Although its frayed petals top 48-inch-tall stems, canna lily (Canna x generalis) is known to withstand heavy wind without staking support. The blossom colors include red and yellow, and they may appear splotched in some varieties during the summer flowering period. The perennial, however, shows clear signs and symptoms of die-back from pests, pathogens and natural life cycles. Canna lily is hardy in U.S. Department of Agriculture plant hardiness zones 8 through 10 or 11.
Although typical garden pests, from snails to spider mites, may negatively affect canna lily periodically, they do not cause severe damage or death to the plant. A specific caterpillar, called the lesser canna leaf-roller, actually uses the flower and foliage stalks as hiding spaces. When new leaves emerge, they are tightly wound. The caterpillar uses that tight space to form protective webbing, where it hides, feeds and reproduces. Because the pest isn't seen easily, your canna may succumb to long-term feeding as it shows signs of growth stunting and wilting. Foliage cannot photosynthesize when it has been damaged by a caterpillar or when it cannot open to receive full sunlight.
Canna lily grows from rhizomes, or underground stems. Growing up to 2 feet long, these storage organs must be in damp, but not soggy, soil. Rhizome rot causes canna die-back if soil drainage is poor. Bacterial blights and fungal leaf spots are also common canna pathogens. If you see reddish-orange bumps along the foliage, canna rust fungi have set in. Water your lily in the morning, watering directly at its root level; wet foliage left overnight encourages fungi pathogens to reproduce rapidly. An affected canna lily is overcome slowly by infected blisters and turns black as it dies back.
You may be alarmed to see canna lily flowers fading after only one day, but canna does not have long-lasting blossoms. Flowers fading is not a sign or symptom of death but a normal flowering cycle for canna lily. Once a flower opens and begins to fade, another blossom unfurls on the same spike. As the spike fades, another spike appears from the long rhizome below. You should see continual flowers from summer to late fall. Encourage more flowers by pruning spent blossoms off the spike; that task improves the overall aesthetics of the plant as well.
Canna lily naturally dies back when cold weather arrives. Its foliage yellows and wilts then. Do not remove the foliage until it has a papery texture. Keeping the foliage in place until then allows the canna to photosynthesize and store energy for next year's flowers and foliage. If your canna has grown well consistently for about four years, dig up its rhizome, divide it and replant the sections separately. When spring or summer arrives, canna lily has a reinvigorated growth period; a large rhizome clump shows signs of poor growth because it is too crowded to produce numerous blossoms.
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