Frost: magic & miracles
Savour sparkling frost in the garden… We explore the art and science behind this icy winter phenomenon
Frost is a magical sight on a winter’s morning, arriving silently overnight to entomb our gardens in its icy grip. The sparkling crystals provide a welcome contrast to leaden skies and dark soil, decorating trees and tracing the outline of leaves, hedges and seedheads.
Frost occurs when water vapour freezes and ice crystals form, and comes in several types. Air frost forms when the air temperature is below zero but the ground is still warm, which is why it’s most common in autumn. It’s not likely to kill plants, but might damage foliage. Ground frost is more dangerous for plants, and occurs when the ground temperature falls below zero.
The sharp crystals you see on trees and fences are often described as feathery or hair-like. This is hoar frost, named because hoar means ‘aged’ and the frost looks a bit like a white beard. These crystals grow when water vapour in the air meets surfaces that are below freezing and it keeps growing as more water vapour freezes, which is why the results can look so striking.
Low-lying dips and hollows, or the bottom of a slope, can become ‘frost pockets’ where cold air gathers. This is because cold air travels downhill as warm air rises. It’s a good idea to avoid putting a screen or hedge across a slope because its shadow allows cold, damp air to linger. To improve conditions for growing, dig organic matter into the soil – this helps drainage and prevents waterlogging, which compounds plant vulnerability. In spring, add a layer of mulch for extra insulation.
East-facing spots are also vulnerable to frost. Here, as the sun rises it thaws frosted plants too quickly and the rapid defrosting can damage flower buds on delicate camellias and magnolias. When planting in these more vulnerable areas, look for native or hardy plants that can shake off our winter weather. These include viburnums, sedums and geraniums such as ‘Rozanne’, which can all cope with cold winters. Golden Hakonechloa macra, black Ophiopogon planiscapus nigrescens, slender green yew, Taxus baccata ‘Fastigiata Robusta’, and red Cornus alba ‘Aurea’ stems make a dramatic contrast rimed in frost
Frost occurs when water vapour freezes and ice crystals form