It’s beginning to look and feel a lot like … fall.

And that means it’s time to understand frost.

In spring and fall, the word frost sends gardeners scurrying for sheets, boxes, jars, floating row covers, the sprinkler system, or whatever can be scrounged to keep plants safe from a possible cold snap.

Just what is frost and what does it do, and how do your protective measures work?

Frost is the sparkling beauty made of ice crystals which cover all manner of vegetation on a crisp morning after a clear night.

Cool air, clear skies and light or calm winds are necessary for frost to occur.

On a warm, cloudy, fall night, we are protected from frost or freezing weather.

The temperature is too high, the clouds act as a blanket keeping the earth warm and the wind mixes all the air together so that the cooler air can’t settle close to the plants.

Cool air, on the other hand, permits the temperature to drop low enough to freeze the moisture in the air which would otherwise form dew.

When skies are clear, the heat from the soil is able to rise, making room for the cool air above to settle down close to the ground without mixing it with warm air as would happen if it were windy.

Don’t wait for frost warnings to move your houseplants indoors.

Temperatures of 50 degrees or lower can damage many tropical plants.

Mulch for Winter: In addition to their weed control qualities and moisture conservation advantages, mulch provides winter protection for shallow-rooted plants.

A mulch serves to prevent rapid temperature fluctuations in the soil and reduces the danger of winter damage to the plants.

Mulch should not be placed around plants just yet.

Wait until after a few frosts. The warmth from the soil will provide protection to the liberal applications (two to three inches) of mulch that has a coarse structure, will decompose slowly, and will complement your landscape.

Wood chips will do an effective job.

Controlling broadleaf lawn weeds: Fall is an especially good time to kill most types of broadleaf weeds.

After the first frost, the leaves drop from deciduous shrubs, annual flowers and similar plants. You, therefore, run less risk of injuring them by drifting spray.

Most seeds of broadleaf weeds have developed into young seedlings, so you can kill the young as well as the old weeds.

Apply liquid or dry weed killers on a day when there is little or no wind.

If you only have a few weeds, use spot treatments, not overall applications.

Combinations of broadleaf killers react slowly. Don’t be impatient.

As always, be sure to read and follow directions on the label.

If your weed problem is severe, apply the herbicide now, and sow grass seed two weeks later.

Don’t be surprised if the seed germinates slowly because of the cooler nights.

If seed has already been planted, wait to apply the weed killer until after the new grass has been cut twice.

“Growing Green” is contributed by Diane Dorn, Lehigh County Extension Office Staff, and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613