Frosts May Start to Bite

By the time we get to November most places have usually experienced a frost or two. Certainly enough to crisp up the ground, throw a layer of ice across the car windows and make us wake up and walk just that bit faster in the morning.

However such events usually do not last more than a few hours and the ground has soon warmed up enough to dissipate the frost, either because of its relatively high soil surface temperature or radiant heat from the sun. Sometimes however, if we get a longer lasting cold spell the ground will freeze enough for it to stay below freezing all day, especially if there is no sunshine or in places that don't see any weak winter sunshine, such as north facing slopes.

Just how deep does the soil usually freeze though? Many will think of the popular Christmas Carol "In the bleak Mid winter" (and we will return to seasonal weather as portrayed in Christmas Carols next month). The first verse, written by Christina Rosetti in 1872 says

In the bleak mid-winter,
Frosty wind made moan,
Earth stood hard as iron,
Water like a stone,
Snow had fallen, snow on snow,
Snow on snow,
In the bleak mid-winter, Long ago..

...which always conjured up images for me of that 'iron like' ground you seldom get, when the temperature has been below freezing by night and day for some length of time. In fact the carol is not technically accurate, in that it suggests that a lot of snow had fallen and this tends to mitigate against exceptional soil freezing acting as an insulating blanket. In such conditions we generally need not be so concerned for tender plants. However, it is usually the very dry and cold set ups, with an anitcyclone aloft on the nearby continent or across the British Isles, especially in conditions where high it is to the north of us, allowing a high wind chill factor as well, with a dry period beforehand and relatively little soil moisture, that generally do the most damage at root level.

In the larger continental interiors for instance, such as the United States, the deepest soil freezing levels are found across the Dakotas, where persistent sub freezing winter temperatures, and relatively little soil moisture and snow cover combine to maximize soil freezing, whilst ample winter snow cover mitigates against soil freezing extremes in areas like the the Great Lakes, northern New England, and the western mountains. Similar conditions occur in the interior of continental Europe such as on the relatively dry Russian steppes. Generally the areas most at risk in the UK are the 'plains' of Lincolnshire and East Anglia and parts of the interior of central England and east Wales.

Although we may blame severe cold weather for the loss of garden plants in winter, it is usually a combination of the persistent low temperature coupled with chilling winds. This tends to especially effect evergreens as generally most deciduous plants are dormant by midwinter, although for them severe November cold can be a problem. The key problem with evergreens is that they lose a lot of water through their leaves throughout the winter and a strong drying wind will only serve to increase the rate of transpiration. The result will be 'scorched' foliage if the plant cannot draw sufficient water up through its roots when the ground is frozen. A sign there's a problem will come when the tree starts to shed needles or leaves but if you can give it extra shelter or wrap the plant with (fine) mesh netting during such periods, it will certainly help it survive unscathed.

Frost can lift the soil and loosen roots, causing exposed stems to die back as well as split the bark on the limbs and trunks of trees. Of course, all non hardy plants need to be protected as best you can from frost, if you can take them into a greenhouse you can protect them by using fleece or other insulating material. Even a leaf blanket around them will help. Plants that have particular problems with frost include camellias, magnolias, pieris and hydrangeas.

In terms of planting trees and shrubs, if you live in the north of the UK or in an upland area it is generally better to plant new tree and shrubs during the spring when temperatures are likely to remain above freezing. However, for those living near to the south and west coasts where winters are milder you can probably get away with autumnal planting. No one year is the same though, and if we have a particular severe winter this can cause problems even in the most normally temperate regions. In the severe winter of 1962-63 for instance, even the sub tropical Scilly Isles saw nights well below freezing at times, though the ground never really froze as such there, and with a snow cover as well this actually helped protect most plants.