The easiest way to eliminate cannas that are already in the ground -- especially if they are in a dedicated plant bed -- is to smother them. This method is done best at the end of the growing season, but it can be done at any time. 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 newspaper. Cover the landscape fabric or newspaper with a layer of organic mulch. Check regularly for signs of new growth; if it appears, remove it at ground level.

If the cannas are in crowded mixed beds or borders, smothering them may not be an option. In such tight situations, digging is the only way to get the cannas out of the ground. Use a sharp-pointed shovel to dig under the rhizomes, taking care to get the entire roots out of the ground. During the growing season the rhizomes or underground swollen stems, are easily located and dug up. Shake off excess soil from the dug rhizomes and place them in full sun to thoroughly dry and die. Offshoots left behind may sprout new growth. Canna rhizomes prosper in warm, moist conditions, including a compost pile. Make sure the rhizomes are fully dry before tossing them into the compost or landfill, especially in mild winter regions where they may sprout and become abundant once again. Reduce remnant re-sprouts of canna rhizomes by creating a soil environment unfavorable for canna growth. Rake away moisture-retentive mulch atop the rhizomes. Do not irrigate the ground or add fertilizers, compost or other organic matter. In essence, you are trying to starve the remaining plants. If digging is not an option, cut off new canna growth and foliage as it appears. Doing so repeatedly probably will kill the plants.

Drying the soil, decreasing soil fertility, digging rhizomes and applying herbicides are ways to rid a landscape of canna.

Last resort: Spray an herbicide with the active ingredient glyphosate according to product label directions onto canna foliage. Roundup® is a product with glyphosate that is appropriate to use in garden beds. However if the canna grows on the margins of a pond, lake or stream, use Glyfos Aquatic®, GlyphoMate® 41 or other glyphosate-based herbicides formulated for use in aquatic environments. Be sure to read the label carefully before using this method. Glyphosate is a non-selective herbicide, meaning that it will kill any plant that is sprayed with the chemical.

Hope this helps!