Question: Last year, my cousin gave me some canna bulbs and I planted them in the garden. They grew to be huge, with beautiful leaves, but they never bloomed. Why?
Answer: Growing cannas (Canna × generalis) is not for the impatient gardener, especially the older varieties. If you plant their rhizomes directly in the ground, they only bloom very late, often well into fall. And where summers are short, like where you live, they may not bloom at all.
That’s why gardeners have traditionally started canna rhizomes (which you called bulbs) indoors about 4 to 6 weeks before the last frost date. Place the rhizomes in a large pot, covering them with 3 to 4 inches (7 to 10 cm) of potting soil, then water well before placing the pot somewhere they can get a bit of heat. 70 °F (21 ° C) is a minimum. Try for 80 °F (27 °C) if you can! As soon as the shoots appear above the surface of the soil, expose the pot to full sun too and continue to keep the growing mix a bit moist. If you can convince the sprouting cannas they’re a tropical swamp, they’ll grow more quickly … and will come into bloom sooner.
When the air has warmed up outdoors (certainly above 50˚F/10˚C at night), put the pots of cannas out and start to gradually acclimatize the plants to outdoor conditions. Try 3 days in shade, 3 days in half-shade and 3 days in sun. Then you can move them their summer spot.
You can plant cannas in the ground or keep growing them in pots. Since cannas love sun and heat, a location protected from wind often suits them very well. Do enrich the soil with compost or fertilizer, as cannas are very greedy plants!
Throughout the summer, continue to water as needed, as cannas don’’t like to dry out. You can even put their pot in a water garden so they soak constantly in water and they’ll still bloom very well, provided the water is relatively warm, that is.
With this regime, you should be able to get your cannas to bloom beautifully!
Or Try Earlier Cannas
The above treatment is the standard one for the big canna plants gardeners have been growing for generations, but there’s a whole range of new, earlier-flowering, smaller cannas you can try.
Why is height a factor? Because the taller a canna is, the later it blooms. Thus, very tall cannas, such as the banana canna (Canna ‘Musaefolia’)—which can reach 10 to 15 feet (3 to 4.5 m) in height—almost never bloom outside the tropics: the growing season is just too short. Classic garden cannas, such as ‘Wyoming’, ‘Red King Humbert’ and ‘City of Portland’, are pretty big plants, most easily reaching 6 feet (nearly 2 m in) high and thus are also quite late to bloom, although they will flower in most climates if they’re started indoors.
But there is also a host of recently introduced dwarf cannas such as the Futurity, Canna Island, Cannova and Cannasol series, which are less than 4 feet (1.2 m) in height and therefore bloom earlier, often as early as the beginning of the summer if started indoors. In addition, most dwarf cannas produce numerous offsets, stems that will flower in their turn over the summer and therefore they often bloom non-stop once the first blooms have appeared.
In short, then, there are two secrets to being absolutely sure to get cannas to bloom in your garden. First, start them indoors to give them a head start on the season … then choose dwarf cannas: they’re just so much easier to bloom than the old standard ones!