For those who live in the northern hemisphere and have not grown Canna from seed before, this may be a good time to start giving it some thought. The best time to plant the seeds is dependant on the facilities that you have available.
If you have access to a propagator or heated greenhouse then seed can be sown as early as mid-January to early February. Given these conditions and reasonable weather in late spring and early summer we have sometimes had young seedlings burst into flower as early as mid-June.
If you are growing in a warm room, don’t sow too early, unless you can provide heat and light to keep the young plants growing nicely through till April-May, when you will be able to transfer them to the unheated greenhouse, as they will likely be too tall for a cold frame.
Cannas have an extremely hard protective seed coat that is impermeable to water, so they will not germinate without some assistance. You can help the germination process by removing a bit of the seed coat through a process called scarification prior to planting.
Once canna seeds have been scarified, they are easy to grow. Only the tiniest amount needs to be exposed and it is better to file too little away than too much. If you look at the seed carefully, you will see on one end a scar (hylem), sometimes called the trap-door. It is through here that the growing shoot will break through. You should scarify the coat on the opposite end to the scar in order to avoid damage to the embryo.
How you go about scarification (nicking) is quite easy. There are several variations on this, and everybody develops their favourite method.
- Take a piece of sandpaper and sand the end of the seed until you get through the black seed coat.
- An alternative is holding them in a pair of pliers and using a file, carefully cutting into the coat until you reach the white endocarp within the seed.
- Or you may want to use a grinding wheel, which I use (a cheap Chinese import). I hold the seed in a pair of pliers and hold the seed to the grinding wheel just for a second or two, until I see can white. If you have a large number of seeds to do, then this is the fastest option, but it can be quite destructive if you are not very steady when you hold the seed to the wheel.
Whichever method you use, you’ll know that you’re through when you see the white endosperm. Basically, what you’re trying to do is make a hole in the seed coat so that water can get through.
Again, there are several techniques available to germinate the seed.
- I get some very warm water, almost hot and I put the seeds in for 48 hours or until I see the embryo coming out one end. I change the water twice daily to keep it from souring.
- Another popular approach is to add some vermiculite to a plastic bag, add water to moisten and then add the seeds. The bag is not locked so that air can circulate. The bag is kept in warm conditions, an incubatotor or airing cupboard. The disadvantage with this is that after a while the water will sour, and while I have used this method in the past I now prefer the first one, because changing the water requires a change of vermiculite as well, and vermiculite costs money!
- Some plant the seeds directly into small pots and water, waiting until a growth of some sort is seen, this might be a growing leaf or a white embryo, depending on the position the seed was placed in the pot. As some seeds will not germinate I always feel reluctant to spend on pot and compost until I see the seed has germinated. Call me a skinflint if you want, I just blame my parents!
Once you see the white embryo coming out you can pot them up. I use 2 inch peat pots filled with a peat-based compost with added fertilizer. I plant the seeds 1/2 inch deep with the embryo pointing downhill, water well and put in direct light. 
Maintain a temperature of 21-24°C (70 to 75°F). In a few days (5-6) you should see growth. As the plants begin to actively grow, the soil will dry out quickly, so check daily and water when needed. Keep the growing medium moist, and do not over-water, as more seedlings die from that cause than from neglect.
Depending on how long a growing period the seedling has indoors it may need planting into a larger pot, and the advantage of the initial peat pot is that this just means planting the whole pot, thus no root disturbance.
Gradually harden off the cannas and transplant them outside, adding plenty of compost to the soil, following the last-frost date. They should flower about 90 to 120 days after the seed was sown.
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, why not go ahead and start cannas from seed?
If you want exact clones of your canna plant, you’ll need to propagate by rhizome 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.
I would be interested in hearing about other techniques you might use.