Many plants need to be protected from cold conditions. For some it’s a necessity – they won’t make it through a frosty winter without extra protection. Others will survive, but will grow much better with protection.
Plants that originate from mild climates are most at risk. New plantings – those that were planted in the previous summer/autumn – are also vulnerable to cold damage.
HOW AND WHY FROST DAMAGE OCCURS
On still winter nights, cold air sinks to the ground and drains and settles on low-lying ground. This is where the heaviest frosts occur.
Frost causes changes to plant cells by freezing the water within a plant. Because water expands when it freezes, plant cells change in shape and burst. The damage quickly shows up as blackened, withered and drooping leaves and stems.
HOW TO MINIMISE DAMAGE FROM FROSTS AND WIND
If you live in an area that suffers from heavy frosts and/or severe cold winds:
- Choose hardy frost-resistant plants – conifers and deciduous plants are reliable in cold conditions. There are also many suitable evergreen plants originating from Tasmania and Victoria, and the colder areas in New Zealand, North America, Europe, etc.
- Plant only after the last frosts have finished in mid spring. Don’t put in any new plants after mid summer.
- Don’t fertilise frost-tender plants in autumn – this encourages tender new growth that is vulnerable to frost and wind damage.
- Plant frost-tender plants in a sheltered area of the garden or halfway up a slope (which is less prone to frosting).
- Create windbreaks – with fencing or plants.
- Use plant guards throughout winter.
- Move frost-tender pot plants into a shadehouse or glasshouse over winter.
- Establish a framework of hardy trees that will protect frost-tender plants (but don’t plant so heavily that the trees create cold, heavy shade throughout winter).
If plants are burnt by frost, don’t cut off the damaged parts. This could encourage new shoots to grow, which are highly vulnerable to late frosts. Wait until mid spring to prune the burnt leaves and stems.
Plant guards are invaluable for protecting small and newly planted shrubs and trees in frost-prone areas. Some of the options are:
- plastic tubes or sleeves – these are placed over the plant and held in place by stakes. Commercial plastic tubes are stabilised against UV rays so can be re-used for a number of years.
- hessian – bags or cloth are held in place by stakes. Hessian can also be used to make a temporary canopy which is placed over the plant on frosty nights.
- milk cartons, plastic soft drink bottles and short lengths of plastic pipes – these are useful for protecting seedlings against wind and frost.
- aluminium foil – this is sometimes used for protecting the stems of young fruit trees. It is also useful for protecting against grazing stock and rabbits. The foil must be removed when the frosts have finished as it keeps the stem damp, increasing the risk of fungal infection.
- shade cloth, newspaper or any other material that can be thrown over the plant before the frost settles is useful as an emergency measure.
If a plant is hit overnight by frost, water the foliage gently and thoroughly before the sun reaches the plant. This minimises the damage that occurs as the plant rapidly thaws.
Some mulches work better than others at insulating the ground. Gravels, stones and even aluminium foil are good – they trap the heat from the sun during the day and raise the soil and air temperature around the plant at night.
Organic mulches such as straw and bark chips attract frost and should not be used around frost tender plants during winter.
Buildings and structures
Any structure which protects the plant from cold is useful. Pot plants can be moved into a greenhouse or shadehouse – take care though not to move the plants back outside too early the following season. They need to be gradually acclimatised to the drier, colder conditions outdoors.
A cold frame or cloche can be placed over vegetables and seedlings. This acts as a mini-greenhouse, and is particularly useful for getting summer seedlings off to a headstart in late winter/early spring.
- Greenhouses are not just useful for magnifying the effects of the sun in the summer, but they are invaluable for providing winter frost protection to many tender plants
- Organic mulches such as woodchips and manure are fine in warmer, sheltered spots, but should be avoided where there is a risk of frost
- Corrugated plastic sheeting will provide protection to plants that are already established in garden beds
- Hessian, sackcloth and other materials can be used to spread over vulnerable plants to give them protection from the cold or from frost
- A cold frame or cloche acts as a mini greenhouse when placed over tender seedlings
- Stone mulches trap heat from the sun during the day and are therefore ideal for protection from frost
- Walls, fences, trees, and hedges can all be included in the garden design as ways of screening plants from frost