As temperatures drop the cells in a plant will freeze, blocking the channels that carry vital fluids and nutrients. As ice is formed the cell walls will dry out and eventually perish. The sharp crystals formed by frost can also damage the cell walls affecting isolated sections of the plant and causing foliage to die back. A fast thaw will also damage plants, often as a result of morning sunshine, rupturing the cell walls as they struggle to rehydrate quickly enough.
Even the hardiest of plants can be affected by extended spells of cold weather as their roots are unable to draw up water from the frozen ground. The plant can boost its hardiness in preparation for a cold period by altering its natural chemical composition. These chemicals are unavailable during the warmer, growing season, reducing hardiness and making the plant vulnerable to frost damage. This is why late frosts cause particular harm to fruit blossom and tender growth.
Signs of Damage.
Plants with frost damage are easily identified. Foliage is usually affected first, wilted and discoloured. The stem will gradually blacken and become distorted while the foliage eventually turns brown and crispy. The damage may affect parts of the plant or in some cases kill it off completely.
Prevention. It is good practice to choose plants suited to your climate and location, with a dependable degree of hardiness. When planting try to avoid frost pockets, where cold air has settled on lower ground usually at the foot of a bank or hollow. Plant young, tender plants in a sunny, south facing position to take advantage of milder conditions. Regularly mulch tender plants and evergreens to provide protection from ground frost and avoid high nitrogen feeds late on in the growing season that will promote frost vulnerable leafy growth. Delay the previous season’s growth from tender plants until Spring to protect any new growth.
Protection. If cold weather is forecast it is advisable to protect plants with a generous layer of horticultural fleece and move portable containers to a sheltered part of the garden. Tender perennials should be lifted and overwintered in the greenhouse and protected with extra insulation or the addition of a hot bed using a soil warming kit.
Autumn cuttings and young plants can be protected in a propagator, providing a warm, humid environment promoting healthy root development. If it is impractical to move plants indoors they can be protected in the garden with a cloche or mini-polytunnel. Fruit and strawberries can be protected from ground frost with a layer of straw or broken twigs.