With their large foliage and stunning colors, cannas lend a dramatic and exotic feel to the garden. They require rich soil and ample water to look their best, which makes them good marginal plants at the edge of ponds and in bog gardens. Varietal variations are widely varied in foliage color, flowers and eventual height.
Almost any canna variety can be used as a marginal plant, however Canna glauca is specially adapted to very wet soils. Most grow only four to six feet tall, a modest height that makes them work well with other bog plants. These cannas are hardy in zones 7 through 11.
Canna glauca is the original species plant. It has green leaves with a bluish tint and white margins. Flowers are creamy yellow and white.
Canna glauca 'Endeavor' displays large bluish green leaves with a distinctive purple margin. Bright red flowers often attract hummingbirds.
Canna glauca 'Erebus' has similar foliage to 'Endeavor'. Bright pink flowers emerge slightly later than most water cannas.
Canna glauca 'Taney' is unusual for the color of its flowers, which are loud shades of orange and pink.
Canna glauca 'Stuttgart' has stunning foliage variegation. Leaves are broadly and randomly striped in white, with much variation on each plant. Flowers are orange, providing a good contrast with the foliage. Some afternoon shade may be needed for this canna to develop optimum color and avoid sun scorch on the paler areas of the leaves.
Canna glauca loves moisture and rich soil. Bog garden media should be a mixture of peat moss for water retention and composted steer or chicken manure for fertility. These cannas will even grow in standing water in the right soil.
Regular garden cannas can be adapted to wet sites. Each canna should be grown in a small pot until at least three or four healthy leaves have emerged. Planting too early can cause root rot. Setting the pot into a container with one or two inches of water for a few days will allow the root system to acclimate. Although they make great marginal and bog plants, garden cannas should never be planted in an area where the crown of the plant is fully submerged.
Cannas grow well with other marginal plants, providing height and structure to the bog garden. Plants such as taro and papyrus provide foliage contrast, and smaller marginals such as variegated calamus and lobelia have brightly colored leaves and flowers. Garden canna varieties with purple leaves look particularly stunning planted with variegated or chartreuse bog plants.
Water cannas are hardy in zones 7 through 11. Overall cold tolerance can change dramatically depending on how the plant is grown. As much as cannas require ample summer water, moisture during the cold of winter can cause root rot and death. Usually in zone 9 and warmer, cannas may be safely left in the ground. In all other growing zones, the safest method is to pot them up and store them for the winter.
Garden cannas grown in areas that are relatively dry during the winter months can be left in the ground with a heavy layer of mulch over the crown of the plant. An extra layer of plastic topped with burlap or canvas will keep winter rains from seeping in and possibly freezing.
In autumn after the foliage has died back, dig up the cannas from their bog garden bed. Moist soils can create long and brittle roots, so use caution when handling. Place the plants in a pot large enough to accommodate the roots, and fill with lightly moistened peat moss. Cannas appreciate winter humidity. If the media surrounding the roots becomes too dry in storage, the rhizomes can wither and die.
Store in a cool place that does not experience freezing temperatures. A garage, porch or chilly closet should suffice. Check periodically through the winter months for mold and moisture levels. Rhizomes with soft spots and discolorations should be removed and discarded.
Starting in late April or early May, growth buds should begin to appear on the stored roots. The plants should not be moved out to the garden until after the last frost. A warm spring can initiate fast, spindly growth on stored plants. Moving them into a sunny area and returning them to shelter if frost threatens will keep them green and healthy.