Overwintering canna plants by digging and storing rhizomes

To overwinter canna plants in a cold climate, dig up the tender rhizomes after the first frost, dry them, and store them in barely damp packing material somewhere cool and dark until spring. Cannas grow from rhizomes that will not survive a freeze, so leaving them in frozen ground turns them to mush. Lifting and storing them lets you replant the same vigorous clumps each year, and the rhizomes multiply, so one plant becomes several over a few seasons.

The first canna I grew came back as a surprise gift of three plants the year after I dug and stored a single clump. I had nearly skipped the digging, figuring one plant was not worth the trouble, but a friend talked me into it. After several winters of storing rhizomes in a box of peat in my basement, I now have far more cannas than I started with, all from that one original clump lifted ahead of a hard zone 5 freeze.

Here are the steps I follow each fall.

The steps cover the whole job, but the details of timing, drying, and storage conditions decide whether the rhizomes come through plump and healthy or shriveled and rotten. The rest of this guide walks through those details.

The canna at a glance

Canna (Canna x generalis and related hybrids) is a tender perennial grown from a thick, fleshy rhizome, in the family Cannaceae, native to tropical and subtropical regions of the Americas. Mature plants reach 3 to 6 feet (90 to 180 cm) tall, with bold paddle-shaped leaves in green, bronze, or variegated forms, and flowers in shades of red, orange, yellow, and pink through summer.

The plant is hardy only in USDA zones 7 to 10, where the rhizome survives winters in the ground. In colder zones the rhizome freezes and turns to mush, which is why cold-climate gardeners must lift and store them each fall. Canna rhizomes are tough and forgiving in storage, and a single clump multiplies over a few seasons of careful digging, so the effort repays itself.

When to dig cannas

Wait for the first frost to blacken the foliage before you dig. Frost is the signal that the plant has finished its season and is sending energy down into the rhizomes for storage. Digging too early, while the leaves are still green and working, can mean smaller, less well-fed rhizomes.

A light frost that kills the tops is fine and actually helps. What you want to avoid is a hard freeze that penetrates the soil and damages the rhizomes themselves. So watch the forecast: dig within a few days of that first frost, before any deep freeze sets in. A canna rhizome that survives a deep freeze in the ground is usually too damaged to store, since the freeze ruptures the cells and the rhizome breaks down into mush.

Lifting and cleaning the rhizomes

Cut the frost-blackened stems back to a few inches above the ground first, so you have less to handle. Then loosen the soil around the clump with a garden fork, working a good distance out from the stems so you do not stab through the rhizomes. Lift the whole clump out gently.

Let the lifted clumps sit and dry for a day or two in a frost-free spot, like a garage or shed. A little drying makes the soil fall away more easily and lets any small nicks in the rhizomes callus over, which reduces rot in storage.

After drying, shake or brush off the loose soil. You do not need to wash the rhizomes, and in fact dry storage works better than wet. Trim the stems short, down to an inch or so above the rhizomes. You can divide large clumps now or in spring, but storing them whole takes less space to heal and protects more surface from rot.

Packing and storing

Pack the rhizomes in a box or bin filled with barely damp peat moss, sawdust, vermiculite, or shredded paper. The packing material should feel just slightly moist, never wet. Its job is to keep the rhizomes from drying out completely without holding so much moisture that they rot. Nestle the rhizomes in so they are surrounded but not crammed tight against each other, which helps air move and stops rot spreading from one to the next.

Store the box somewhere cool, dark, and frost-free, ideally around 45 to 50 degrees Fahrenheit (7 to 10 degrees C). A basement, insulated garage, or cool closet usually fits the bill. Too warm and the rhizomes sprout or shrivel early. Too cold and they freeze. Dark keeps them from trying to grow before spring.

Checking through winter

Check the stored rhizomes once or twice over winter rather than forgetting them until spring. Look for two problems. The first is rot, shown by soft, mushy, or moldy rhizomes. Remove any of these right away before the rot spreads to their neighbors.

The second is shriveling, when a rhizome dries out too much and goes wrinkly and light. If you see this, lightly mist the packing material to add a little moisture. The aim is to keep the rhizomes plump and dormant, neither rotting from too much moisture nor drying to a husk from too little.

Overwintering cannas in pots

If you grow cannas in containers, you have an easier option than digging. Rather than lifting the rhizomes, you can bring the whole pot indoors and store it as is. After the first frost blackens the foliage, cut the stems back to a few inches, move the pot somewhere cool, dark, and frost-free, and let the rhizomes rest in their own soil through winter.

Keep the soil in a stored pot barely moist, never wet. A dormant rhizome uses almost no water, and soggy soil over a cool winter rots it, the same risk as overpacked storage boxes. Check the pot once or twice over winter and add only a splash if the soil has dried out completely. An unheated garage, shed, or cool basement around 45 to 50 degrees Fahrenheit (7 to 10 degrees C) suits a stored pot just as it suits a box of rhizomes.

This method saves the work of lifting and packing, but it takes more space and the rhizomes do not get the inspection that digging allows. If you have only a few pots and the room to store them, it is the simplest route. If you grow cannas in the ground or in large numbers, digging and boxing the rhizomes is more compact and lets you check each one.

Avoiding the common storage mistakes

A few errors account for most canna storage failures. The first is digging too late and letting a hard freeze reach the rhizomes in the ground. A light frost on the foliage is your signal to dig, but a deep freeze that penetrates the soil damages the rhizomes themselves. Watch the forecast and lift within a few days of that first frost.

The second is storing the rhizomes too wet or too clean. Washed, damp rhizomes packed into a closed container are a recipe for mold and rot. Knock off only the loose soil, dry the rhizomes for a day or two, and pack them in barely moist material. Dry storage beats clean storage every time with cannas.

The third is forgetting them entirely until spring. Stored rhizomes need a check or two over winter so you can pull out any that have started to rot before the problem spreads, and lightly moisten any that have shriveled. A few minutes of attention in midwinter saves a whole box from being lost to rot or desiccation by spring.

Canna cultivars and their rhizome behavior

Different canna cultivars grow rhizomes of different sizes, which affects how they handle storage. Smaller cultivars need less space but also dry out faster, while larger types store more energy and tolerate longer winters.

Whichever cultivar you grow, the storage method is the same. The differences are mostly in how much space the rhizomes take up and how vigorously they multiply, which matters if you are dividing in spring.

Replanting in spring

Bring the rhizomes out in spring once the danger of hard frost has passed and the soil is warming. You can start them early indoors in pots a few weeks before the last frost to get a head start on flowering, or plant them straight into the garden once the weather settles.

This is also the easiest time to divide. Each piece of rhizome with a growth bud, or eye, will grow into a new plant. A clump that went into storage as one plant often divides into several healthy pieces in spring, which is how a single canna becomes a drift over a few seasons.

Plant the rhizomes a few inches deep in rich, moist soil and full sun, and they leaf out and grow fast as the weather warms. By midsummer they are back to their full height with bold leaves and flowers.

Overwintering canna plants this way means a little digging in fall and a check or two over winter, and it returns the same vigorous clumps year after year, multiplying as it goes. For a tender plant in a cold climate, that is a fair trade.