leaf tips turn yellow and white and then dry to a silver color. Also, the leaf tip will look shredded for awhile. This is very different from the condition described for annual ryegrass.

When the lawn is frozen (no matter what grass you have) do not walk on the lawn. The leaves will snap with the ice. When the ice melts, the grass then wilts severely. The grass wilts because the leaves have been broken and the water in the leaves exits very quickly. This is sometimes called "frost printing." It may take a considerable time for the grass to regrow upright again.

The best way to keep the grass green in December is to . . .

1. Mow properly. Do not scalp the lawn. The scalped lawn will grow back yellow and take up to 3-4 weeks for recovery.

2. Fertilize the lawn with 1/4 lb. -N-/1000 ft. from a high nitrate fertilizer. Examples include potassium nitrate (KNO₃) which is 13.75-0-44 in analysis, or ammonium nitrate (NH₄ NO₃) which is 34-0-0.

3. Do not use urea or slow release fertilizers in December or January. Spoon food with nitrate forms once every three weeks at the prescribed rate of 1/4 lb. -N-/1000 ft.

4. Use iron regularly before it gets cold and during cold snaps. You can use iron sulphate powder at the rate of 4 ounces per 1000 ft². You can mix four tablespoons per gallon in warm water and stir it vigorously. Then apply the iron to the lawn as a spray with a hand sprayer. Make sure the sprayer did not have a herbicide in it before. Wash the sprayer with detergent and rinse three times for safety sake. It's best to apply the iron at 10:00 a.m. or so in the morning. Do not irrigate the lawn or mow the lawn that day. Irrigate the lawn the next day and before mowing.

Iron cheleates are more expensive, but they last longer. For small lawns, the expense is offset by good performance. Most of these are liquids. Apply according to label directions. Usually the rates are from 1-3 ounces of liquid product per gallon of water to be sprayed.