Frost can be a misleading term, as there are varying degrees of frost ranging from light frost to hard killing frost, according to The Garden Helper. Hardy plants can withstand most degrees of frost, but delicate garden plants that are sensitive to frost can be damaged even by light frost.
Frost Sensitive Plants
Garden plants with tender fruit, vegetables or delicate stalks and leaves are the most susceptible to frost damage. Some of the most affected are strawberries, melons, cucumbers, squash, pumpkins and tomatoes. Any plant with a new blossom that is damaged by frost will not produce fruit from that blossom, according to the Ontario Ministry of Agriculture Food and Rural Affairs' article, “Freeze Protection Methods for Crops.” Potted plants of all types are weak to frost because their roots are relatively unprotected; the layer of soil surrounding them is not enough to insulate them when the temperature reaches below freezing. These should be moved indoors whenever possible if frost is forecasted.
How Frost Damages Plants
Frost actually damages plants in two stages. The first takes place at night, when temperatures dip to their lowest. Cold air sinks below warm air, so the lowest part of a garden is usually the coldest. Frost causes water within the plant's cells to freeze, the first half of the damage. The second step takes place when the sun rises and hits afflicted plants, defrosting them all at once and rupturing the cell walls.
Signs of Frost Damage
Frost damage is identified by blackened plants or distorted growth, especially if stalks become limp; leaves may even become translucent. Not all frost damage is immediately evident; sometimes plants with frost-damaged roots show damage a few days or weeks after the initial frost. These plants are unable to efficiently draw water through their roots and may eventually die from lack of moisture.
Avoid planting frost sensitive plants in the lowest areas of the garden; these areas become the coldest and should be reserved for hardier plants. In the early and late parts of the year when frost is around the corner, avoid applying nitrogen-rich fertilizer to frost sensitive plants. This type of fertilizer encourages new growth like leaves, which are tender and especially susceptible to frost damage. When there is a frost warning of any kind, ranging from light to hard killing frost, cover sensitive plants with old blankets or apply a thick layer of manure, old leaves or straw to the ground around the plant to prevent the soil itself from freezing, which will then prevent the plants' roots from freezing.