Canna is a genus of tropical, summer-flowering rhizomes that contains more than 55 species and cultivars of the plant, according to Washington State University. As tender tropicals, canna cannot tolerate temperatures below 25 degrees Fahrenheit. USDA zone 7 experiences temperatures well below freezing that hover between 0 and 10 degrees Fahrenheit, at their lowest. Protection from cold damage, along with adequate water and good soil nutrition, are keys to successfully growing canna in zone 7 climates.
Use in Container Plantings
Canna plants growing in USDA zone 7 need protection from the cold. The easiest way to achieve this is to grow the plants in large containers that can be moved indoors in the fall when temperatures drop. A garage, greenhouse or insulated porch with ambient temperatures between 41 and 50 degrees Fahrenheit will suffice.
Containers give you flexibility to place the plants in new locations as you like. This management method is also a time-saver, preventing you from having to dig up the rhizomes in fall, package and store them and then replant the bulbs in spring.
Dig and Overwinter Bulbs Indoors
Growing canna in ground plantings will require harvesting the rhizomes from the soil in early fall, and storing them indoors in a protected location until mid-spring, when the soil has thawed and warmed and ambient outdoor temperatures have warmed to 60 degrees Fahrenheit or so.
Cut back all of the foliage in the fall and compost it. Dig the rhizomes, gently removing the bulk of the dirt around them. Nestle them in a container filled with dr,y shredded peat moss or vermiculite. Place the container in a spot where temperatures remain in the range of 41 to 50 degrees Fahrenheit.
Water and Fertilize Well
Cannas thrive in moist, nutrient-rich soil. As the growing season is shorter than the plant is designed for, water and fertilizer help speed the growth of the plant and deliver the longest, lushest bloom cycle possible.
Water the plants once a week, deeply, to keep the soil very moist. Never allow the soil to dry out beyond the top 1/2-inch crust. Feed the plants once a month from spring through fall with a complete fertilizer designed for flowering plants with a guaranteed analysis of 10-10-10 or 5-10-10. Apply according to the product label dosing directions and water in well with each feeding.