Things You'll Need
Herbicide with glyphosate
The tropical foliage and brilliantly colored flowers of the canna (Canna spp.) are a beloved in gardens that have warm, moist and fertile soils. However, in mild winter regions the canna will spread its rhizomes indefinitely if growing conditions are ideal, potentially becoming a weed. Drying the soil, decreasing soil fertility, digging rhizomes and applying herbicides are ways to rid a landscape of canna.
Dig out canna plants with a shovel. 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.
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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.
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.
Repeat steps 1, 2 and 3 in patches of the garden that are particularly ridden with canna rhizomes. Steps 2 and 3 require the least amount of hard labor, and repeated tandem attacks of dry soil environments with herbicide should kill the canna without a need for physical plant removal.
If you live in a harsh, cold winter region, the canna will likely be killed by the frozen soil temperatures. In USDA Hardiness Zones 1 through 6, chances are the cannas will be killed each winter.
Herbicides with glyphosate are only effective if applied to canna foliage and stems; the chemical cannot be applied to the soil.
Glyphosate is a non-selective herbicide, meaning that it will kill any plant that is sprayed with the chemical.
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.