Can I Leave Pots Outside in the Winter?
If you want to understand what to do with your pots in the Winter...read below.
CAN I LEAVE MY POTS OUTSIDE IN THE WINTER?
You can leave them safely outside IF (and only if) you have a situation where the part of the pot that is not glazed (usually the inside of the pot and the bottom of the outside of the pot) will always be dry when the temperature goes below zero.
The surfaces of the pot that are PROPERLY and completely glazed do not absorb any of that water when wet...it just rolls off. Unglazed surfaces do absorb water though, and, if wet when the temperature goes below zero, will be stressed a little by the expansion of that water turning to ice within the clay. As a result a wet clay wall is weakened at least a little each time the temperature falls below zero. You might get away with allowing this to happen for a pretty long time...even years maybe. Or you might find your pot doesn't even make it through the first winter. We don’t take that chance.
Glazed Pots: Starting in late September, early October, we turn all our hundreds of glazed pots left over at the end of each season upside down and then, with a regular paint brush, brush a coat of cement sealer onto the bottom of each pot. Cement sealer can be bought at Can Tire or any hardware store...maybe $18 / 4 litre jug. A litre would do hundreds of pot bottoms though and is way more than you likely would ever need. We buy it by the 20 litre pails and rebottle it here in small quantities for $3.00 also to sell to customers to solve that problem for them.
Terra Cotta Pots: We have given terra cotta pots 2 or 3 coats of sealer in the past in order to have some Christmas décor (cedar boughs, etc.) in them over the Xmas holidays. This can be done without problems but, as a generality, terra cotta pots are even more at risk of frost damage than glazed pots and we don't recommend leaving them outside over winter. We put all of them under shelter...still below zero, but DRY and under shelter.
Sometimes after explaining this we get silly responses like "Oh we're in the Windsor area though and your winters are much colder up here in in this part of the country so we wouldn't have that problem." Or, "We're in Edmonton and our winters are much colder than your winters here so that solution wouldn't work in our area." Water freezes at any temperature below zero and the damage is done as soon as that happens. FROZEN IS FROZEN! Things don't get more frozen at the north pole than they do in Windsor....they just stay frozen longer. In fact, if an unglazed clay surface is wet, the greater damage is likely to come in late fall and early spring when the temperature goes repeatedly above and below zero.
We’ve seen clay pots claiming to be "frost resistant". Depending on how and who prepared the clay mixture, some clays will be more porous than others. It’s true that a really dense clay would be more frost resistant than a really porous one but to think that any clay pot is “winter proof” is just nonsense…Don't believe it! If your pots are not too heavy and you a have a place to put them...we recommend just bringing them inside from Oct 'til April...doesn't have to be above zero...just dry...(they won't likely get dry if full of wet dirt). If you don’t have the indoor space, turn them upside down and apply a coat of cement sealer each fall to protect them.
What To Do With Really Big or Heavy Pots?
Essentially a really large potted plant outside in the winter simply puts the pot too much at risk of cracking but there is still a way to have really large pots. The answer is to use a plastic insert inside the ceramic pot... More often than not people just want to have annuals in their large pots and so really don’t need the pot to be full of dirt at all. The way to do this is to fill the pot with bits of wood or styrofoam or plastic pop bottles (whatever light filler you can think of…) and just use the few inches of soil needed in the plastic insert. At the end of the season just remove the insert and either turn the pot upside down and seal as described above, or tie heavy plastic sheet around the mouth of the pot on top of which a plywood sheet weighted down with a rock completes the job.