A Canna makes a perfect “thriller” for your outdoor planters. However, there are a few thing you should know about keeping them blooming and growing all season long.
First, when a Canna is done blooming the leftover petals can be quite ugly. It is best to dead head any spent blooms to keep the plant putting out new flowers. On the same note, this plant will start to grow seed pods rather quickly. If you want to encourage more blooms, make sure and cut back the seed pods. This will encourage the Canna to spend its energy in putting out blooms rather than making seeds.
Canna Seed Harvest:
Personally, I love finding seeds and experimenting with growing seeds indoors and outdoors. I allowed my Canna to seed so I could harvest its large round seed pods. Every pod can carry 2-6 seeds and are easily removed from the pods after they are dry and ready to harvest.
The seeds have a very hard outer coating which will one will need to nik or scar to aid in germination. Soak the niked seed for 24 hours before placing in soil. Start your Canna seeds 8 or so weeks before your last frost. Canna is hardy in zones 8-11, but one can dig up the rhizomes and store over winter. This overwintering practice is much more common than growing from seed.
These plants like full sun and do really well in the hot heat of summer, but may require extra water during those really hot days. Other resources claim that Canna is not really bothered by pests or disease, but I know first hand it is loved by the invasive Japanese Beetle.
Also, here is a link to download your very own seed packet template. Store your seeds in style!