Canna is a genus of tropical plants that contains about 10 species. The plants, which develop from tuberous rhizomes instead of real bulbs, are broadly used as ornamentals, with most popular hybrid varieties sturdy in U.S. Department of Agriculture plant hardiness zones 8 though 11. In areas that have cold winters, cannas are treated as annuals, with the rhizomes lifted and stored at the end of each growing season and replanted at the beginning of the next growing season. Killing in-ground canna rhizomes isn’t difficult.
Before ruining your cannas, consider why you would like to achieve that. If they’re healthy and are in a garden area where you wish to plant something else, consider moving them or growing them in containers. Should you decide you don’t need cannas on your garden or don’t have room for them, then get the word out to gardening friends or gardening classes. Other gardeners may be delighted to dig up the cannas and provide them good homes elsewhere.
The simplest way to get rid of cannas that are already at the ground — particularly if they’re in a committed plant bed — would be to smother them. This way is done best at the end of the growing season, but it might be performed at any given moment. If foliage is present on the cannas, cut it off at ground level. Cover the cannas’ area with landscape fabric or at least a 2-inch thick layer of paper. Cover the landscape fabric or paper with a layer of organic mulch. Check regularly for signs of new growth; if it appears, remove it at ground level.
In case the cannas are in crowded combined beds or borders, smothering them might not be an alternative. In these tight situations, digging is the only way to get the cannas out of the ground. Use a sharp-pointed trowel to dig beneath the rhizomes, taking care to find the whole roots out of the ground. Offshoots left behind may sprout new growth. If digging isn’t an option, cut off fresh canna increase and leaf since it appears. Doing this repeatedly probably will kill the plants.
Sometimes cannas can be infected with the mosaic virus, which causes misshapen leaves and yellowish streaking or a mosaic pattern on their foliage. The issue has no treatment, and the affected plants have to be destroyed. That circumstance is another instance where it’s ideal to dig up the plants, making sure to get every bit of each rhizome out of the ground. Don’t include the infected plants or rhizomes to your compost pile.