Growing from fresh rhizomes.
This section covers plants received as fresh rhizomes via mail order or your own splits. Dried rhizomes sold in garden centres, by bulb dealers or online are not a good way to grow a quality plant for a few reasons. They are in an unnatural state as canna rhizomes do not naturally dry out for dormancy and at best are unreliable. Most virused cannas occur on bulb farms which is where canna virus has spread from. Also the dried rhizomes could be incorrctly named.
Early or dormant season plantings, January-March will need to be grown on with warmth and light such as on a sunny window sill or in a heated propagator or in a warm room or the rhizome may rot due to the damage done when split from the parent clump. Initially light is not required until the shoot has grown 3-7cm tall. The easiest 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 and is grown for its foliage and cannot be relied on to flower. 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 or half buried as new roots will form from the underside. Your rhizome may be long like a piece of root with a bud at the end and possibly other developing buds further down if so plant horizontally partly or fully covered. If it is like a bulb I would plant it either just below the surface or with the growing point just protruding. Allow enough space for the growing tip to grow without touching the side of the pot. 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.
Sometimes the first leaf of the season may have some brown or transparent patches. The second leaf may also have issues with its appearance on a much lesser scale. This issue, when it occurs, grows out quickly. Also the young leaves are vunerable to scorch and burn from being on the windowsill/ underglass or a hot dry environment in direct very strong sunlight and can quickly cause discoloured patches. When the plant has a few leaves these early ones, if too unsightly, can be removed. The first leaf or two especially grown indoors can have many kinds of imperfections. One slug or snail hole before a leaf unfolds becomes a line of holes when the leaf opens. The key point to realise is that one unsightly leaf today will become insignificant quite quickly due to the fast growth of these plants. There is much paranoia about Canna virus and it should be a concern to a point. There is more about canna virus further down this article.
GROWING FROM YOUNG PLANTS/EARLY SEASON PLANTS
Much of the advice on growing from rhizomes applies, especially early in the season when the plants need to be kept growing in warm and light conditions. From April a cool greenhouse or kept frost free is fine and they will grow away. These small potted or bare rooted plants want potting into a 2 or 3 litre pot with a very good or enriched multi purpose or potting compost. Larger early plants need to be kept frost free and the warmer it is the bigger and faster they will grow.
GROWING ON/PLANTING OUT
Canna are clump forming and will produce multiple stems from soil level throughout the growing season. If planting large varieties to make a large group plant about 50cm apart. 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 but it's often mid June by the time all the planting is done. The foliage on young plants will be easily killed or badly damaged by late spring frost. 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 ground, raised beds and in pots. Potted specimens are great for moving around and providing interest/ adding drama in dull areas throughout the season. Lots of manure used for all the cannas. Unusually, you can use fresh horse manure for growing cannas and that is what I use as it is cheap and easy to get hold of.
Plant in full sun or dappled shade. Flowering is affected in full shade unless growing very vigorously. For foliage they can be grown in shade but as mentioned they thrive on warmth. It is said that darker foliage types loose leaf colour in full sun and it may be so but not by a huge amount. 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. This much manure will last the plants at least a couple of years. Those in the ground like 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 as well as flowers. Flowering can go on into early November here. In a sunny conservatory the plants will keep growing and not die off.
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 pots too large to move. Dig out trying to keep the clumps as intact as possible and again store the same. Light and warmth to bring into growth in the Spring, this is the best time to split them. However here I have mulched the plants or covered the pots and left in situ and for frosts down to -3c this works for me. Some damage may occur but the plants survive. Levels of frost vary, a more penetrating wind frost is able to cause more damage. A deep mulch and plastic sheeting can help. Deep snow before the frost gets severe is helpful too. For pots try and store out of the chilled winds and wrap the pots in polythene or pond liner. In Winter 2021 Cannas in the ground were left until 04/05 January 2022, before being lifted with dispatch of rhizomes commencing the following week. I like to leave the cannas in the ground for as long as possible. 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.
PROBLEMS AND PESTS
Canna are pretty robust, tough, plants with few issues.
The most common problem I hear about is canna's not flowering. This is almost always due to insufficent feeding of the plants. Except for musifolia which is generally not expected to flower.
Pest issues I have experienced are with aphids in the early part of the season but never to serious levels. Spider mite are often a problem especially under glass/polythene and in our warming climate these pests can be an issue in the garden. I always use the recommended biological control and never pesticides. Sometimes caterpillars cause minor damage.
CANNA VIRUS is still is a major problem especially affecting dry rhizomes supplied by bulb traders/garden centres and growers who produce plants for sale from them. It's best to avoid buying the dry rhizomes/bulbs, although some people must have had success with them. I have sought to increase my range in 2021 and bought in a few plants in order to achieve this and it quickly materialised that there are lots of virus infected plants offered for sale out there including from some surprisingingly well reknowned outlets. Some plant sellers especially general nurseries will pot on their unsold dry canna rhizomes and sell virus infested plants. There are a lot of unscrupulous sellers out there and many opportunities to pick up canna virus so it is really worth buying from a trusted source. Of which, in my endeavours to acquire new varieties for my own collection, I have discovered there are very few. And being careless can lead to some or all of ones cannas becoming infected. All my canna are grown from scratch on site and are healthy , free from virus. It is easy to be very concerned that a new plant might have the virus. Signs of virus vary but the most common and worst one shows yellow striping in the leaf veins at an early stage and or widespread yellow spotting. Later the plant may distort, flowers distort, leaves rot, plant rots. The key is really the yellow striping/spots in the veins and can be difficult to detect in the variegated cannas but mottling, yellowing, distortion are all possible signs, browning and rotting during the growing season are signs of some of the devasting virus taking hold, this would be after the yellow stages described otherwise the browning or distortion are likely to be other issues like waterlogging in oxygen starved substrate. An extreme example of this would be growing in a bucket of mud with no drainage holes exposed to the heat of the sun. Suspect plants should be isolated or destroyed. That said if plants are grown stressed they may show some erratic leaf growth through rapid temperature fluctuations and yellow patches can be down to a patch of chalk in the soil or some other soil contaminent and things like that. Pale dots in areas of leaf can be after or during pests such as aphids and spider mite. Browning or crisping of the leaf edge can be down to wind burn. When I get a new variety I get as few as possible and monitor closely. Even then I have burnt a couple on unpacking that may have been virus free and burnt others that certainly had virus. Any new plant coming in is grown in isolation and checked for aphids regularly so as not to spread the virus to my other plants and monitored closely for the first year. Check out the dedicated page for canna virus and common leaf damage symptoms. Suspect plants can be isolated/observed/burnt.