Is overwintering a canna lily in a heated greenhouse a good idea?
I can find no source that states Canna must have a dormant period. Apparently, they grow quite nicely all year round in sunny climes like Florida. So, keeping them growing in pots in a heated greenhouse until taking them out next summer seems perfectly reasonable.
Here are my only concerns. If you are just bringing the pots of Cannas into your greenhouse now, they will suffer a bit of shock. If you are in Toronto, or further North, it is much colder, especially at night, than sunny Florida. So bringing them suddenly inside a climate that is much warmer, the leaves may curl, yellow or the tips may brown and die. The plants won’t die, they just won’t look happy. If you want to avoid such shock in the future, you should bring your plants in when the temperature range of your greenhouse is the same as the temperature range outside. That’s usually the end of August, or beginning of September.
Truthfully, I never remember, or never have the time, and all my geranium and hibiscus leaves turn yellow and fall off when I finally bring them into the greenhouse. They pout with bare branches except for the leaves with white fly, until I can put them outside again the following summer when they perk up beautifully.
My other concern is that although Cannas like to be kept in consistently moist soil, if it gets too cold and damp inside the greenhouse, and the soil gets too cold and damp, the bulbs may rot. I don’t know your greenhouse, but even though mine is heated throughout the winter, it gets quite inhospitable for tropical plants at night. Only the tough ones with low temperature tolerance survive. I have a banana plant, a relation of Cannas, that always rots around the base by the middle of winter. So make sure the soil drains well, and keep the pots in the warmest area of your greenhouse.