Winter Protection for Container Trees

Why container trees feel winter more keenly

A tree in the ground has the whole earth as a blanket. A tree in a pot has nothing but a thin wall of terracotta, plastic or glazed ceramic between its roots and the air, and air on every side is a merciless thing in January. Compost in a container can freeze solid within a few hours of a sharp frost, and because the volume is small, it thaws and refreezes repeatedly as the temperature swings between day and night. That cycle is what does the damage: it heaves the rootball, shears fine feeding roots and can crack pots that were never sold as frost-proof.

Wind matters as much as cold. Evergreens keep transpiring through winter, and a drying east wind strips moisture from leaves that the frozen compost cannot replace. Add winter wet — our other national speciality — and you have compost that sits sodden for weeks, suffocating roots and rotting crowns. So the job is not simply to keep pots warm. It is to insulate, drain and shelter, in that order.

Wrap the pot, not the tree

Bubble wrap and hessian are the two reliable standbys, and each has its moment. Hessian is breathable and ideal in wet, mild-winter regions where trapped moisture is a bigger risk than frost. Bubble wrap, especially the large-bubble sort, insulates better and suits colder gardens and smaller pots, but it does seal in damp, so use it once growth has stopped and check beneath it now and then.

• Wrap from just below the rim right down to the base, leaving the drainage holes clear.
• Secure with twine rather than tape, so you can unwrap easily on a mild day.
• Leave the compost surface open to the rain and air — covering it traps mould and sourness.
• Do not wrap the trunk. Bark needs to breathe, and a cosy collar of plastic around a stem is an open invitation to rot and fungal problems.
• For a belt-and-braces job, stand the pot in a slightly larger pot or a hessian sack and pack the gap with straw or dry leaves.

Timing matters. Wrap in late autumn once growth has genuinely stopped, not at the first chilly evening in September, and unwrap in early spring. If we get a mild, damp fortnight in the middle of winter, peel the wrap back for a few days to let everything air.

Get pots off the ground

This single step prevents more losses than any amount of wrapping. Pot feet, bricks, timber offcuts or even two battens laid side by side will lift the base clear of cold paving, let surplus water drain freely and stop the pot freezing to the ground.

Empty saucers before winter. A saucer full of ice and water around the base of a pot is a slow execution. If your garden is on heavy clay and water pools in winter, move pots onto gravel, a paved slope or a raised plank, or tilt them very slightly so rain sheds off the surface. For dormant, drought-loving subjects such as agapanthus and many alpines, tipping the pot onto its side under the eaves is a traditional and effective trick — but only for plants that are fully asleep.

Use the shelter your garden already offers

A south- or west-facing house wall is a genuine microclimate: the masonry releases stored warmth overnight and the wall blocks the worst of the wind. Move tender specimens — olives, citrus, bay, cordyline, phormium, tree ferns and figs in pots — against it before the first hard frost, while they are still easy to shift.

• Group pots shoulder to shoulder, with the largest and hardiest on the outside as a windbreak for the smaller, softer ones.
• Cluster them close so they share warmth, but not so close that air cannot move between them.
• Keep pots away from paths where de-icing salt is used, and out of the drip line of a downpipe.
• Erect a temporary fleece screen on the windward side if you have no wall to spare.

Water carefully, feed not at all

Winter drought kills more container evergreens than frost does. On a mild morning, if the compost is dry a few centimetres down, give a modest drink and let it drain. Never water onto frozen compost — it simply sits there and freezes into a block. Stop feeding entirely from mid-autumn until growth restarts in spring; a soft flush of new growth in December is the last thing you want.

Knock heavy snow off evergreens, conifers and bay with a broom before it weighs branches down and splits them. Keep an eye out for slugs and vine weevil grubs, which stay active in mild spells, and check stored pots for mould.

Which plants need the full treatment

Assume any borderline plant in a pot needs help. That means anything rated as needing a sheltered spot: olives, citrus, pomegranate, fig, Japanese maple, camellia, choisya, cordyline, palms, phormium, agapanthus, tree ferns and young, newly planted specimens of almost anything. Small pots freeze faster than large ones, so a two-litre pot of lavender deserves more attention than a half-barrel of hornbeam.

Fully hardy natives in generous containers will usually sail through, but even they will thank you for pot feet and a sheltered corner. A little effort in November saves an awful lot of replanting in May.