Why Winter Hits Containers Harder Than Borders

A plant in the ground has a vast reservoir of soil, steady warmth at depth and drainage stretching away in every direction. A plant in a pot has a few litres of compost, exposed on all sides to whatever the weather throws at it. That is why the same plant can sail through winter in a border and struggle in a container. Two things do the damage: cold and water. Frost that repeatedly freezes and thaws compost can heave roots and crack pots, while compost that stays sodden for weeks starves roots of oxygen. Cold, waterlogged compost is the single most common reason container plants rot away before spring.

The good news is that almost all of the risk is manageable with a few simple jobs, none of which take long. Do them before the first hard frost is forecast and you will save yourself a great deal of heartache.

Raise Pots Onto Feet

If there is only one thing you do this autumn, do this. Sit every container on pot feet, three bricks, wooden blocks or even a couple of flat tiles stacked at the edges. You are aiming for 2–3 cm of clear air beneath the base.

This matters for two reasons. First, it guarantees that the drainage hole is never blocked or sitting in a puddle, so surplus water can escape freely after heavy rain. A pot sitting flat on paving can effectively become a bucket. Second, it stops the compost freezing to the cold ground and limits the freeze-thaw cycle that lifts plants out of their compost and splits pots from the base up. It also denies slugs and worms an easy route in through the drainage hole.

• Take saucers and drip trays away for winter, or turn them upside down and use them as a stand instead.
• Check drainage holes are clear — a pencil or a bamboo cane is ideal for poking out compacted compost.
• In a gravel tray, keep the water level well below the pot base rather than letting it sit in a pool.

Wrap Terracotta, but Wrap It Wisely

Terracotta is beautiful and porous, which is exactly why winter is unkind to it. It absorbs water, the water expands as it freezes, and the pot flakes, crazes or splits. Unglazed terracotta is the most vulnerable; glazed pots can craze, while plastic, fibreglass and wooden containers generally cope well.

Wrap vulnerable pots with two or three layers of bubble wrap, working from just below the rim down to within a couple of centimetres of the base. Leave the base and drainage hole uncovered so water can still escape, and secure the wrap with twine or a couple of strips of tape. Hessian, hessian-backed fleece or even an old woolly jumper does the same job and looks far more attractive.

Do not seal the top. Plastic sheeting over the compost traps damp and encourages mould long before it keeps any frost out. If you want to protect foliage on a tender evergreen, drape fleece over the top on the coldest nights and take it off again during milder spells.

Group pots together against a house wall or fence, ideally in a sheltered corner. Clustering them creates a pocket of stiller, slightly warmer air, and the wall gives off a little stored heat. Move the most tender containers into a porch, greenhouse or unheated garage if you have the space.

Cut Back on Watering

Most container plants need dramatically less water between November and February. Compost that would dry out in a day in July can stay damp for weeks in winter, because growth has slowed and evaporation has almost stopped. Watering on autopilot is the fastest way to kill a plant that has survived the cold perfectly well.

Water only when the compost feels dry about 2–3 cm below the surface, and then water in the morning on a mild day, so any surplus has time to drain before night frosts arrive. Never water compost that is already frozen, and never water when a hard frost is forecast. Plants under the eaves of a house, or in the rain shadow of a wall, may still need the occasional drink — evergreens in particular lose moisture through their leaves during mild, windy spells. Small pots dry out far faster than large ones. Remember that a plant sitting wet and cold is in more danger than one that is slightly dry.

Mulch, Tidy and Hold the Feed

A layer of bark chippings or horticultural grit over the surface of the compost insulates the roots and stops the top layer freezing solid. It also keeps the crown of the plant dry, which helps guard against rot. Clear fallen leaves off the surface of pots as you go — they hold moisture against the compost, shelter slugs and encourage mould.

Resist the urge to feed. Fertiliser in winter sits unused, builds up in the compost and can damage roots that have no way of flushing it away. Save it for the first signs of growth in spring. Likewise, leave pruning until the worst of the frost has passed, as fresh cuts are vulnerable to cold damage.

Check and Repair After the Frosts

Once the risk of hard frost has passed — usually mid to late spring, depending on where you garden — unwrap your pots and inspect them properly. Tap each one and look for hairline cracks, particularly around the base and rim. Empty any compost that has stayed wet all winter, check the roots and repot in fresh, free-draining compost if they have started to rot. Then put the pot feet away for next autumn, and give yourself credit: a little preparation in October and November is what carries container plants through to a healthy spring.