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Home  > Question of the Week

Question of the Week



Permalink Is there a low-toxicity herbicide that kills thistle as well as thistle roots?

Answer: Research has suggested that application rates of 20 to 160 gallons per acre of 20% acetic acid solution applied in combination with additives such as orange oil, non-ionic surfactant, and crop oil concentrate can be effective for controlling weeds. However, the addition of the additives were effective only in the low application rate. Higher application rates did not show improved control than lower application rates. Broadleaf weeds are controlled more easily with acetic acid than are grassy weeds.

Low-toxicity herbicides are available from several suppliers. Scythe, produced by Dow AgroSciences, is made from fatty acids. Scythe acts fast as a broad-spectrum herbicide, and results can often be seen in as little as five minutes. It is used as a post-emergent herbicide, sprayed directly on the foliage. It is primarily a burn-down herbicide, has no residual activity, and is not effective on non-green, woody portions of plants.

Vinegar is an ingredient in several new herbicides on the market today. Burnout and Bioganic are two available brands. Both of these are post-emergent burndown herbicides. They are sprayed onto the plant to burn off top growth—hence the concept "burndown." As for any root-killing activity with these two herbicides, I cannot say. The label on Burnout states that perennials like thistle may regenerate after a single application and require additional treatment.

Researchers in Maryland tested 5% and 10% acidity vinegar for effectiveness in weed control. They found that older plants required a higher concentration of vinegar to kill them. At the higher concentration, they got an 85 to 100% kill rate. A 5% solution burned off the top growth with 100% success. Household vinegar is about 5% acetic acid. Burnout is 23% acetic acid. Bioganic contains 10% acetic acid plus clove oil, thyme oil, and sodium lauryl sulfate. AllDown contains acetic acid, citric acid, garlic, and yucca extract. Matran 2 contains 50% clove oil. Vinegar is corrosive to metal sprayer parts—the higher the acidity, the more corrosive. Plastic equipment is recommended for applying vinegar.

According to a study conducted in California by the UC Statewide IPM Program comparing several non-synthetic herbicides with Roundup Pro, the following herbicides might prove effective in controlling broadleaf weeds like thistle.

Eco-Exempt is a contact, non-selective, broad spectrum, foliar-applied herbicide that will only control actively growing green vegetation. The active ingredients are 2-phenethyl propionate and eugenol (clove oil). 2-phenethyl propionate is considered a minimum risk pesticide by the EPA, is exempt from EPA pesticide registration (as are the following products, with the exception of Roundup Pro), and is in the same risk classification as cinnamon oil, citric acid, clove oil, and corn gluten meal. In a California study, Eco-Exempt was reported to have minimal effect on broadleaf weeds (including thistle).

Matran 2, like Eco-Exempt, is a contact, non-selective, broad spectrum, foliar-applied herbicide that will only control actively growing green vegetation. The active ingredients are clove leaf oil and wintergreen oil.

For more information on reducing weeds, consult the ATTRA publication Sustainable Weed Management for Small and Medium-Scale Farms. This publication discusses several strategies, both proactive and reactive, as alternatives to conventional tillage systems. Options include mulching, competition, crop rotations, and low-toxicity control alternatives.

Note: The mention of specific brand names is for educational purposes only and does not constitute endorsement by NCAT, ATTRA, or USDA.

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