Why containers feel the cold more than borders

A plant in the ground has a vast reservoir of soil around its roots, and that mass of earth holds warmth and buffers the worst of a frost. A plant in a pot has none of that. Its rootball sits above ground, exposed to cold air on every side, and on a balcony it also has to contend with wind that rips across the railings and chills compost far faster than still air ever would. Add the freeze-thaw rhythm of a British winter — mild, wet days followed by sharp overnight frosts — and you have conditions that crack terracotta, heave rootballs and rot roots in sodden compost. The good news is that an afternoon's work in late autumn will carry most of your pots through to spring.

Start with the roots: wrapping and raising pots

Roots are far more vulnerable than top growth, and they are the part you can protect most easily. Insulate the pot itself rather than the plant, and you will keep the rootball a crucial few degrees warmer through the coldest nights.

• Bubble wrap is cheap, light and remarkably effective. Wrap two or three layers around the sides, tape it in place, and leave the drainage holes clear.
• Hessian or an old blanket looks softer, breathes well and can be tied on with twine. Wool blankets from a charity shop are excellent for larger pots.
• Cardboard and newspaper will do at a pinch, though they sag once wet — tuck them inside a bin bag with the bottom cut away to shed rain.

Raising pots off cold paving matters just as much. Pot feet, bricks or offcuts of timber stop the base sitting in a puddle and reduce the chance of the rootball freezing to the ground. Grouping pots together helps too: cluster them shoulder to shoulder, with the most tender specimen in the middle, and they insulate one another. Top the compost with a thick mulch of bark chips, straw or scrunched newspaper, keeping it clear of stems so it doesn't encourage rot.

Move tender specimens into shelter

Anything borderline hardy — citrus, olives, bay, tree ferns, agapanthus, pelargoniums and most tender herbs — will thank you for a move. Against a south- or west-facing wall is the next best thing to a greenhouse: brick and stone release stored heat overnight, and the wall also blocks the worst of the wind. A spot under deep eaves or a balcony overhang keeps rain off the compost as well.

If a plant has to stay where it is, wrap it in horticultural fleece rather than plastic sheeting. Plastic traps condensation, and a sunny morning can scorch foliage trapped behind it. Fleece breathes while still shaving a degree or two off the cold. A cold frame, unheated porch or shed will see most borderline plants through, provided it is light and frost-free.

Don't drag a plant from a cold balcony straight into a warm living room. The shock of dry, heated air is often worse than the frost you were trying to avoid. Aim for a cool, bright spot instead.

Cut back the watering

This is the piece most of us get wrong. Growth slows to a crawl, evaporation almost stops, and plants need a fraction of their summer ration. The biggest killer of container plants in winter isn't cold — it's wet. Soggy compost starves roots of oxygen and rots them from below, and the damage is usually invisible until the plant collapses in spring.

Water only when the top few centimetres feel dry, and only on a mild morning so any surplus can drain before the night frost. Empty saucers and drip trays so pots never stand in water, and check that drainage holes aren't blocked by fallen leaves. Evergreens under shelter — a bay in a porch, a rosemary against a wall — still lose moisture through their leaves and may need a little water every few weeks, especially during a dry, windy spell. Don't let them dry out completely; just don't drown them.

Handle wind and snow

Wind does more damage than most gardeners expect. On a balcony it is relentless, rocking pots, snapping stems and drying out evergreen leaves faster than the roots can replace the moisture. A simple windbreak of trellis, willow screening or even a row of sturdier pots takes the sting out of it. Stake anything tall before the gales arrive, and check ties are firm but not strangling.

When snow settles, brush it off gently with a soft broom before it becomes heavy enough to splay an evergreen or snap branches. Don't try to knock off ice — you will break stems that would otherwise have come through unscathed. After any storm, walk the balcony and check pots haven't blown over or shifted against the railings.

Your winter container checklist

• Wrap or insulate the pots of anything staying outdoors.
• Raise every pot onto feet, bricks or timber.
• Group pots together and mulch the compost surface.
• Move tender specimens to a sheltered wall, porch or cold frame.
• Fleece anything that cannot be moved.
• Water sparingly, in the morning, and never leave pots standing in water.
• Stake tall plants and screen the windiest spots.
• Shake snow off evergreens before it settles.

Do these few things and your balcony will still be full of life when the first green shoots of spring push through. Winter is a slow season, not a lost one — and a little shelter now means far less replanting later.