Keep Your Plants Alive in Winter
Winter is a time when the garden "sleeps".
Plant growth slows or stops altogether.
Hardy plants have evolved various methods to be able to cope with the cold.
These will grow where there are frost, but may still require a bit of protection, especially when young.
More tender plants will require more protection in one form or another.
Hardy plants employ a variety of ways to cope with the cold.
Tender organs, such as leaves and twigs, may be shed.
Certain plants produce anti-freeze that stops their sap from freezing.
Others cope by burying storage organs (such as roots, tubers and bulbs) underground.
The soil protects them from the cold.
Plants have different degrees of hardiness.
Those that come from colder areas will be able to withstand lower temperatures.
The USDA defines a number of "hardiness zones" based on the minimum winter temperature and range from 1 for artic conditions to 11 for sub-tropical climate.
New Zealand Zones
||-10 to -7
||-7 to -4
||-4 to -1
||-1 to 2
||2 to 4
zones 8b to 11
New Zealand has a mild and more moderate climate, and only zones 8b through 11 apply to us.
The far North is zone 11, and Northland, Auckland and the top of the Coromandel Peninsula fall into zone 10b.
The rest of coastal North Island, the top of the South Island and Westport are in zone 10a.
The other coastal areas are zone 9a and 9b, and inland and high country areas are mostly zone 8b.
The sparsely populated high alpine areas are not shown in the map but are simply lumpped into zone 8b.
These zomes are only approximate and your local microclimate will need to be taken into account.
What the zones show is most places in New Zealand will have frost in the winter.
New Zealand is surrounded by ocean and it has a moderating influence on the climate.
Most areas do not experience extremely cold winters or extremely hot summers.
However, our weather is very changeable and it is not uncommon for it to be very warm and then to suddenly turn cold.
Parts of the country also receive a lot of rain, in particular the west coast of the South Island and Fiordland.
||11 and 10b
||10a and 9b
||8b and 9a
These USDA hardiness scale needs to be read in the context of New Zealand conditions.
Quite simply, most of the zones do not apply to us because our winters are not as severe.
Our winters are typically cool and wet.
Plants that may be accustomed to cold dry winters may not like the wet feet.
Our changeable weather means we do not have long periods where the soil is permanently frozen.
Except for the high country and alpine areas, it is perhaps easier to think of New Zealand hardiness zones as:
- Warmer areas - tend to be frost free or only get very light frost several days in the year,
- Cooler areas - will get moderate frosts for most days in the winter,
- Cold areas - can experience more severe and prolonged frosts
Picking Suitable Plants
The best plants for your garden are those that match your hardiness zone.
Overseas web sites and foreign literature normally express plant hardiness as a range corresponding to the USDA hardiness zones.
So for example, a plant that is quoted as suitable for zones 9-10 means it will grow and possibly even flower and fruit in these zones.
If grown in zone 8, the plant will need protection or it may not grow at all.
If grown in zone 11, the plant may grow but not flower or fruit satisfactorily, or a deciduous plant may fail to lose its leaves.
You can use the above map and hardiness chart to loosely translate these USDA zones to pick plants suitable for your garden.
Plant hardiness on this web site is not expressed in terms of USDA climate zones.
Instead, plants are labelled as either warm, tender, half hardy or fully hardy:
||Hardy plants will grow in warmer, cooler and cold areas but may not flower or fruit satisfactorily in warmer areas.
||Half hardy plants will grow in warmer areas but will require some protection from the frost. More protection will be needed in cooler areas and may be grown in an unheated greenhouse or conservatory in cold areas.
||Tender plants may be grown outdoors during the summer but must be kept away from the frost. May be grown in an unheated greenhouse in warmer areas. In colder areas, the greenhouse will need some heating in the winter to keep frosts away.
||Plants from tropical origin that are best grown as indoor plants or in a heated greenhouse / conservatory.
Simple horticultural practises can extend the range of plants that will grow in your garden without having to invest heavily in protection.
A thick layer of mulch is all that is needed to protect more tender roots and other underground organs from frost damage.
The mulch can also come from fallen leaves.
These will breakdown slowly and improve the soil.
New shoots are less hardy than mature growth.
In the autumn, cease fertilising and reduce watering.
This allows plant growth to harden in preparation for winter.
Some plants benefit from an application of Calcium Nitrate.
The extra calcium hardens up the plant tissues, making them less susceptable to frost damage.
In areas where late frosts are common, it is best to delay pruning until the spring.
Trees and shrubs that are prunned early may start to grow too early.
Late frost can burn to burn their new growth.
