Tropical in origins, canna lilies (Canna spp.) provide lush foliage and colorful flowers during the frost-free growing season. If you live where winter temperatures rarely drop below 10 degrees Fahrenheit, chances are the underground tubers of canna lilies will survive winter just fine and re-sprout in late spring. If your winters are colder, you need to dig up the roots and bring the plants inside where frost cannot harm them.
Allow the first fall frost to kill back the foliage of the canna lilies. In fact, because the root tubers are underground, you have up to one month past your first fall frost to dig up the tubers and get them indoors. You can dig them up as long as the soil is workable and snow or frost hasn't crusted the soil surface.
Lifting the Tubers
To better view the area, cut back all dead canna lily foliage with a hand pruners to a stem height of 3 to 6 inches. Pierce the soil with a garden shovel or potato-digging fork no closer than 10 inches from the stem bases and 360 degrees around the plant. This allows you to loosen the soil without cutting into healthy tubers accidentally. Dig deeply, too, to at least 6 to 10 inches, so you sever the small roots that emanate from the larger tubers to better lift them. With one hand on the stems and another jostling the soil, you can more readily pull tubers from the crumbling soil.
Prepping for Storage
Once the tubers are dug, gently break away any larger clumps of clinging soil. Move the tubers to a dry, well-ventilated area to further dry out of direct sunlight. Lay down a tarp or thick layer of newspapers in the garage or back garden shed to place the tubers, for example. Allow the clumps to gently dry for 1 to 3 days. After this drying, further break away soil off the tubers with your hands or by shaking the tuber clusters. Alternatively, you can hose the soil off the freshly dug tubers. Wet the soil with the garden hose, allow the soil to soak the water in for 30 minutes and return to washing the muddy soil off the tubers. Then, allow these cleaned tubers to air dry, again out of direct sunlight for up to three days.
Canna tubers appreciate a cool but slightly moist condition while in winter storage, between 40 and 50 degrees F. Use a box, large bucket or heavy plastic garbage bag and fill it with damp sand, sphagnum peat, coarse sawdust or vermiculite particles. Rest the tubers in the medium, nestling them so they are protected and slightly moistened by the medium. Place the bin of tubers in a dark location so there is no reason for the tubers to prematurely re-sprout leaves or stems over winter while dormant.
Monitoring Dormant Tubers
Every three to four weeks, check on the tubers. Mist the medium with water if it feels bone dry, but take care to only make it damp, not wet, when done. Examine tubers, looking for any signs of mold, disease, fungal rot or smelly, soft tubers. Immediately remove damaged tissue with a sharp knife and throw it away. Sprinkle ground cinnamon on cut wounds to deter fungal infection during the rest of the storage season. Any shriveled tubers should be discarded.
If you wish, you can lift and pot the tubers of cannas immediately from the garden in fall to spend winter as a houseplant in a sunny window. Use caution with this approach, as you don't want to bring in any insect pests in the plant foliage; carrying a pot of soil and canna lilies can be cumbersome. If you do bring plants indoors, keep the soil moist over the winter and provide abundant direct sunlight so the foliage doesn't become leggy or die back.