Cold damages but when it combines with wet conditions, plants can be decimated. Hardy plants will quite happily sit through the winter, but steps can be taken to protect those with a slightly less robust character.
A dramatic cold spell will damage some plants. The easiest way to protect them is to wrap them in horticultural fleece.
The trick however is to unwrap when the weather warms, even the morning after a frost. The moisture within a tent of fleece can cause fungal problems so get the air circulating as soon as the weather allows.
A pane of glass will not protect tender plants from a penetrating, plunging frost. Greenhouse heaters are easy to get hold of and use.
Old fashioned, and much loved, paraffin heaters can be lit at night and extinguished in the morning, but beware of smoky flames as the fumes can physically colour leaves. Electric heaters are great if you have electricity running to your greenhouse – connect them to a thermostat so you can plug in and forget.
A large greenhouse is expensive to heat so think about consolidating your plants in one area, away from the door, and partitioning this off with bubble plastic to insulate through winter.
Seedlings can be vulnerable to cold and a simple covering with a sheet of newspaper will keep a degree of frost away. Remember to take the paper off in the morning and move trays away from the window to keep them warmer.
Pots and containers
Water underneath the pot will often freeze and prevent more water from flowing freely - raise pots off the ground with pot feet or half bricks (just enough to allow water to move) to prevent this from happening. Whole pots can be wrapped in fleece to prevent freezing but often simply moving them to a wall will afford enough protection to keep frost at bay.
If shoots of shrubs get nibbled back by frost cut them off down to where new growth can be seen. This will prevent diseases getting in and rotting back into new growth.
Bulbs are tough and can survive a warm spell followed by a cold snap. Daffodils will flop in the cold but will rise in line with increasing day temperatures.
A frozen pond will stop fish from feeding and gases to be exchanged with the air. Either gently melt holes in the ice by resting a jug of hot water on the surface or float a small football on the water. The ball can be removed after a frosty night leaving a hole for fish to breathe and feed. Don't smash any ice as the shock waves can damage the fish.