How to Kill Tuberous Rhizome Weeds

Keeping a garden free of weeds is a tough enough challenge without having to contend with an underground network of hard-to-kill rhizomes that can lie dormant for years before sprouting again. Eradicating these stubborn weeds takes patience and persistence, but if you understand what they need to survive, you can take steps to stop them dead in their tracks.

Not all weeds grow from rhizomes or form tubers, but the types that do are more persistent and harder to kill. With a reputation as "the world's worst weed," purple nutsedge (Cyperus rotundus), a perennial in U.S. Department of Agriculture plant hardiness zones 3a through 10b, and its invasive cousin, yellow nutsedge (Cyperus esculentus), which grows in USDA zones 8a through 10a, are named for their nutlike tubers. These weeds spread by rhizomes, or horizontal stems, that creep along the ground, just under the soil surface. Along these rhizomes grow tubers, which store food for the plants and form underground colonies. Some colonies may grow to more than 10 feet in diameter, with one mother plant capable of producing 7,000 tubers in one year. The key to killing tuberous nutsedge weeds is not to simply kill the grassy growth above ground -- because new growth can sprout from the tubers -- but to eradicate the tubers.

All weedkillers don’t kill all types of weeds. If you use a product labeled for grassy or broad-leaf weeds, it will not effectively travel through a sedge weed to kill the formidable tubers. Using a chemical that translocates, or moves through a plant, all the way to the tubers is an effective strategy. Products that contain the chemicals imazaquin, halosulfuron and bentazon may kill tuberous weeds, but they may also kill the roots of nearby plants. According to the University of California Statewide Integrated Pest Management Program, glyphosate is the only nonselective chemical available to homeowners that provides effective post-emergent control against nutsedge. Glyphosate requires repeat applications and may not be completely effective.

Glyphosate is most effective when it is applied to young plants that are actively growing. After the plants are full grown, their tubers are also mature and the chemical cannot move from the leaves to the rhizomes to kill them. Apply a ready-to-use premixed product that contains glyphosate on sunny days by spraying tuberous weeds until they are thoroughly wet. The chemical will move more quickly through the plants when the temperature is above 60 degrees Fahrenheit. Dormant tubers, or tubers that are unattached to the foliage you spray, will not be killed. As more weeds sprout from these tubers, you must reapply glyphosate and continue this treatment until the weeds are under control.

A nonselective herbicide, glyphosate can kill any plant it touches, including trees. If you spray tuberous weeds on a windy day, the wind can blow the chemical onto nearby plants. Make sure your prize-winning flowers aren’t downwind of the weeds you’re spraying. If you prefer, use a paintbrush or sponge to paint or dab the chemical onto the weeds, which eliminates the drift from spraying. Or, cover your nearby plants with cardboard or a tarp while you spray. Wear gloves and goggles when you apply herbicides to protect your skin and eyes. Keep children and pets out of the area until the chemical has dried.