Problem oxalis species include wood sorrel (Oxalis corniculata) which grows in U.S. Department of Agriculture plant hardiness zones 4 through 8, and Bermuda buttercup (Oxalis pes-caprae), which grows in USDA zones 8 through 11. Oxalis weeds have either delicate, long roots or tangled bulb roots that easily break off, remain in the soil and grow into new plants. Use the right methods to get rid of these weedy oxalis while protecting your garden's plants.
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Remove Established Plants
Hand-pulling and hoeing can help get rid of established oxalis weeds. Make sure you remove all of the rhizomes -- the underground stems -- along with the leaves and stems. Tiny pieces of rhizome that stay in the soil can grow into new plants. Pulling the weeds by hand is easier when the soil is damp, but grub out weeds with a hoe from dry soil. When using a hoe, be sure you dig deeper than the plant's root system to get all pieces of the plant.
Bermuda buttercup spreads through small root bulbs. Kill established Bermuda buttercups by repeatedly cutting off the tops of the plants before they flower. This deprives the weeds of the nutrients needed to regenerate and propagate. Dispose of all oxalis plant debris in a plastic trash bag and discard it in a covered trash can. Don't just leave pulled weeds laying on the ground or the small pieces might root or grow.
Large patches of oxalis weeds or those throughout a lawn might be tough to dig up, so it may be better to use a chemical weedkiller. Spray before the plants flower and seed to prevent future infestations. Effective chemicals include 2,4-D, dicamba, triclopyr, fluroxypyr, 2,4-DP and mecoprop-p. Many herbicide manufacturers sell those active ingredients in ready-to-use combinations. Following the label instructions, uniformly spray the herbicide over all the oxalis weeds. One lawn product recommends giving your lawn 1/4 to 1/2 inch of water about 60 minutes after treatment to help the chemicals work down into the soil. Repeat the application in 14 days, if necessary. Avoid mowing for 24 to 48 hours after treatment, if you use it on the lawn. Leaving treated grass clippings on the lawn can help reduce the number of weeds in your yard, but don't use the treated clippings as compost or mulch in vegetable gardens and flower beds.
Once you get established oxalis weeds under control, prevent any seeds already in the soil from germinating by using a preemergent herbicide containing pendimethalin. Read and follow the instructions on the product's label for effective, safe weed control. One product recommends mixing 1 to 2 ounces of pendimethalin concentrate and 1 gallon of water for every 1,000 square feet of lawn. Use a small garden sprayer to treat your lawn, and repeat application after five to eight weeks, if necessary.
Playing It Safe
Before using any type of herbicide on your lawn, read the label carefully to make sure the chemicals can be used on the type of grass you have in your lawn. Some herbicides remain active in the soil, so take care not to spray around any tree and shrub roots in your yard. Only use weed killers on calm days so the chemicals don't drift onto desirable plants. Keep people and pets off a treated lawn until the herbicide solution totally dries. Reduce your risk of chemical exposure by wearing goggles, latex gloves, a face mask and protective clothing when mixing and spraying herbicides.