Overwintering Plants: UK Frost Protection Guide
Overwintering protects tender plants from UK frosts between October and April. Geraniums, dahlias, citrus trees, and Mediterranean herbs all need winter shelter when temperatures drop below –5°C. Methods range from greenhouse storage at 1–7°C to mulching hardy perennials with 75–100mm of bark. We have fitted over 12,000 greenhouses since 2008, and proper overwintering prevents 90% of winter plant losses.
Key Takeaways
- Tender perennials (RHS H2) need greenhouse or indoor protection at a minimum of 1°C.
- Lift dahlia tubers after the first frost and store them in dry compost at 5°C.
- Mulch hardy perennials with 75–100mm of bark, straw, or leaf mould.
- Cold frames protect Mediterranean herbs and seedlings down to –10°C.
- Ventilate your greenhouse on days above 10°C to prevent grey mould.
- Never prune spring-flowering shrubs before winter. Wait until after they bloom.
Installer's Note
When we prepare customer greenhouses for winter, insulation is the first job. A single layer of bubble wrap cuts heat loss by 40–50%. We always check glazing clips and door seals before October. Draughts kill more overwintering plants than cold does. Get the structure right, and the plants look after themselves.
What Is Overwintering and Why Does It Matter?
Overwintering means protecting plants through the coldest months of the year. In the UK, that is roughly October to April. Not every plant needs help. Hardy perennials survive our winters without any intervention at all.
The problem comes with tender and half-hardy plants. Geraniums, fuchsias, dahlias, and citrus cannot tolerate prolonged frost. Without protection, they die back to nothing. That means replacing them every spring at full cost.
The Royal Horticultural Society uses hardiness ratings from H1 to H7. H1 plants need heated glass year-round. H7 plants survive anything the UK throws at them. Most garden centres label plants with these ratings. Check them before you buy.
Which Plants Need Overwintering in the UK?
The table below shows common plant types and their overwintering needs. All temperatures are in Celsius.
| Plant Type | Examples | RHS Rating | Overwintering Method | Minimum Temp |
|---|---|---|---|---|
| Tender perennials | Geraniums, fuchsias | H2 | Move to greenhouse or indoors | 1°C |
| Half-hardy bulbs | Dahlias, cannas | H3 | Lift and store tubers | –5°C |
| Hardy perennials | Hostas, daylilies | H5 | Mulch in situ | –15°C |
| Citrus trees | Lemon, orange, lime | H2 | Greenhouse or conservatory | 1°C |
| Mediterranean herbs | Rosemary, lavender | H4 | Cold frame or sheltered spot | –10°C |
| Potted shrubs | Hydrangeas, acers | H4–H5 | Cluster against south wall, wrap pots | –10°C |
Tender perennials are the biggest concern for most gardeners. One hard frost below –5°C can destroy an entire collection. Half-hardy bulbs survive if you lift them before the ground freezes solid.
How Do I Overwinter Tender Plants in a Greenhouse?
A greenhouse is the single best tool for overwintering tender plants. The aim is to keep temperatures between 1°C and 7°C. You do not need tropical warmth. Just frost-free conditions.
Move tender plants inside before the first forecast frost. In most of the UK, that is late October or early November. Space pots so air can circulate freely between them. Crowded plants develop grey mould within weeks.
Ventilation matters more than heating in a mild winter. Open vents or doors on days above 10°C for two to three hours. Stagnant damp air is the real killer, not cold temperatures alone.
If temperatures regularly drop below –5°C overnight, you will need some heating. A small electric fan heater with a thermostat set to 2°C works well. From our experience, paraffin heaters are cheaper to run but need more ventilation. You can heat a greenhouse on a budget with the right setup.
Bubble wrap insulation makes a big difference. One layer on interior walls reduces heat loss by 40–50%. We always recommend this to customers. It costs under £20 and saves far more in heating bills. Read our guide on how to insulate a greenhouse for step-by-step instructions.
How Do I Protect Potted Plants from Frost?
Potted plants are more vulnerable than those in the ground. Roots in pots freeze faster because soil volume is smaller. Terracotta pots can crack when ice expands inside them.
The simplest method is clustering. Group pots together against a south-facing wall. The wall radiates stored heat overnight. Clustering also creates a shared microclimate between the pots.
Wrap individual pots with horticultural fleece or hessian. Two layers give better insulation than one. Avoid plastic wrapping directly around pots. It traps moisture and promotes root rot.
For valuable specimens, consider a cold frame. A 4×4ft cold frame provides ground-level protection down to –10°C. We fit a lot of these for customers with collections of alpines and herbs. Browse our full range of cold frames for options that suit your space.
Our potting sheds also work well as temporary winter shelter for frost-sensitive pots. The enclosed walls block wind chill, which causes more damage than still air at the same temperature.
Matt's Tip: Overwintering on a Budget
I tell every customer the same thing. Before you spend anything, cluster your pots against the house wall and wrap them in hessian. That alone gets most plants through a normal UK winter. The only kit I always recommend buying is a min-max thermometer for your greenhouse. Knowing your actual overnight lows tells you exactly when to act. I bought mine for £8 from a garden centre and it has saved me hundreds in lost plants.
What Is the Best Mulch for Overwintering?
