Frost and freeze are not the same, and generally do not happen at the same time. Frost and freeze are due to the moisture in the air and the surface air temperature.
You can see a frost. They are the white layer of icicles. Your car is covered with a thin layer of ice crystals, during freezing cold, but not below freezing point.
You do not generally see a freeze. Freeze occurs when the temperature in the air is below the freezing point. There may or may not also be a frost.
A frost can occur when the surface air temperature falls below 36 degrees Fahrenheit, whereas a freeze occurs when the surface air temperature reaches 32 degrees Fahrenheit or lower. Frost depends on the moisture in the air. Frost won’t happen if it’s windy or cloudy, or when it is raining.
When temperatures fall and frost or freezing occurs, water in the plant cells turn to ice crystals and expand, causing the cell walls to burst. Leaves and new growth tend to be the first affected.
Cold-hardy vegetables can survive light frosts and freezes since they start to produce sugars when the temperature drops. Sugar water freezes at a lower temperature than water, so the water in the plant cells does not freeze and no cell damage is caused. Sugar defense in cold weather is why frost-tolerant vegetables improve their flavor after a freeze or two.
Water plants a few days before the temperature drops to keep the soil is moist. Damp soil holds the heat better and as the water in the soil cools at night it releases the heat and protects the plant. A jug of water beside each plant can give off more heat as the temperature drops.
Mulch has many benefits but the main reason to mulch in winter is to protect from freezes. Wood chips, straw, fallen leaves, various barks and pine needles are examples of suitable winter mulch materials. Spread a layer about 2 to 3 inches thick leaving a space around the base of the plant.
Semi-hardy plants will need some kind of cover, as will hardy vegetables when temperatures drop below 28 degrees Fahrenheit. The simplest and easiest way to protect your plants from frost is using old sheets, fabric tabletops, towels and plastic. Floating row covers, garden fabric and plant covers made of non-woven materials such as polypropylene or polyester.
