Winter is coming! Ill.: http://www.pngfly.com & clipart-library.com, montage: laidbackgardener.coom
I hope your cannas did well this summer, providing great tropical-looking foliage and spectacular, colorful flowers. But as fall sets in, and certainly before winter hits, you have some choices to make.
Cannas, you see, are essentially tropical plants and won’t overwinter outdoors in cool temperate climates. If you live in hardiness zones 9 to 12, that is, in a warm temperate to tropical zone, you can leave them in the ground. In fact, in zones 10 to 12, they may even continue growing and flowering through the winter as long as they receive sufficient rain. In zone 8 and in some parts of zone 7, you might want to risk leaving them outdoors if your soil doesn’t freeze more than superficially, especially if you mulch them (a good 6 to 8 inches/15 to 20 cm of chopped leaves or grass clippings would be perfect), as their “bulbs” (actually rhizomes) are underground and out of reach of light frost.
Elsewhere, you can either let your cannas freeze and replace them with fresh rhizomes next year, or bring them in. Here’s how to do the latter.
After Frost Hits
Logically, you’d want to leave your cannas outdoors until frost hits, partly because they’ll keep growing and blooming until then. However, this also gives them longest possible growing season over which the plant can store food for next year’s growth. But when frost does kill them back (it will literally blacken the leaves!), their growing season is officially over and it’s time to act.
If there is no frost before mid-November (in the Northern Hemisphere; mid-May in the Southern Hemisphere) though, bring them in any way: you don’t want them to be still in the ground when it does freeze solid!
The easiest cannas to handle are those growing in containers. Simply cut them back to 1 to 2 inches (3 to 5 cm) above the soil and bring them in. (More about where to store them later.) You can even pile pots one on top of the other. If you feel the need to repot them or divide them, you can do that in the spring.
Otherwise, before digging your cannas, cut the leaves and stems back to about 8 to 12 inches (20 to 30 cm) so they won’t be in your way. Now, dig up the root ball with a shovel or garden fork, using the leaf stubs as a handle to shake off the excess soil. Spread the rhizomes on newspaper, cardboard or an old blanket for about a week in a frost-free spot (a garage or shed, for example). Now, cut the foliage back completely (1 to 2 inches/3 to 5 cm from the rhizome) and trim back the thick roots. Clean the rhizomes roughly with a brush to remove most of the dirt. Don’t rinse them, though: they need to stay dry.
Do make sure you label the rhizomes, with the cultivar name if possible; if not, at least as to flower color, leaf color and size (i.e. “tall red w purple leaves.”). Canna rhizomes all look pretty much alike when they’re naked and certainly, by spring you’ll have forgotten which is which! Either write on a plastic label with a twist tie and fix it to the rhizome or write on the rhizome with a garden marker. I find just placing plastic labels with the tubers risky: they always seem to end up being mislaid.
Some people like to divide their cannas before they store them, but I only do this if the clump is really big and difficult to handle, in which case I simply break the clump in two or three. (It will separate along “natural lines.”) I do any precision dividing in the spring before starting a new growth cycle.
If possible, store the rhizomes in a cardboard box or a plastic container (the latter should have a few holes so there’ll be a bit of air circulation), covering them with vermiculite, peat moss, wood shavings, or shredded paper. Or wrap them in newspaper. One friend simply tosses his into a series of plain trash bags (air circulation be damned!) and uses a twist tie both seal the bag and attach the label. Whatever works for you!
The ideal location for winter storage is a cool but not cold spot that remains between 40 and 55˚ F (5–12°C) for much of the winter. Spots to consider are root cellars, unfinished basements, crawlspaces and barely heated garages.
You have nowhere cool to store the rhizomes? Don’t worry: they can be kept at room temperature if necessary, but if so, the rhizomes will tend to dry out over the winter, so check them monthly, giving them a spritz of water if they start to shrivel.
You need to check the rhizomes occasionally at any rate, even if you can keep them cool. Just remove any rotting rhizomes and lightly spray any shriveling ones.
You’ll see that the rhizomes may start to sprout well before spring, especially when conditions a bit warmer than they should be. Just ignore their pushiness and put them back into storage until you’re ready. After all, who’’s the boss here, you or the rhizomes?
By earliest spring, the rhizomes will be itching to grow and starting to sprout. This is the time to divide them, if necessary. Rhizomes stored in their pots can be dumped out for division at this point. Most will be too pot-bound to leave in the same container more than a year or two.
In areas where spring comes early, you can plant canna rhizomes directly in the garden as soon as there is no more risk of frost and the soil temperatures rise above 55˚ F (12° C). Don’t plant them while the soil is still cold: that will just stunt their growth.
Plant them in full sun in rich soil (add slow-release fertilizer), covering the rhizomes with about 3 to 4 inches (8 to 10 cm) of soil and setting about 12 to 18 inches (30 to 45 cm) apart, depending on the size of the plants.
In colder climes, cannas will need a head start indoors if you want to see them bloom before summer’s end, so, about 4 to 5 weeks before the expected last frost, pot them up temporarily in pots about 12” (30 cm) in diameter, barely covering them in soil (you can plant them deeper when you move them to the garden). If they’ll be growing in a pot all summer, you might find it practical to plant them directly into their final container, probably a large tub.
Place the pots in front of a sunny window indoors, in a cold frame or an unheated greenhouse: somewhere there is plenty of light. Start watering lightly, increasing as sprouts appear and start to grow. Then plant your cannas out once the soil warms up (again, 55˚ F/12° C is the minimum) and there is no danger of frost.
💡Helpful hint: If you’ll be placing pot of cannas in a water garden (cannas can be grown as semi-aquatic plants), cover the container’s soil in about 2 inches (5 cm) of gravel. That will help keep the soil from floating away and dirtying the water.
And there you go: successful overwintering for cannas!