Wrap the Glass, Not the Air: A Smarter Greenhouse Winter Plan

Bubble wrap has a reputation problem. Say you are spending a Saturday afternoon taping it to a greenhouse and someone will make a joke about wrapping up a fragile relative. Ignore them. In a British greenhouse in November, a decent layer of horticultural bubble wrap is less of a comedy prop and more of a thermal curtain — cheap, removable and surprisingly effective at taking the edge off a cold snap.

The catch is that a greenhouse wrapped like a parcel and then left alone becomes a damp, still box. That is not protection; that is a mould incubator with good intentions. The aim is to slow heat loss while keeping air moving, so plants come through winter healthy rather than merely alive.

Start with the leaks, not the roll of bubble wrap

Before you reach for the tape, spend ten minutes checking the structure. Run a hand around door frames, vent seals and glazing clips. Look for cracked panes, loose putty, gaps where the frame meets the base and draughts under the door. A lot of winter heat disappears through small, boring faults rather than through the glass itself.

Replace broken panes, firm up loose clips and fit foam draught strip around the door if it rattles in the wind. Do not silicone every vent shut in a fit of enthusiasm. You will need to open them on mild, breezy days, and a sealed greenhouse in a British winter is a recipe for condensation running down the glass by lunchtime.

Insulate the glass you can afford to cover

Horticultural bubble wrap is worth the small extra cost over repurposed packaging because it is UV-stabilised and less likely to go brittle and shower you with plastic flakes by January. Fix it to the inside of the frame with proper greenhouse clips or wide-headed clips rather than a heroic amount of tape. You want it to come down easily in spring.

Concentrate on the north and east-facing sides first, plus any exposed end wall that catches the wind. Those surfaces lose heat without contributing much winter light. The roof and the south-facing side are different: cover them with something clear or leave them be, because light is the one thing a winter greenhouse cannot spare. A double layer with a small air gap works better than a single tight sheet, but do not turn the whole house into a padded envelope. Leave vents, door edges and any automatic openers clear.

Condensation is the quiet problem

Cold glass plus damp compost plus still air equals grey mould, rotting stems and that unmistakable greenhouse smell of regret. Bubble wrap makes condensation easier to miss because water gathers behind it and between the layers. Check behind the wrap every week or so, especially after a run of mild, wet days.

Ventilate whenever the weather allows — a little and often beats a dramatic midday airing. Open a vent or the door for twenty minutes on a dry, breezy morning, then close up before the temperature drops. Water plants in the morning rather than the evening, keep foliage dry and avoid cramming pots so tightly that air cannot move between them. If you are overwintering a lot of stock, a small battery-powered fan moving air gently is often more useful than another layer of insulation.

Decide which plants have earned the space

Not everything deserves the sheltered spot. Tender favourites such as pelargoniums, fuchsias, citrus and young cuttings are obvious candidates. So are pots of herbs that would sulk outdoors and any late-sown seedlings you are nursing through. Hardy shrubs, established perennials and anything that actually prefers a cold dormant spell can stay outside or go in a cold frame with plenty of air.

Raise pots off the floor using staging, slatted shelves or even a couple of bricks. Cold air pools at ground level, and roots sitting in a puddle of chilly condensation are far more likely to rot. Keep leaves from touching the glass, because that is where the coldest, wettest surface is. Group plants by their need for warmth and air, not by whatever space is left.

Heat only where it earns its keep

If you run a small electric heater, bubble wrap reduces how often it has to switch on. Use a thermostat rather than leaving it running constantly, and avoid paraffin heaters in a sealed, wrapped house unless you are serious about ventilation — they add moisture as well as warmth. For most gardeners, the realistic goal is frost protection, not tropical temperatures. A max-min thermometer tells you what actually happened overnight, which is far more useful than guessing from the kitchen window.

The cheap checks that pay you back

Before the first hard frost, stock a few basics: spare glazing clips, a roll of horticultural bubble wrap, some fleece for extra-cold nights and a max-min thermometer. Check the door hinges and vent mechanisms now, while your fingers still work. Clear fallen leaves from the gutter and base so water does not back up. On snowy days, brush heavy snow off the roof carefully rather than letting it sit and block the light.

By February 2026, the payoff should be obvious: cuttings that did not collapse, tender pots that did not turn to mush and a greenhouse that still feels usable on a bright, cold morning. Bubble wrap will not perform miracles, and it will not replace a proper heater if you are growing exotics. But as a cheap, reversible winter upgrade, it earns its place — as long as you wrap the glass and keep the air moving.