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Snow
Snow protects plants creating a duvet-like cover that shields them from very low temperatures. Unfortunately heavy snow can break branches and literally flatten plants. Many conifers from snowy climates have a cone shape to protect branches from breaking, these collect less snow and allow it to easily slide onto the ground.
Snow that lies for a long time can prevent plants from photosynthesising and, when the snow finally melts, leaves will have yellowed. In areas with high snowfall many plants typically adapt by dying back in winter.
Frost
Frost is the winter killer cold climate gardeners are all too familiar with.
Frost occurs when the temperature drops below 0oC (32oF) and thin ice crystals form on the ground or other surfaces. The temperature can vary between ground level and the air, and soil may not freeze as quickly as moisture lying on its surface. At -5oC (24oF) the soil freezes. The depth of frozen soil depends on the length and severity of freezing, as well as soil type and moisture content.
Spectacular hoar frosts occur when air moisture levels are high and air temperatures fall below freezing, causing air moisture to freeze, dangling icicles from trees, fences and buildings.
Freezing and Plant Damage
When it freezes, the moisture inside plants begins to freeze. Water is forced by osmosis through the membranes and into the fluid sacs. This transfer dries out the plant's protoplasm results in dehydration, the cause of most damage. A severe frost can rupture the plant membranes, and more damage occurs.
If the ice melts within the fluid sacs before the sun warms the plant and daily activity begins, plant tissue can escape damage. But if the sun warms the plant while the fluid is still frozen, then damage results from dehydration.
An early or sudden frost catches plants before they can develop winter hardiness and have little ability to withstand cold temperatures. Mild weather in midwinter is also destructuve as it destroys the resistance to cold.
New spring growth is not cold hardy. Spring frosts are devastating, destroying soft new shoots that have little or no frost hardiness.
Soil
Soil moisture is a major factor in plant hardiness. Why? Wet soils are colder than drier soils and the roots freeze in the ground. Many alpine plants survive cold as they grow in rocky screes that drain freely. In a garden loam, with higher moisture levels, these same plants succumb to freezing soil temperatures.
Many plants can cope with severe cold in well-drained soils. In clay or other soils with high moisture content these plants may succumb. These heavier, wetter soils are also slower to warm in spring, and plants will come again and seeds germinate earlier in a well-drained soil.
Beating the cold
Plants are survivors, adapting to ensure survival from one season to next. Gardeners can work with plants, using their ingenious strategies for surviving all that winter can throw them.
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Snow can protect - and break - plants
Frost rimes leaves in winter
Hoar Frost
 Hoar frost, or white frost, forms when air with a dew point below freezing is brought to saturation by cooling. It is a deposit of interlocking ice crystals formed directly from vapour, without first becoming liquid.
Hoar frost forms on objects usually of small diameter freely exposed to the air, such as tree branches. The temperature of the be-frosted object must be below freezing. |
A rhododendron 'burnt' by frost
Plants survive severe cold in free-draining soils
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Winter Care in the Garden
 If you garden in a cold area or one that experiences several hard frosts in winter you need to prepare the garden for winter. |
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 More Garden Botany
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