Celebrate frost: magic and miracles

Savour sparkling frost in the garden… We explore the art and science behind this icy winter phenomenon

Crocuses in snow

by Liz Potter |

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.

Frosted rose
Frosted rose

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.

Tender plants need extra care when frost and snow are forecast. Frost occurs at 0-4C (32-39.2F) and in Britain, winter temperatures can dip to around -10C (14F) at night, sometimes even to -17.7C (0.1F) in the Scottish Highlands.

Plants that object to such extremes include dahlias, cannas and pelargoniums, bay trees, agapanthus and olive trees, tender fuchsias, bananas and tree ferns.

In cold, northerly and mountainous areas, it’s safest to bring tender plants under cover in winter. Lift dahlias and cannas from borders and store their tubers, labelled, in a frost-free place. Bring potted pelargoniums under cover too: trim them back by about half and keep them on a windowsill or in a conservatory.

If you can’t bring tender plants indoors, wrap their containers in bubble wrap to prevent the roots from freezing. Thick, lined and dark-coloured containers tend to have good insulation. In borders, insulate tender perennials such as penstemons with a layer of mulch, around 7.5-10cm (3-4in) deep. This is also a useful way to protect tubers and rhizomes that you aren’t able to lift. For bigger plants such as bananas, tree ferns and palms, protect their crowns by tying the fronds upright with twine, packing the centre with straw and wrapping the whole thing in fleece.

Keep an eye on the forecast in spring and be ready to cover fruit trees and new plants with fleece to protect delicate buds.

Protect tender plants from frost
Protect tender plants from frost
Fleece wigwams help protect tender plants in winter
Fleece wigwams help protect tender plants in winter

How to repair frost damage:

Black, brown or floppy leaves can indicate frost damage.

• Don’t prune off the damaged bits until all danger of frost has passed because it offers some protection.

• In late May or early June, prune back to a healthy bud or sideshoot.

• Some plants may look dead but can bounce back. Watch out for new shoots emerging from May.

• Plants may heave out of the ground, so firm them back after a hard frost.

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