Furthermore, in areas with very cold winters, the unpruned branches of trees and shrubs can provide an additional level of protection for plants growing below them.
Growing less hardy plants under trees and shrubs is a particularly good way of providing shelter.
In warmer areas, often this is all that is required to grow tender sub-tropical plants.
In cooler areas, the extra protection can often mean the difference between survival and death.
Apart from trees, growing close to the shed, garage or house, in particular under the eaves, can also impart protection.
Plants growing under eaves will need to be watered more often as they are sheltered from the rain.
Buildings, walls, fences and hedges can shelter plants from cold southerly winds.
A north facing brick wall will absorb warmth from the sun and release it at night.
However, it can get too hot in the summer for certain plants.
A cloche allows tomatoes
to be started earlier.
A water cloche is a transparent plastic device that is placed around one or more plants that need protection.
It that holds an amount of water.
As a frost is settling down, the water inside the cloche will start to freeze.
Freezing water releases latent heat, and this protects the plant from freezing.
A cloche is only effective againsts light frosts.
Make sure there is room for the water in the cloche to expand or it will burst.
A frost cloth works by trapping warm air and blocking clond air from reaching the plant.
It is only effective if the whole plant is covered and there is an air gap between plant and cloth.
Heavy frosts can still penetrate the frost cloth.
Cover lemon trees with frost cloth to prevent the new shoots from being burnt by the frost.
Lemon and other citrus fruit on new growth so it is essential that these are protected or the plant will not bear fruit.
Frost cloth can also be used to protect greenhouses.
Tropical plants in the Auckland Domain Winter Garden warm house.
The ultimate in protection is a greenhouse or covered conservatory.
Even an unheated greenhouse will provide several degrees of frost hardiness.
A small greenhouse with an electric heater with the thermostat set on frost watch will be sufficient even in colder areas.
Pots raised on a bench will be warmer than pots sitting on the ground.
Cool air pools on the ground and it can be several degrees warmer just one metre from the ground.
With a heated greenhouse, a wide range of tropical plants can be grown, although in very southern areas, the lack of light in the winter becomes an issue.
The problem with greenhouses is temperature regulation.
Day time temperatures tend to be too high in the summer and night time temperatures too low in the winter.
A larger and taller greenhouse will have better temperature regulation, so will a greenhouse that has good insulating materials.
Greenhouse insulation has traditionally been horticulture glass.
This is a thin glass that is very prone to breakage.
Modern greenhouses are also insulated with plastic films.
The best insulation comes from having two or more skins, eg. double glazing.
These trap a layer of air between the skins, providing superior insulation.
Vents, fans, shade cloth and paint, humidifiers and foggers can be used to help keep summer temperatures down.
Growing in the basement is not very popular in New Zealand.
We like to think that we have the space and sunlight to garden outdoors, but for those living in very cold areas, this is a viable alternative.
There is a higher setup cost in terms of procuring equipment such as lights, fans and heaters but once setup, it can provide virtually any growing environment.
In Canada, parts of the US and Europe, this is the only way to successfully grow tender plants.
Deciduous plants can be
grown in pots and over-
wintered in the garage.
Containers open up a whole new dimension of plants to gardeners.
Plants can be displayed when in flower and shifted out of the way when the flowers fade, especially those with untidy foliage.
Those who do not have a lot of room will be able to grow more plants if they are in pots.
Deciduous plants can be grown outdoors in the summer and tucked away in the garage to over-winter.
Containers culture allows plants to be grown indoors.
The warm temperature that we enjoy in our homes is also suitable for tropical plants.
While the traditional indoor plants tend to be foliage plants such as begonias and bromeliads, there is no stopping the gardener from growing other exotics such as orchids and gingers.
Containers also allow plants to be kept on the drier side during winter.
Often plants are killed not solely from the cold but a combination of cold and wet feet.
Growing in pots allows the gardener more precise control over watering and feeding.
Growing in containers does have its down side.
Pots tend to dry up rather quickly and will require more frequent watering.
Frequent watering tend to leach out fertilisers, meaning potted plants will require more frequent fertilising.
Pots can be sunken into the soil during the summer months as a way to keep them moist.
A mulch can also be used to stop the surface of the potting mix from drying out too fast, and keeping the roots cool over the summer.
As simple as it may sound, growing annuals is a good way to avoid winter damage.
Annuals grow during the warmer months and will die either before or on the onset of winter.
The plants survive as seeds and are renewed every year.
See also: Saving Seeds for the Home Gardener.
Certain tender perennials such as Chillies and Peppers can be grown as annuals.
The only criteria are the perennial must be quick growing and will flower or fruit in their first year.