Container gardening remains popular, for various reasons though I wonder how much longer the trend will last what with bags of compost being controversial and so few that are available made with ‘peat alternatives’ falling short when it comes to actually growing plants. But, that aside, winter brings the problem of cracked pots. Pots crack in winter because of the action of freeze-thaw on water. Water is odd that it expands as it becomes a solid, which is why those ice cubes in your gin and tonic float. Terracotta and ceramic pots, which are porous, are full of water in winter and when this water freezes it expands and the pots flake or crack. Some ceramic pots are glazed on the outside and this glazes frequently shears off as the material underneath freezes.
Of course, plastic pots do not suffer this damage. Wood is flexible and so wooden containers do not show this kind of damage. Terracotta pots tend to be either ‘frost-resistant’, which is more or less useless, and ‘frostproof’ which generally means they are fired to a higher temperature and are not damaged by this kind of frost damage. But they are more expensive.
A simple way to avoid this kind of frost damage, if you don’t move the pot and plant to a sheltered place, is to wrap the pot with insulation. Bubble plastic is useful for this but bin bags, filled with shredded paper, can be tied round the pot too. Effective, but not very attractive.
The other way that pots are cracked by frost is when the compost in the pot freezes and expands. the pressure increases until the pot is cracked. The worst affected are those lovely round pots with narrow tops. In a ‘normal-shaped’ pot the compost can move vertically as it expands- like a cork out of a bottle – but in a rounded pot this is not possible and the pot will crack into two halves, a top and a bottom. Because of the issues of repotting perennials, I have grown out of this kind of pot or, if I do use them, I find a pot that fits into the mouth of the pot and never fill the round pot with compost anymore.
But you can help prevent pots cracking if you line them with bubble plastic. This needs a bit of forethought but is well worth doing. Most of us have some bubble plastic around the place and simply lining the inside of the pot with plastic, before filling with compost and planting. When frost causes the compost to expand the ‘slack’ is taken up by the bubbles and the pot is safe. After planting you can tuck in or cut away any excess plastic. A secondary benefit is that the plant roots are also ‘slightly’ protected from frost. Plants in pots are more vulnerable to damage by frost than in the garden where their roots rarely get exposed to frost.