Frost refers to the freezing of the liquid solutions within the plant cells as the night temperature drops close to zero, or below. Plants containing plentiful fluids within their leaves and stem structures are more prone to frost damage. On freezing and thawing, the plant cells burst and so destroy the plant. The following morning the plant will show signs of stress and may collapse and die completely.
Prevention is better than cure
Before planning your garden, check neighbourhood gardens during winter and see what is surviving the frosts.
Frost settles in the lower parts of the garden, so plant frost-hardy plants in these areas. Plant frost-tender plants on garden slopes as the cold night temperatures will roll down the slope without harming the plants.
Ensure your plants are ready for winter frosts. Stop fertilising in late summer and allow the foliage to mature. Plants stimulated by feeding in late summer will have tender foliage unable to withstand the winter cold.
Frost protection is simple
Protection can consist of tunnels of frost cloth placed over rows of plants, or simply a plant pot or upturned bucket popped over a single plant.
Larger plants that have a sturdy structure, such as a citrus, can have a blanket, a sheet or a piece of paper draped over them. Allow the material to drape down to the ground but don’t gather and tie it around the trunk. The cover traps the warmth rising upwards from the surrounding soil. Do not use plastic as it cannot breathe. Remove covers each morning so the plants can benefit from the air and sun during the day.
Frost cloth, now available at garden centres and hardware stores, is an exception. It can remain in place throughout the winter as its woven structure enables the plant to breathe and receive light during the day. It is lightweight and is sold by the metre.
There is also liquid frost cover than comes in a spray can. The wax-like liquid forms a waxy coating that will protect plants from freezing. It needs to be applied every four to six weeks. It is organic and non-toxic.
When your plant has been hit by frost
Do not prune away the frost-affected parts. These unsightly burnt leaves and twigs can provide a protective shield to the remainder of the plant during future frosts. Pruning may also result in the plant commencing new growths and these will be particularly vulnerable to further frost damage.
Once all danger of frosts has gone, prune away the frost-affected parts. The changing seasons show frosts can occur as late as spring or early summer. New growth affected at these times will quickly be replaced.
The benefits of a good frost?
A frost or two will quickly kill many garden pests that are hopeful of overwintering in your garden.
Hawke’s Bay and Central Otago are successful fruit-growing regions due to heavy frosts in winter. Frosts assist in the formation of fruit buds.
Many winter vegetables improve with a good frost or two. Underground crops such as parsnips, carrots and swedes increase the sugar in their cells as a means of preventing freezing. Winter cabbages, Brussels sprouts and broccoli do likewise. They increase the sugar in their leaves to ward off freezing. In doing so they become more flavoursome.
Types of frost
Frost can take many forms: hoar, rime, air, black and feathery. The three common frosts known to Kiwi gardeners are:
A ground frost occurs when the temperature of the ground drops below freezing. This results in water freezing and ice forming. The common sight of a lawn covered in frost is a ground frost.
This type of frost occurs between 1-1.5m above the ground. When the air temperature falls to below zero, then an air frost has occurred. Air and ground frost often occur simultaneously.
A frost without the icy crystals forming on the ground. It occurs when clouds arrive shortly before sunrise, once the formation of the ground frost is well underway.