A tropical-looking garden filled with colorful canna lilies (Canna x generalis) takes regular fertilizer, consistent moisture, disease and pest control and some winter care. With the right care, canna lily plants, which grow in U.S. Department of Agriculture plant hardiness zones 7 through 10, reward your efforts with large, colorful flowers on erect, leafy stalks up to 8 feet tall. The foliage, which comes in bright green to deep purple, adds a visual element of its own.
Fertilizer and Water
Feed canna lily plants once a month with 5-10-5 fertilizer. For a 25-foot garden bed, use 1/2 cup of granular fertilizer. Sprinkle it onto the soil around the base of each canna lily then water the garden bed. Provide about 1 inch of water per week. If it has rained, cut back on watering. Soil drains at different rates depending on the structure of the soil and the climate. Aim to keep the soil consistently moist but not waterlogged and adjust watering to keep it moist.
Deadheading and Trimming
Cut down each flower stalk as the final flowers start to die. A pair of sharp pruning shears work to get through the fleshy stalk. Make the cut at the soil line. After the stalk flowers, it produces seeds and starts to die. Cutting out the spent stalks encourages more flower stalks. Dip your tools in a solution of equal parts rubbing alcohol and water to disinfect them to prevent disease spread.
You can grow canna lilies outdoors even if you live in USDA zone 6b or below. In fall, cut back all the foliage and any remaining flower stalks and dig up the rhizomes. Keep them in a dark, cool spot, ideally between 45 and 50 degrees Fahrenheit until spring. In USDA zones 7 through 10, you can leave canna lilies in place year-round but remove any dead foliage as it appears at the base. In areas where winter frosts are common, canna lilies die back to the ground. Cut out the dead foliage before spring but leave the rhizomes in place.
Several pests find canna lily leaves delicious. Watch for slugs, caterpillars and snails. Pick them off by hand. Spider mites -- tiny sap-sucking insects that weave a fine webs over affected leaves -- can also affect cannas. Spray affected canna lily leaves with water to wash off the spider mites. If they persist, use insecticidal soap spray -- look for a ready-to-use product -- and spray the spider mite infestation. Spray both sides of each affected leaf. To avoid damage to your canna lily, pick a day when the temperature is below 90 degrees Fahrenheit and spray early or late in the day when it's not in the sun. Follow up every four to seven days until the pests disappear. While soap sprays aren't highly toxic, avoid getting the product in your eyes. If it gets on your skin, rinse the area.
Two fungal diseases can affect canna lily plans: rust and fungal leaf spot. Rust appears as light spots on the leaves, often yellow or rust colored. Fungal leaf spot causes small damaged spots where the leaf dies. Cut off affected leaves as soon as you spot a problem. Treat rust and leaf spot with fungicide that contains copper compounds if they don't disappear after removing affected leaves. Use 2 fluid ounces of concentrated mix in 1 gallon of water. Spray the leaves, making sure to saturate both sides, every seven to 10 days until the the condition clears up. Keep people, children and pets away from cannas until the fungicide dries completely. Avoid spilling or getting it on your clothes or skin. Wash any areas that come into contact right away. When you're done with the bottle, punch a hole in the bottom so no one can put consumables in it.
Root rot, caused by soggy growing conditions causes overall decline. If you suspect root rot, dig up a rhizome and look for mushy roots. Mild cases can improve with reduced watering, but if the roots are rotten through, replace the canna lily plants with fresh rhizomes.