How do I protect a greenhouse from storms and frost?

An aluminium greenhouse weighs very little and presents a large flat area to the wind, which is why they take off. Most losses start at one point of failure: a pane whose clip has dropped out, a door left swinging, or a frame that was never bolted down to anything. Frost damage is a separate problem with a separate answer — glass gives almost no insulation, so an unheated greenhouse is only a degree or two warmer than the garden on a still, clear night.

Safety

What you'll need

• Greenhouse glazing clips (a bag of spares)
• Ground anchors or a bolted base
• Horticultural bubble wrap (large-bubble)
• A spanner or socket set for the frame bolts
• A frost-protection heater and thermostat (optional)
• A maximum-minimum thermometer (optional)

Step by step

1. Bolt the frame to its base, and the base to the ground.A greenhouse standing loose on paving will move. It needs bolting to a proper base — a timber or aluminium base frame on a level, solid perimeter — and that base pinned into concrete or held by ground anchors at each corner.
2. Replace every missing glazing clip before autumn.Clips are the single most common failure. Walk the outside and the inside, look along every overlap, and fit spares wherever one is missing or has rusted. A bag of them costs very little compared with a pane.
3. Sort out the door.A greenhouse door caught open by the wind pressurises the inside and lifts the roof off. Fit a catch that holds it both open and shut, check the runners slide freely, and shut and latch it before any forecast wind.
4. Clear gutters, and check the downpipe goes somewhere.A blocked greenhouse gutter overflows down the inside of the glass and into the border, which is how the base rots and the soil at one end becomes waterlogged.
5. Take the shading off and clean the glass in autumn.Winter light is the scarce resource. Shade paint, netting and a summer's worth of algae between the panes can cut what gets through by a quarter, and netting left up is one more thing for the wind to grab.
6. Line the inside with bubble wrap, not the outside.Horticultural bubble wrap clipped to the inside of the frame creates a still air gap and can hold two or three degrees. On the outside it flogs about, catches the wind and is gone by January.
7. Group tender plants into one corner rather than spreading them out.Heating a whole greenhouse to keep six plants alive is expensive. Group them, curtain that section off with more bubble wrap, and heat only that. A cold frame inside a greenhouse is an old trick that still works.
8. Set any heater on a thermostat, at the lowest useful setting.For most overwintering, frost-free — around 3 to 5 degrees — is all that is required, not a warm greenhouse. A thermostat that switches off is the difference between a manageable bill and an alarming one.
9. Take the pots off the cold floor.Roots freeze before shoots do, and a pot on concrete or paving loses heat downwards all night. Staging, or even a sheet of polystyrene under the pots, makes a real difference.
10. After a gale, look before you walk in.Check the roof line and each pane from outside first. If glass is cracked but in place, or a pane has gone, wait for the wind to drop, then clear it with gloves and eye protection before doing anything else.

Tips

• Polycarbonate panels blow out more easily than glass because they weigh less. They need more clips, not fewer, and a strip of tape over the open flutes at the top stops algae growing inside them.
• A greenhouse in the lee of a hedge or an open fence survives better than one behind a solid wall, because a solid barrier creates turbulence and downdraughts on the sheltered side.
• Keep a maximum-minimum thermometer inside. It tells you what actually happened overnight, which is usually different from what the forecast said, and it stops you heating for a frost that never comes.
• Ventilate on mild winter days. A sealed, damp, unventilated greenhouse grows botrytis on everything, and grey mould kills more overwintered plants in the UK than cold does.

Common mistakes

• Bubble wrapping the outside of the glass — It has nothing to hold it in a gust, it tears at the fixings, and it shades the inside just when light is shortest. Inside, it insulates and stays put.
• Leaving the door open on a mild autumn day and forgetting it — Wind gets under the roof from inside. Most greenhouses that lift in a storm had a door or a roof vent open when the gust arrived.
• Heating the whole greenhouse to keep frost off a few plants — Glass loses heat as fast as you put it in. Partitioning off a small heated section costs a fraction of the same protection across the whole structure.

If it doesn't work

Questions people ask