How to protect plants from frost: a UK guide

To protect plants from frost, cover tender plants with a double layer of horticultural fleece or an old sheet on nights when frost is forecast. Move pots against a sheltered wall and wrap them, mulch the roots of borderline plants, and bring frost-tender plants such as pelargoniums under cover before the first frost arrives.

That covers most gardens on most nights. The details decide whether it works: which plants need it, how cold is too cold, what to do with pots, and what to do the morning after. This guide is written for UK gardens, using RHS and Met Office guidance.

When is the first frost in the UK?

There is no single date. The RHS advises getting winter wraps on at the first warning of frost, which usually comes somewhere between September and November, and the timing varies a lot with where you garden.

• North and high ground usually get frost earlier than the south and the coast.
• Towns and cities tend to stay a little milder at night than open countryside, so a city garden can stay frost-free for a while after rural gardens nearby have had one.
• Frost pockets matter more than region. The RHS explains that cold air drains downhill and collects at the lowest point, or behind a barrier like a solid fence or hedge. The bottom of a sloping garden can be noticeably colder than the top.

There are also two kinds of frost. The Met Office defines an air frost as the air falling to freezing or below, measured at least a metre above the ground. A ground frost is ice forming on surfaces below freezing, and it can happen while the air stays above 0°C. Your plants live at ground level, so a forecast low just above freezing is no guarantee.

The Met Office's own rule of thumb is useful here: forecasts of 0°C to 4°C with little or no cloud and light winds suggest frost may form. Clear, still nights are the ones to watch.

Which plants need protecting from frost?

Anything tender or half-hardy, anything in a pot, anything planted this year, and the flowers and soft new shoots of some hardy plants. The RHS hardiness rating, on many plant labels and the RHS website, tells you which group a plant is in.

Three groups catch people out:

• Plants in pots. Their roots are exposed on every side, which is why the RHS recommends wrapping pots. A plant that is fine in a border can struggle in a container.
• New plantings. The RHS notes that young, newly planted plants are more susceptible than established ones.
• Hardy plants caught early. Plants build frost tolerance over several cold days, so a sudden frost after a mild spell can damage plants that would shrug it off in January.

The RHS names dahlias, cannas, tomatoes and potatoes among the plants most likely to suffer, along with the new growth and flowers of shrubs such as camellia, magnolia, pittosporum, bay and fuchsia.

How do you protect plants on a frosty night?

Cover what is tender, shelter what is in pots, and do it in the afternoon rather than at midnight in your dressing gown.

1. Check the forecast after lunch. A clear, calm evening with a low in single figures is your cue.
2. Cover tender plants. Use a double layer of horticultural fleece, or an old sheet in a pinch. The RHS says heavier fleece gives about 2°C of protection and does not need hoops to hold it up. Weigh the edges down so wind cannot lift it.
3. Move pots into shelter. Against a house wall, under a porch, or into a shed or greenhouse. Group them together so they shelter each other.
4. Wrap the pots, not just the plants. Bubble polythene or straw around the pot insulates the roots. The RHS also suggests plunging pots into the ground with just the rim showing.
5. Mulch the roots of borderline plants. A thick layer of organic matter over the root area of tender shrubs and tender perennials stops the ground beneath freezing as hard.
6. Think about the morning sun. The RHS explains that plants in early morning sun can thaw too quickly after a frost, which damages flowers and young growth. Camellia and magnolia blooms can be ruined by a single frost for exactly this reason.

Take sheets off once the frost has lifted, so the plant gets its light. Fleece can stay on through a cold spell, but remove covers in mild spells so plants do not sweat and rot underneath.

What should you bring in, lift or wrap before winter?

Some plants need more than a sheet on a cold night. Do these jobs before the frosts settle in, or straight after the first one.

Houseplants count too: the RHS warns that cold windowsills behind closed curtains can damage some plants. The guide to watering houseplants in winter covers the rest of their winter care.

What does frost damage look like, and can plants recover?

On tender plants, frost-damaged leaves look dark and water-soaked, then turn black. On new spring growth it shows as scorched, pale brown patches between the veins. Woody plants can take months to show damage at all.

