protection is less wherever the cover touches the plant. Any material can be used to cover the plants, however woven fabrics are better insulators then plastics or paper. The best time to apply covers is in the late afternoon after the wind has died down. Remove covers the next morning before the sun hits them.
Irrigate during the potential frost
Many people claim that watering the frost off plants prevents frost damage. This is partially true.As mentioned above, watering plants helps raise their temperature and the air around them to that of the water. In addition, as water freezes, heat is released; 80 calories for each gram of water that freezes. Therefore,watering plants before they are injured from frost can help keep their internal temperature above freezing. A single application at the coldest part of the night (generally just before sunrise) may be enough on 30- to 32-degree F nights. On colder nights it may be necessary to apply overhead irrigation for an extended period of time, allowing actual ice formation on the plants. In this case, irrigation must continue until morning temperatures rise above 32 F and the ice melts. It is important to note that once frost damage occurs, watering does not help.
If this is cold enough to break cell walls or disrupt cell constituents beyond repair, damage, wilting, and
death will occur in affected tissue
How much cold will kill a plant?
Some plants can survive sub-freezing temperatures for months. while others cannot take temperatures below 50 F. (10 C.) more than two or three hours.
Damage to plant tissue can be detrimental to plants.
Light frost typically will not cause major damage, unless it is a very tender plants, A really hard frost, will freeze water in plant cells, causing dehydration and damage to cell walls. Cold injury can then occur when the sun comes up, because the plant defrosts too quickly, killing leaves and stems.
A good read on frost from Cornell