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Plants have a number of defences for cold, adapting in ways that enable them to survive from season to season. Understanding these helps us to carry our plants through the cold months.
Most plants are not hardy to cold year round. Hardiness develops as a reaction to shorter days, decreasing light levels and falling temperatures, as summer becomes autumn and then winter.
Winters are never quite the same and plants cope with the changeable seasons with greater and lesser degrees of success. The rate of cooling, and the length of a winter, wind and soil types will all affect plants.
Cold Weather Strategies
Many plants protect themselves from cold by dropping their leaves and reducing activity, and thus moisture, to very low levels. Other plants hide out - herbaceous perennials die back to blow ground, bulbs disappear underground and annuals lie dormant as seeds. These plants start into growth and emerge again when temperatures rise.
The layer of fallen leaves under trees, shrubs and from herbaceous plants is nature's mulch, a covering that modifies soil temperature changes and helps to protect over wintering plants beneath its protective covering.
Some plants cope with cold by literally hugging the ground. Celmisias, our mountain daisies, have a low, mounding form that copes with heavy snow. When there is no snow cover, this shapes protects them from cold, desiccating winter winds.
Many mountain and cold climate plants are evergreen, as the short growing season cannot be wasted on producing new leaves each year.
Most plants cover their apical bud, the growth point, through the colder months. Bud scales, actually modified leaves adapted to survive periods of cold and dehydration, cover this vital growing point.
Developing Hardiness
Plants that need to develop hardiness to survive above ground develop 'ripe' wood or tissues to withstand frost. These are tougher and have a lower moisture content, enabling the plant to cope with winter temperatures.
The early onset of winter or an early frost severely affects plants as their tissues are 'soft' and have a high moisture content. Plants that naturally do not develop ripe wood will be more affected by.
Herbaceous perennials, bulbs and other plants that over winter below ground may well re-emerge even if their top growth is blackened by an early frost.
Wind
Plants can suffer damage from dehydration if exposed to cold, drying winter winds. Broadleaf trees tend to be deciduous, dropping their leaves to avoid winter's desiccating winds.
Evergreen trees from very cold climates tend to have thin, needle like leaves that have little surface area to naturally reduce transpiration and dehydration caused by wind.
Many NZ native alpine shrubs have small leaves with providing only a limited area for transpiration, and grow in mounding shapes to minimize moisture loss in winter winds. Evergreen broadleaf trees are likely to come from mild climates.
Prostate and ground hugging plants lie low to the ground, taking advantage of natural contours and hollows, clinging in rock crevices to that deflect or lie out of drying winds. Many of these plants will have leaf shapes or forms, even tiny hairs, that minimize exposure to wind.
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Cold Weather Strategies for Winter
Dormancy and a Natural Anti-freeze
 In low temperatures plant metabolisms slow down to a virtual standstill, slowing chemical activity.
As the plant reduces moisture content and stores sugars to provide for growth in the coming season, the concentration of level of sugars increases. This forms a natural anti-freeze and protects the plant from moisture within cells freezing, rupturing membranes and killing that part of the plant. |
Winter protection from leaf loss
See Autumn Leaf Changes
Nature's Mulch, a layer of fallen leaves
Celmisias hug the ground to avoid snow damage and drying wind
Hebe topiara - small leaves, rounded shape reduce moisture loss
Thin, needle leaves reduce dehydration | |