Many plants recover, and the RHS advice is to be patient:

• Do not prune straight away. Wait until no more frost is expected, then cut damaged growth back to a healthy bud or sideshoot.
• Give it time. The RHS notes that recovery can take until early summer, so it may be worth waiting until mid-summer before deciding a plant has not made it.
• Re-firm new plantings. Frost can lift recently planted shrubs out of the ground. Press the soil back around them after a thaw.

Not sure it is frost damage at all? Here is how to identify plant problems from a photo.

What frost-protection mistakes should you avoid?

• Bubble wrap or clear polythene directly over foliage. The RHS warns that unpierced plastic can cause moisture build-up and overheating on sunny days. Use it on pots, not over leaves.
• Leaving covers on through mild weather. Plants sweat and rot under wraps that stay on when it is warm.
• Feeding nitrogen late in the season. It encourages soft new growth, which is the growth frost damages most.
• Planting tender things in a frost pocket. The low corner, or the spot behind a solid fence, will always be the coldest part of the garden.

Where does Mali help with frost?

The hard part is not the fleece. It is knowing, on the right afternoon, that tonight is the night, which is the same timing gap behind most plant problems.

• Frost alerts: Mali checks the forecast for your garden's location, and when it shows temperatures dropping below 2°C over the coming day, it sends an alert with the expected low and how many of your outdoor plants may be affected. Frost alerts can still arrive up to 10pm, an hour after other weather alerts go quiet, so there is time to get the fleece out. Weather alerts are on by default and can be turned off.
• Frost dates: Mali works out your hardiness zone and typical frost dates from your location, and uses them to time sowing. Suggestions flag when a sowing window is closing, with a sow-by date, and Sidney only offers plants that will finish in time.
• Ask Sidney: Sidney, Mali's AI plant expert, can look at your actual garden, your local weather and your frost dates, so you can ask whether a specific plant needs covering tonight.

The forecast is for your area, not your frost pocket, so you still know your garden best. Mali is free to use, with an optional Mali+ subscription that lifts the limits on the AI features. Get it on iOS or Android, or see the rest of what it does on the Mali home page.

Sidney, our resident purple snail, deals with frost by sealing herself into her shell until spring. She is aware this option is not available to your dahlias and is, frankly, smug about it.

FAQ

At what temperature should I cover my plants?

Cover tender plants when a clear, calm night is forecast to drop into low single figures. The Met Office says forecasts of 0°C to 4°C with little or no cloud and light winds suggest frost may form, because ground-level surfaces can be a few degrees colder than the air. Mali's frost alerts fire when the forecast drops below 2°C.

Does horticultural fleece protect plants from frost?

Yes, by a couple of degrees. The RHS says heavier fleece gives about 2°C of protection, and a double layer gives more. That helps on light frosts, not severe ones, which the RHS defines as below -5°C (23°F). Truly tender plants need to come under cover.

Can I use bubble wrap to protect plants from frost?

Yes, around pots, where it insulates the roots well. Be careful about putting it over foliage: unpierced plastic traps moisture and can overheat plants on sunny days. Fleece is the better choice for covering leaves.

When should I take frost covers off?

Take sheets off once the frost has lifted the next morning. Fleece can stay on through a run of cold nights, but remove it during mild spells to stop plants sweating and rotting. In spring, the RHS suggests removing winter wraps in stages or on a dull day so plants can adjust.

Will my frost-damaged plants recover?

Often, yes. Leave the damaged growth in place until there is no more frost forecast, then cut back to healthy buds. The RHS notes that recovery can take until early summer, so wait before deciding a plant is lost.

Do plants in pots need more frost protection?

Usually, yes. Roots in a pot freeze far more easily than roots in the ground. Move pots somewhere sheltered, group them, and wrap them in bubble polythene or straw.

Keep reading: how often to water houseplants in winter, how to identify plant problems from a photo, or browse all posts.

— Akhil Madan, founder of Keeda Studios. I learned to garden in a greenhouse in Henley-on-Thames, and still think the best frost protection is a roof.