STARTING YOUR RHIZOME INTO GROWTH
This section covers plants received as rhizomes via mail order or your own splits. Early or dormant season plantings, January-March will need to be grown on with warmth and light such as on a window sill or in a heated propagator or the rhizome may rot due to the damage done when split from the parent clump. The easy way is to buy from April onwards when you can plant in a cold greenhouse/polytunnel etc. Getting the rhizome into growth early will result in a bigger plant during the growing season which can be very beneficial in the first season. Planted early in the season one rhizome would typically produce 5-8 stems by the seasons end and will flower in the first year (except musifolia). Canna musifolia bulks up faster than other types I grow but rarely flowers . Unpack your rhizome on receipt and pot on as soon as you can into a pot just large enough, with moist but not wet compost of a type for general use. Initially it doesn't matter what kind, be it bought/homemade or soil based or indeed garden soil. It should be easy to work out what way is up, the rhizome may be just buried, I lay the rhizome horizontal and just cover with the growing tip horizontal or pointing upwards. I prefer to plant with the growing tip just below the surface which makes it easy to recognise growth starting when it breaks through. You do not want to plant it with the growing tip pushing against the sides of the pot but toward the middle of the pot if you can. Adding fertilser at this time may be harmful but if the fertilser is already in the bought compost that's fine. When the rhizome is in growth and you can see some roots you can repot with a little fertiliser or manure added. Manure is most effective at the bottom of the pot. If started early season keep growing in a light and warm environment. If starting the plant in April or later a cool greenhouse is fine if kept frost free.
GROWING ON/PLANTING OUT
Once your plant is growing strongly you can feed regularly or pot on with manure or both. Canna are very greedy plants and once growing strongly cannot be overfed or over watered but this doesn't mean keep the pot standing in water as it is best to be able to drain to avoid stagnation of the soil. In the early stages you need to take care but they are easy plants to grow. There is no point planting out too early and the warmer they are kept in the spring the faster they will grow. I aim to plant out here at the end of May. Plants can grow strongly in a small pot and they will distort a plastic one if not potted on. Warmth food and water are the key to large plants. If keeping in pots best results will be obtained in a pot size of at least 30 litres for the final container for all the large types. I grow in the soil and in pots. Potted specimens are great for moving around and provding interest/ adding drama in dull areas throughout the season. Lots of manure used for all the canna's. Unusually you can use fresh horse manure for growing canna's and that is what I use as it is easy to get hold of.
Plant in full sun or dappled shade. Flowering is affected in shade unless growing very vigorously. It is said that darker folige types loose leaf colour in full sun but I haven't found it so. Flowers can fade in strong sunshine. Canna do not do well in a windy location as the leaves shred and look tatty.
Planted with manure around 50/50 in pots I still feed with vitax Q4 every six weeks or so. The same goes for those in the ground with plenty of manure added around and in the planting hole. Water frequently for best growth however they are drought tolerant. When anticipating flowers, as a general rule after seven leaves on a stem a flower spike should be visible. Dead heading can be done by shaking the flower stem if they are too tall to pick off. The flowering stem normally branches to produce more flowers so avoid dead heading by cutting down the stem. When the stem has absolutely finished flowering I cut it off above the top leaf, as I grow for foliage and flowers. Flowering can go on into early November here.
The general advice is to cut down the stems to the ground after the first frost blackens the leaves and dig them up and keep them as intact as possible, storing frost free and damp with or without light through the winter. Pots, bring in and store the same or dig out the plants from unmovably large pots and again store the same. Light and warmth to bring into growth in the Spring, However here I mulch the plants or cover the pots and leave in situ and for frosts down to -3c this works very well for me. Winter 2019 the canna's remained in situ throughout. I watch the weather forecasts and bring in if -3c or colder is expected. It is 01 January 2021 and this winter the canna are now coming in having only been cut down a month earlier. Canna left outside are slow into growth initially due to the cooler temperatures experienced compared to those brought in. They catch up eventually but you get a longer season from those brought in and grown in the greenhouse during the spring.