Soil Temperatures for Canna Bulbs

Growth Threshold

• As tropical plants, cannas actively grow and prosper when growing conditions are adequately warm. Canna seeds won't germinate until the soil is at least 70 degrees Fahrenheit. The rhizomes do not actively sprout new roots or leaves unless the soil is no colder than 61 F. A dormant canna rhizome persists as long as the soil does not freeze. Cool soil temperatures along with slightly dry soil does not harm the rhizome when dormant.

Hardiness Zones

• Gardeners in the southern half of the United States may leave canna rhizomes outdoors in the ground year round. Here, in U.S. Department of Agriculture plant hardiness zones 7b and warmer, winters are neither cold enough nor have long-lasting bouts of subfreezing temperatures to kill canna rhizomes. Fall frost and freezes kill back all aboveground tissues. New growth emerges late in spring once the soil warms above 60 F. The warmer the soil, the faster and lusher the growth. In USDA zone 7, placing a 12-inch-deep layer of shredded leaf mulch over the soil ensures cold air temperatures cannot penetrate as deeply into the ground, thereby preventing rhizomes from freezing and dying.

Summer Growth

• Warm, moist soils prevent cannas from prematurely aborting leaves or stems or fully becoming dormant. Planting rhizomes outdoors too early in spring, when frosts still occur or the soil isn't at least 60 F, prevents growth and may lead to root rot if the soil remains too chilly and wet. Cannas grow their best when the soil is over 70 F at the 2- to 4-inch depth, or where the rhizome is situated. Soil temperatures in the 75- to 85-degrees range is ideal. The warmer the soil, the greater the need for moisture to prevent dehydration in all of the rapidly growing foliage.

Winter Storage

• In regions where outdoor winter air and soil temperatures are too cold for the survival of cannas in the ground, you must dig up and store rhizomes indoors. Dormant rhizomes are not overwintered in soil. Instead, the soil is crumbled away after digging and the rhizomes air dry. This prevents any fungal growth or mold to occur during dormancy. Place the dormant rhizomes in barely damp peat moss, sawdust or shredded newspaper. They need to be exposed to temperatures between 35 and 60 F. Temperatures that are too cold cause the rhizomes to suffer freeze damage, but temperatures above 61 F begin to induce root or shoot growth again.