Canna After the first light frost turns foliage brown, cut plants down to 6 inches and dig up rhizomes, corms or tubers. Rinse, separate "bulblets" and allow to air dry completely, then place in peat moss in a box in which you've cut out some holes for ventilation. If you have many, use a milk crate or other large ventilated box. Place in a cool, dark place, such as a crawl space or cellar. Check monthly and spritz with water if they look like they're starting to shrivel. Discard rotted roots. Plant outdoors around Memorial Day, or to give them a head start, pot them up in potting mix in April and set them by your sunniest window or under grow lights, keeping the soil lightly moist. Transplant outdoors in May.