Mulch insulates the soil and protects roots from freeze-thaw cycles. Apply it in a layer 75–100mm deep around the base of plants. This is the single cheapest form of winter protection available.
The three best mulch materials for overwintering are:
- Bark chips — long-lasting, widely available, and slow to decompose. Best for shrub borders.
- Straw — excellent insulator and free if you know a local farmer. Best for vegetable beds.
- Leaf mould — homemade from autumn leaves collected in wire bins. Ready in 12–18 months.
Keep all mulch 50mm away from plant stems. Mulch touching stems traps moisture and causes collar rot. This is the most common mulching mistake we see in customer gardens.
Apply mulch after the first hard frost. Mulching too early keeps warmth in the soil and attracts rodents looking for a cosy spot to nest.
Should I Prune Before Winter?
Prune dead and damaged growth only. Remove broken branches, diseased stems, and any dead wood. This stops fungal spores spreading through the plant over winter.
Do not prune spring-flowering shrubs like forsythia, lilac, or camellia. These set flower buds in autumn. Pruning them now means no flowers next year. Wait until immediately after they finish blooming.
Roses are a special case. Most gardeners prune roses in late February or early March. Pruning in autumn encourages soft new growth that frost kills. We always advise waiting until you see buds beginning to swell.
Herbaceous perennials can be cut back to 100–150mm above ground level. Leave the stubble standing through winter. It protects the crown and provides habitat for overwintering insects.
How Do I Use Cold Frames for Overwintering?
Cold frames act as miniature greenhouses at ground level. They trap solar heat during the day and release it slowly at night. A well-positioned cold frame adds 5–10°C above the outside temperature.
Position your cold frame facing south for maximum sunlight. Place it against a wall if possible. The wall acts as a thermal mass, storing heat during the day.
Ventilation is critical on mild days. Prop the lid open when temperatures rise above 10°C. Close it again by mid-afternoon to trap warmth for the night. Forgetting to ventilate causes condensation and fungal problems.
Cold frames are ideal for overwintering Mediterranean herbs, alpine plants, and autumn-sown seedlings. They also harden off spring seedlings before planting out. Rosemary, lavender, and thyme all thrive in cold frame protection through a typical UK winter.
For larger spaces, consider greenhouse heaters with a built-in thermostat. Electric fan heaters are the most reliable option.
How Do I Overwinter Dahlias and Other Tender Bulbs?
Dahlias, cannas, and gladioli are the most common tender bulbs in UK gardens. None of them survive prolonged soil temperatures below –5°C. The traditional method is to lift and store them.
Wait until the first frost blackens the foliage. Cut stems back to 150mm above the tuber. Carefully fork around the plant and lift the entire rootball. Shake off excess soil but do not wash them.
Dry the tubers upside down for two weeks in a frost-free shed or garage. This prevents moisture sitting in the hollow stems and causing rot. Once dry, store them in trays of dry compost or vermiculite.
The ideal storage temperature is 5°C. A cool garage, unheated spare room, or frost-free greenhouse all work. Check stored tubers monthly through winter. Remove any that show signs of mould or soft rot.
In mild southern regions, some gardeners leave dahlias in the ground. This works if you apply 100–150mm of bark mulch over the crown after cutting back. In our experience, lifting gives more reliable results across most of the UK.
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Matt's Pick for Overwintering ProtectionBest For: Protecting tender plants, seedlings, and herbs through winter Why I Recommend It: I have fitted dozens of these and the raised design sets it apart. The 4×4ft bed sits above ground level, keeping roots away from ground frost. Twin-wall polycarbonate panels hold heat without blocking light. The hinged lid makes ventilation and watering easy every day. Price: £329 |
Frequently Asked Questions
When should I start overwintering plants in the UK?
Start moving tender plants indoors by mid-October in most UK regions. Watch your local weather forecasts from late September onwards. The first frosts usually arrive between mid-October and early November depending on your location. In northern England and Scotland, start two weeks earlier than the south.
Can I leave dahlias in the ground over winter?
Yes, in mild areas with well-drained soil and 100mm+ bark mulch. Gardens in southern England and sheltered coastal spots can get away with it. Cover the crowns with 100–150mm of bark chips after the first frost. In heavy clay or frost-prone areas, lifting and storing tubers at 5°C is safer.
What temperature should I keep my greenhouse at in winter?
Between 1°C and 7°C for most overwintering plants. Tender perennials like geraniums and fuchsias survive at just above freezing. Citrus trees prefer 5–7°C. Tropical plants need 10–13°C but these are not common in most UK collections.
Is hessian or fleece better for wrapping plants?
Horticultural fleece is better for most situations. Fleece is lighter, lets more light through, and is easier to secure around awkward shapes. Hessian is tougher and better for wrapping large pots or tree trunks. Both outperform plastic, which traps moisture and causes rot.
How often should I water overwintering plants?
Water sparingly, only when the top 25mm of compost feels dry. Overwatering in winter is the most common cause of plant loss. Most dormant plants need water once every two to three weeks. Always water in the morning so excess moisture evaporates before nightfall temperatures drop.
Do I need to feed plants during winter?
No, stop all feeding from October until March. Plants are dormant or semi-dormant in winter. Fertiliser encourages soft new growth that frost damages immediately. Resume feeding in early spring when you see active growth starting again.
