Frost Damaged Plants: What to Do the Morning After

• Frost
• Outdoor
• Problems
• Winter

Photo by DominikRh on Pixabay

Leaves blackened by frost look dead but the plant often is not. Here is what to do today, what to leave alone, and how to tell when to give up.

What frost does to a plant

Frost freezes the water inside leaves and stems. Ice crystals form between and inside the cells, the cells burst, and when the plant thaws they leak. That is why a plant can look fine at dawn and sag within a few hours of the sun reaching it.

The first sign is usually leaves that look dark and wet. The RHS describes damaged foliage as water-soaked and dark green, turning black in time. On tender new growth, the same page says to look for scorching and pale brown patches between the leaf veins. Flowers brown and drop. What you see is the damage at the surface. The roots and the lower buds of most established plants are insulated by soil, so they came through. Whether the plant survives depends on how hardy it is, how hard the frost was and how long the cold lasted, not on how bad the leaves look this morning.

If the frost caught a pot you meant to bring in, our guide to bringing houseplants back indoors covers how to settle one in once you have rescued it.

What to do this morning

You have a few jobs, and all of them are about slowing things down.

1. 1

   Let it thaw slowly

   The RHS warns that plants in early morning sun can thaw too fast, damaging flowers and young growth. If you can, shade frosted plants with a sheet, or move potted ones out of the sun until the air has warmed.

2. 2

   Move pots somewhere sheltered

   A garage, porch, shed with a window or the lee of a house wall. A frozen pot cannot be watered, and roots in frozen compost are in more danger than the leaves. Group the pots together so they shelter each other.

3. 3

   Leave the damaged growth on

   It looks awful, and it is also a layer of insulation over the buds beneath it. Cut it off now and the next frost reaches tissue that was protected. Resist the urge to tidy.

4. 4

   Check the soil, then water only if it is dry

   Push a finger in. If the compost is frozen, leave it. If it has thawed and is dry below the surface, water lightly. A frost-damaged plant takes up very little, and soggy compost makes things worse.

5. 5

   Do not feed

   Feeding pushes soft new growth that the next cold night will kill. Wait until the plant is making fresh leaves of its own accord.

How to tell whether it is dead

Be patient, because recovery can be slow. The RHS says recovery may not be seen until early summer, which is around June in the northern hemisphere and December in Australia, New Zealand and South Africa. If no regrowth has appeared by then, the same page says to replace the plant.

While you wait, test it. Scratch a small strip of bark from a stem with a fingernail or a knife. Green and moist underneath means the stem is alive. Brown and dry means that stem is dead, so try one further down. Work your way towards the base, because the tips die first and the roots are the last to go.

Look at the buds too. Plump and firm buds on a woody plant are a good sign. Shrivelled, papery ones are not.

Some plants give you the answer sooner:

• Dahlias and cannas. Blackened tops look like the end. The RHS says that on these, the roots are alive and can still be protected or lifted and stored. Cut the foliage down once it has collapsed and mulch the crowns thickly or lift the tubers.
• Basil, courgettes (zucchini), tomatoes and beans. Finished. They have no frost tolerance, and the plant will not come back from a black, collapsed stem.
• Hardy perennials. Leaves flop and brown, but the crown below ground is fine. Cut back in spring.
• Evergreen shrubs. Brown, scorched foliage can drop in the weeks that follow. Most reshoot from the stems.

The damage that shows up late

Frost does not always show its hand in a day. Woody plants in particular can look unharmed for weeks and then drop their leaves, or leaf out in spring only to have one side of the plant stay bare. Newly planted shrubs have another problem. The RHS notes that frost can lift them out of the ground, so after a hard night, look at the base and re-firm the soil around any that have risen.

This is also where it pays to know what you are looking at. Browning, wilting and leaf drop have a long list of causes, and frost is only one. Overwatering, drought, root problems and some diseases all produce the same dead-looking foliage, and treating the wrong one wastes the plant's chances. If the pattern does not fit what you saw the night before, for example browning that is patchy rather than on the exposed top of the plant, look for something else too. Our guide to saving a dying houseplant works through the checks for a plant that is declining for no obvious reason.

What to do in spring

Once the days warm and the risk has passed, the sequence is easy.

1. Wait for new growth to show where it will. Buds break first from the live wood, and that tells you where to cut.
2. Prune dead material back to an undamaged sideshoot or bud, using clean sharp secateurs (pruners) and cutting just above the bud.
3. Water as the weather dries out, and feed once the plant is clearly growing.
4. Give a weak plant a season to recover. Do not heavily prune a shrub that is already stressed.

Tender plants you lost can be replaced with something hardy for the spot, or kept under cover next year. Geraniums (pelargoniums) are the classic case, and our guide to overwintering geraniums covers three ways to bring them through. For next time, our guide to how to protect plants from frost sets out what to cover and how.

Timing depends on where you live. In the northern hemisphere, the first damaging frosts typically arrive in autumn (fall) and the last come in spring. In Australia, New Zealand and South Africa the pattern is reversed, with the risk running from the southern autumn into late winter. The advice is the same on either side of the equator: wait for the last frost before you cut.

Which plants are most at risk

Find out what the damage really is

Frost damage looks a lot like other things. Blackened leaves, brown edges and a wilted stem can come from frost, from a root problem or from a disease, and the right response is different for each.

Photograph the affected leaves and Plant Doctor gives you the likely cause, what it looks like, what causes it, how to prevent it and what to do first. It draws on 500+ diseases and pests, and the diagnosis is free on every plan. That is the quickest way to tell a frost casualty that will recover from something else that will not.

If you grow in more than one place, a balcony and an allotment, for instance, each growing site in Folly has its own location and weather. Folly's local weather alerts, part of Pro, name the plants at risk from frost, heat, wind and storms and give you a protection checklist, so next time you cover the right plants before the night, not clear up after it.