Canna are herbaceous perennials of tropical origin. They have a rhizomatous rootstock and they spread slowly outward from where they are planted. Each individual stem consists of a central herbaceous stalk with 10 to 12 leaves arranged alternately or spirally on it. Each plant may be 2′ to 3′ wide. In nature, the plants tend to be quite tall (7′ to 16′) but many shorter selections have been made for gardens. Once the plant has 6 to 9 leaves, it forms an inflorescence at the tip.
Canna Rhizome
The rhizome is a thickened underground horizontal stem that sits just below the soil surface. As it elongates, it produces shoots that grow upward to produce leaves and flowers and fibrous roots at each node. The thickness of the Canna rhizome varies by species. The cultivars grown for food have very thick rhizomes, but some species (especially the water-loving species) have thin or wiry rhizomes. When a person purchases a bare-root Canna rhizome they should expect to receive a section that has at least 3 prominent stalk buds (eyes) on it. In some species, the rhizome grows up to 24″ (60cm) long and has many branches. Eventually the older parts of the rhizome die off naturally.
Canna Leaves
Canna leaves are large, banana-like, tropical-looking and bold. Most cultivars have rich emerald-green leaves. However, there are also purple/red leaves and variegated leaves. The purple or red leaf color is usually quite dark and may cover the entire leaf, just the outside margin, or occasionally just the midrib. Variegation in Cannas has two forms. It may consist of white or red splotches/sectors on green leaves (e.g., Canna ‘Stuttgart’). Or, it may consist of narrow stripes of color in-between the minor veins (e.g., Canna ‘Phasion’ or Canna ‘Bengal Tiger’).
The leaves arise alternately (1/2 phyllotaxy) or spirally (1/3 phyllotaxy) from the stem. When they first emerge they are rolled up and unfurl over the course of a day or two (unfurling occurs only at night). The leaves are broad and flat…roughly 6″ wide and 1.5′ to 2′ long. Sometimes the leaf edge (margin) may be wavy. The leaves are generally waxy (glaucous) and may have a dull or shiny finish depending on the type of wax. Cannas are monocots, so the leaves have a prominent midrib that is pinnately veined with many medium or fine side veins. The Water Canna cultivar group generally has very narrow leaves compared to most others. The leaves have rounded sides that taper to a point at the tip (acute or short acuminate). The leaf blade tapers gradually into a sheath that merges with the stem and thus there is no petiole.
Canna Fruit and Seed
The Canna fruit forms from a warty ovary that subtends the floral parts. The fruit swells after ferilization and looks somewhat like a small horse-chestnut. When mature, it turns brown and splits open (dehisces) along 3 seams revealing pea-sized brown or black seeds that are exceedingly hard. The embryos are mature at dehiscence but the seed will not germinate because the seed coat is extremely thick (prevents imbibition) and contains germination inhibiting chemicals. This thick seed coat allows Canna seed to survive for a very long time. In 1969, Canna indica seed was found in a 550 year old archaeological dig in Argentina and was successfully germinated. The reason that the seed coat may be so thick is that fire plays a part in Canna seed germination in its native habitiat. In the wild, Canna seed germinates best in places burned by fire. Fire not only weakens the seed coat, but destroys any competition for the emerging Canna seedling.