Jan 17, 2013
Today's weather is a stark contrast to summer's sweltering days and warm evenings. Winter is here with cooler days and cold evenings- cold enough to cause frost. Frost occurs when the ground temperature goes below the freezing point (32F).
Frost damage in plants occurs when the liquid inside the individual cells freezes and forms ice crystals. The ice crystals then rupture the cell walls draining the fluid. The leaves turn limp, dry out, and turn brown and black resembling a burned appearance. This is often mistaken as a lack of water. While the leaves show damage, the buds and stem may not until a later point in time. Some plants lose their foliage and are presumed dead, but this is not always the case.
Plant survival depends on a few key factors including: stage of growth and development, age, species, overall health and water content. Recently planted, young plants are more sensitive to frost than older, established material.
There are 2 types of frost:
1. Advective frost occurs when a cold front moves into an area. The wind is usually gusty, clouds may occur, and the cold air layer may be more than a mile high.
2. Radiation frost occurs when the winds are calm and the sky is clear. During these conditions the radiant heat from the Earth rises to the upper layers in the atmosphere. A cloud cover at night traps the radiant heat from the ground and if there's any wind, it mixes and forms a uniform temperature. With the lack of wind air does not mix and an inversion layer develops where the air is opposite of daytime conditions and the temperature of air decreases with height. This inversion layer collects cold air near the ground while warmer air is trapped above this cold layer.
Cool air is heavier than warm air and collects at the bottom of slopes like retention basins. Pockets of frost form in these valleys where this cool air is trapped.
Plant Damage Prevention
The very first step begins with choosing frost tolerant plants and appropriate plant material for the climate zone. Frost sensitive plant materials should be planted in sheltered locations and in the warmest locations - western and southern exposures.
Plant material can be covered with woven fabric or paper. Plants should be completely covered from the top all the way to the ground avoiding any openings where heat can escape. This provides 2 - 5°F protection which may be just enough to prevent damage. For best results, covers should be supported on stakes avoiding contact with plants and should be removed during the day when temperatures have increased. Large developments and businesses may consider covering focal points such as the main entry and high traffic areas.
Irrigating before the frost can also help the soil retain four times more heat than soil that is dry. It also conducts heat to the soil surface quicker than dry soil. Plants in moveable containers should be sheltered under a tree canopy or covered area.
Pruning of the damaged growth should be postponed until the threat of frost has ended; dead foliage protects the rest of the plant from additional damage from the cold by insulating the plant with a pocket of empty air space and aids in a quicker recovery. Many plants rejuvenate and recover, but this takes time and patience.