Cannas have an extremely hard protective seed coat that is impermeable to water, so they will not germinate without some assistance. To propagate cannas by seeds:
1. Help the germination process by removing a bit of the seed coat through a process called scarification. "Prior to planting, take a piece of sandpaper and sand the end of the seed until you get through the black seed coat. You'll know that you're through when you see the white endosperm," says James F. Harbage, research division leader at Longwood Gardens, in Kennett Square, Pennsylvania. "Basically, what you're trying to do is make a hole in the seed coat so that water can get through." Once canna seeds have been scarified, they are easy to grow.
2. Sow the seeds a quarter-inch deep in a tray of vermiculite 8 to 10 weeks before your last frost date. Keep the vermiculite moist and the room temperature between 65°: and 70°F during the day and at 60°F overnight. The seeds should germinate in about one week.
3. When the seedlings reach about 6 inches tall, transplant them into individual pots filled with a premoistened, well-draining potting soil. Add a granulated organic fertilizer to the potting soil (follow application recommendations on the fertilizer label). Keep the potting mix moist but not wet, and place the plants in a sunny (southern exposure) window. As the plants begin to actively grow, the soil will dry out quickly, so check daily and water when needed.
4. Gradually harden off the cannas and transplant them outside, adding plenty of compost to the soil, following the last-frost date.
Keep in mind that most cannas are hybrids bred for their flamboyant blooms and foliage. Seeds from hybrids will not come true to type, which means they will exhibit characteristics that are different from those of the mother plant (but not necessarily unattractive). If you're feeling a bit on the wild side and don't mind growing a few mystery plants, go ahead and start cannas from seed. If you want exact clones of your canna plant, you'll need to propagate by division. The book The Gardeners Guide to Growing Cannas, by Ian Cooke (Timber Press, 2001), has excellent information on propagating cannas by seed and division, as well as information on cultivars and the history of cannas.