The risk of spring frost in any area determines the date after which it is safe to sow or plant out tender plants and half hardy bedding plants. You always find notes on tender plants labels that say that they can be planted out after all risks of frost have passed. The onset of the autumn frost determines the end of their growing season. If you want to keep your tender plants from one year to another then you should take them indoors or give them the adequate protection over the cold weather.
Frost pockets and frost damage
Where dense, cold air flows downwards, pools of frost may collect. This way, any valley or hollow is therefore a potential frost pocket. Cold air accumulates in the depression, increasing the area of potential damage as it backs up the sloping sides of the valley. To avoid formation of frost pockets in front of a barrier formed by established trees or shrubs along a hillside you need to thin or remove the trees or hedges to allow cold air to flow through down the slope.
When the ground is frozen, water is no longer available to the roots of plants. Deep-rooted trees are not affected by heavy frosts because their roots penetrate the ground well below the frost-line. Plants with shallow roots, specially evergreen ones, may be affected by ground frosts, because they will no longer be able to replace the moisture lost by continuing transpiration. Severe ground frosts often also cause newly planted, young or shallow-rooted plants to rise or lift out of the soil, and when this happen you should be carefully re-firm the soil around them as soon as the thaw begins.
Freezing and thawing
The frost itself does not cause such great damage to the plants as it does the alternate of frost with thaw, this may be more devastating. Cell sap expands on freezing and destroy the plants cell walls, causing death to tender plants. This also damage the flowers, shoots, buds and leaves of hardier plants. It may even harm the roots sometimes or cause the bark of some woody plants to split open.
Repeated hard freezes followed by rapid thaws and subsequent waterlogging of the soil causes great damage to plants roots. Late frosts in spring are particularly damaging to new top-growth, causing blackening of the leaves and injury to the new buds and flowers.
Duration of frosts
The duration of frosts is also important to the amount of damage caused to the plants. If the temperatures below freezing level, for example -3 Celsius degrees (27 F), are maintained for a short period, lets say a quarter of an hour, it might cause no damage, while when the same temperature is maintained over a longer period, lets say few hours, the result could cause substantial losses.
The risk of frost damage to plants that are kept permanently outside in the garden may be reduced by various methods and for tender plants grown in pots is important to bring them indoors before early autumn frosts are forecasted.
Despite the danger to plants, frost may be helpful sometime in cultivation. Soil water expands when it freezes, shattering clods into much smaller soil particles and this is particularly useful on clay soils, in aiding the development of tilth. Low soil temperatures also reduce the numbers of some soil-inhabiting pests.