James Whitcombe
There is something quietly hopeful about a collection of pots. They let us grow lemons in Leeds, olives in Oban and agapanthus on a windy balcony. But come the first hard frost, those same pots become the most vulnerable things in the garden. A plant in open ground has the vast thermal mass of the soil beneath it; a plant in a container has a few litres of compost that can freeze solid from the top down in a single cold night. The good news is that a couple of hours of preparation in late autumn will carry most tender plants safely through to spring.
Why Pots Are So Much Riskier Than Borders
Frost damages plants in two ways. The first is the obvious one: ice crystals form inside the cells of leaves and stems, puncturing the cell walls. The second is subtler and often more deadly. As compost freezes, the roots are physically lifted and crushed, and the plant loses its ability to take up water at exactly the moment when drying winds are pulling moisture out of the leaves. Pot-grown plants suffer both at once, and they have no deep soil to retreat into.
The other factor unique to containers is that frost can attack from every direction. In a border the soil insulates the rootball from below and the sides. In a pot, cold air circulates all the way around the rootball and underneath, and terracotta and glazed ceramic conduct that cold straight into the compost. A pot sitting on a paving slab on a clear January night can be several degrees colder than the soil a spade's depth away.
So the strategy is straightforward: insulate the rootball, cut the wind, and stop adding water that will only freeze. Do those three things and most tender plants will come through.
Move Containers Before the Frost Arrives
Position is your cheapest form of protection, and it works far better than any wrapping. Move tender pots to the most sheltered spot you have, ideally against a south or west-facing wall that has absorbed warmth during the day and releases it overnight.
- Group pots tightly together, shoulder to shoulder. A cluster of containers traps air between them, and that shared microclimate can be two or three degrees warmer than a pot standing alone.
- Push pots as close to the house wall as you can. The wall radiates stored heat through the night.
- Avoid low spots at the bottom of a garden or at the foot of a slope, where cold air pools like water in a bath.
- Move pots under an overhang, a porch, or the eaves of a shed. Keeping rain and snow off the compost is half the battle.
- If you have a cold greenhouse, unheated conservatory, or even a bright garage, use it. Light matters, but for most hardy-evergreen tender plants, dryness matters more.
- Raise pots off the ground on pot feet, bricks or timber battens so that air and drainage can pass beneath.
Do this in October or early November, before the plants have been stressed by cold. A plant that has already had its roots chilled is more likely to succumb later, and moving a large pot in the middle of a freeze risks cracking it.
Wrapping and Insulating the Rootball
Insulating the pot itself is the single most effective thing you can do for a tender plant in a container, because it protects the roots rather than the foliage. Bubble wrap is the traditional and still the best material, and a few metres of it costs very little.
- Wrap the pot, not the plant. Trapping the foliage does more harm than good, encouraging damp and rot.
- Use two or three layers of bubble wrap around the sides of the pot, with the bubbles facing inwards for maximum insulation.
- Secure it with string or tape, tucking the top edge just below the rim so the compost surface stays open to the air.
- For extra protection on the coldest nights, pack the gap between the wrap and the pot with straw, scrunched newspaper or old woollen jumpers.
- Alternatively, stand the pot inside a larger one and fill the gap between them with bark chippings or straw. This is tidier than bubble wrap and works very well for specimen pots.
- Plunge smaller pots right into the ground or into a raised bed, rim and all, and mulch over the top. Soil is a superb insulator.
- Wrap terracotta and glazed pots with particular care. They are more prone to cracking in hard frosts even when the plant inside is fine.
For marginally tender plants like tree ferns, cordylines or phormiums, you can also tie the leaves loosely upwards and stuff the crown with dry straw. Just do not seal it with plastic, or you will create a rot trap.
Reduce Watering and Keep the Compost on the Dry Side
Overwatering in winter kills more potted plants than cold does, and the two problems work together. Wet compost freezes harder and takes longer to thaw than dry compost, and cold, soggy roots rot quickly. From around late October, cut watering right back.
- Water only when the top few centimetres of compost feel dry, and then sparingly.
- Water in the morning, never in the evening, so surplus moisture has a chance to drain before nightfall.
- Water less often in dull weather and stop entirely during a sustained freeze.
- Check that drainage holes are clear and not blocked by a saucer or a layer of frozen gravel. Empty saucers so pots are not sitting in water.
- Never feed tender plants in winter. They are not growing, and unused nutrients will simply build up in the compost.
- If you have a greenhouse, ventilate on mild, bright days to keep the air moving and prevent grey mould.
Aim for compost that is barely moist. Most Mediterranean and subtropical plants would rather be dry and cool than damp and cold, and a dry rootball is far better equipped to shrug off a hard frost.
Watch the Details Through to Spring
Once everything is wrapped, moved and watered sparingly, the main job is vigilance. Keep an eye on the forecast and, on the coldest nights, drape an old blanket, a duvet, or a layer of horticultural fleece over the top of the plants. Remove it again in the morning, or once the temperature lifts above freezing, so the plant can breathe and get whatever light there is.
Shake heavy snow off evergreens and wrapped pots before it settles, as the weight can snap branches and tear covers. Check wrapped pots occasionally through the winter for slugs, vine weevil grubs and patches of mould, and ventilate on mild days.
Then be patient. Unwrap in stages as the weather eases, usually from late March, keeping an eye out for late frosts which can still catch new growth. Leave the fleece to hand until May, water gently as growth restarts, and your pots will reward you with another full season with barely a gap. A little preparation now means far fewer sad surprises when the sun finally returns.
