The relaxed gardener's guide to growing cannas
In recent years, a canna renaissance has taken place, with many new and exotic varieties being released and more gardeners incorporating them into planting schemes. Cannas have long been treasured elements in southern gardens and have become increasingly popular in northern zones where they aren’t reliably hardy. Cannas might be used even more widely in colder climates if there wasn’t so much misinformation about growing them, a problem I’ll address here.
First of all, gardening websites and so-called experts often mischaracterize cannas as “tropical” plants, referring to them as “tender perennials” which have to be carefully coddled before they can be set out into gardens. In reality, cannas are tough, hardy perennials that reliably return every spring in gardens as far north as U.S.D.A. zone 7b (average lowest winter temperature 5-10F). Gardeners in zone 7a (where an average winter can see temps as low as 0F) commonly succeed with these plants as well, as do growers well up into zone 6 (0 to -10F average winter nadir), using protected locations such as beds on the south wall of homes and ample winter mulch. Cannas are generally native to tropical locations but have been naturalized in the southeastern U.S., parts of Europe and the rest of the world.
Misconceptions like this result in bad advice on how to raise cannas. Gardeners in zones 6 and colder are commonly urged to start canna rhizomes in pots a couple of months ahead of the last expected frost to “get a jump on the season”, or to wait until all danger of frost is past, planting rhizomes at the same time tomato plants go out. Now, there’s nothing wrong with starting a few cannas indoors in pots - if you only need a few plants and have the sunny windowsill or light garden space to accomodate them. It’s a different story if you want a real show from a large planting. Even if you have ample growing space, you’ll be faced with the likelihood of repotting plants into larger containers, then having to lug them outside to accustom them to outdoor conditions (and inevitably needing to drag them back inside when cold, frosty weather threatens - unless you’re fortunate enough to have a sizable coldframe or greenhouse in which to keep them). During this shuffling process, the plants also have to be gradually accustomed to full sun, which inevitably is more intense than experienced even in a southern exposure window indoors.
The claim that cannas cannot be planted until after all danger of frost is past is similarly misguided, and merely serves to delay and shorten the growing season. I’ve collected several articles from northern-based sources to illustrate wrongheaded advice that plagues potential canna enthusiasts.
So what do I have to offer that’s better?
My experience has shown that direct planting of canna rhizomes need not wait until dependably frost-free weather. Typically, I plant around the date of the average last frost, which in both central Kentucky and my former central Ohio garden is in late April. I keep a close eye on the extended weather forecast to either advance or delay the planting date. If all goes well, the first sprouts will be nudging above ground after the final, usually mild frost has occurred. In case of an unexpected cold front, I keep lightweight frost cloth or mulch on hand to cover the emerging sprouts. A couple of years ago, we had an unseasonable freeze with lows in the upper 20s around May 12. I used lightweight frost cloth adding several degrees of protection to cover the canna bed, removing it after several chilly nights. About 80% of the young foliage was undamaged, with the remainder showing signs of frost burning. No matter - the plants quickly recovered and flowered on schedule starting in mid to late June. After all, even in the South there are instances where late frost damage occurs in spring, yet plantings continue thriving.
Cannas are fairly undemanding, needing only sun, an ample supply of moisture (some varieties even do well in shallow pond settings) and regular feedings. My plantings are mostly untroubled by a common pest further south, the canna leaf roller which can be controlled by spraying with BT. Cannas’ major enemy here is the Japanese beetle, which devours flowers and feeds on leaf edges, creating an unsightly mess if you don’t have a regular preventative spraying program. In the past I’ve used pyrethrin or its derivatives. These are broad-spectrum sprays which are relatively non-toxic to humans (don’t spray when bees are active). Last year I tried a Bioadvanced “3-in-1” spray (for insects like JBs, mites and certain diseases) and it permitted me to go up to two weeks between sprayings (pyrethrins and derivatives generally have effects that wane after a week or so).
Deadheading of cannas (removal of fading flowers) improves their appearance and prevents seed setting, encouraging further bloom.
At the approach of cooler temperatures in fall, it’s time to plan for overwintering of canna rhizomes and/or winter storage. Here we see another parade of bad advice from “experts”. We’re told that once the first frost shrivels and blackens the topmost growth, we need to immediately dig up rhizomes for safekeeping, or at most delaying only a few days, to protect these “tender perennials”. Not so. You can safely wait up to a few weeks, during which time there’s usually several frosts plus a hard freeze or two to finish off the foliage entirely and send the protected rhizomes below ground into dormancy. I generally dig up rhizomes in late October or early November, timing the process for when soil isn’t too damp and mild weather is expected. Laying out rhizomes on a tarp or lawn for a couple days allows them to dry out prior to storage, and permits the gardener to gloat over the increase in rhizomes from the prior season. I typically use large planting tubs, adding a layer of peat moss followed by a layer of rhizomes, then more peat moss etc. until the tub is full. The containers go into cool storage (optimum 35-45F) with no supplemental water until the rhizomes have had a chance to dry out, usually for about a month. Subsequent waterings sufficient to keep the rhizomes plump and viable occur at intervals of about a month. The growing process repeats in spring.
I do start a few varieties in 6 to 8 inch pots in early spring, moving them into a coldframe sometime in early to mid-April, being careful to gradually adjust sun exposure.
For the past several years, my major canna planting has been “Tropicanna Black”, which combines dark purple foliage and hot scarlet flowers. I’ve also enjoyed classics like “Wyoming” (bronze leaves, orange flowers). the giant “Red King Humbert” and “President” (both red-flowered) and “Bengal Tiger” (yellow-striped leaves, orange flowers). I go for bold and spectacular cannas over subtle pastels and small-flowered delicate types, but some of those find their way into the garden too.
Growing cannas from seed can be fun, and first-year plants often flower, with success depending on the variety. For many years I maintained a planting of “South Pacific Scarlet”, a semi-dwarf. Seeds can be started in late winter/early spring. The hard-shelled seeds need to be scarified before planting. To save wear and tear on your hands, try gripping a seed with pliers and using a serrated knife or file held in the other hand to scratch past the outer seed coat layer, probably no more than about 1/32 inch. Then, seeds are soaked in initially warm to hot water for up to 48 hours, after which they can be planted to their thickness in a freely-draining but water-retentive soilless medium. Sometimes, dropped seeds will surprise you by emerging in spring in the border. Aside from the “South Pacific” series in multiple colors, there’s a dwarf “Tropical” series that includes a bronze-leafed variant. You can also grow seeds produced by various hybrids; what you ultimately get will depend on an unpredictable genetic lottery.
Some sources for canna rhizomes:
Plant Delights Nursery offers a good variety including new hybrids. Rhizomes and shipping are expensive.
Karchesky Cannas. Many hybrids, often found nowhere else.
Horn Canna Farm. This used to be my go-to place for ordering older, classic varieties cheaply. Unfortunately prices have gone way up. I once could order “Wyoming” for a few bucks per rhizome; it now costs at least three times as much.
Brent and Becky’s Bulbs.
Relatively inexpensive packaged bulbs can be found at garden centers and home improvement stores. Quality is something of a crapshoot; make sure rhizomes are firm and not mushy and starting to rot.