Issue: April 10, 2004

Vinegar as herbicide

Do you know of any solution that I can spray on thistles to kill them? I've heard about vinegar? Does it work?

Several years ago I heard that vinegar was a good herbicide. I tried it at home without success and filed that information away in my mind until recently when I learned of a research report from the U.S. Department of Agriculture. USDA researchers were studying the use of acetic acid (a component of vinegar) as an herbicide. Their report explained why my attempt failed.

USDA research indicated that at low concentrations acetic acid had variable success in controlling weeds, and then any success was only when the acetic acid was applied to young weed seedlings (less than two weeks after germination). I had used kitchen vinegar with a concentration of 5% acetic acid on mature plants. To kill adult weeds, the concentration of acetic acid needed to be much higher. At higher concentrations, the results indicated that 85-100 percent of the weeds treated were killed. This has not been tried on all weed species.

Some nurseries are now selling herbicides made with vinegar at higher concentrations of acetic acid, or other substances that qualify as organic weed controls. Sometimes these materials are combined.

To succeed it is advisable to purchase the vinegar at the nursery/garden center that is made and labeled for weed control. Can you use the kitchen vinegar? Well, that is a violation of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), so I wouldn't recommend that you use your kitchen vinegar. In addition, kitchen vinegar would be less effective than that made specifically for weed management. Keep the kitchen vinegar in the kitchen.

Another factor to consider in New Mexico and the rest of the Southwest is that our dry climate results in rapid production of a waxy cuticle to protect plant leaves, especially weed leaves. This coating prevents the absorption of vinegar by the plant. Newly germinated seedlings have a thinner waxy cuticle protecting their leaves. This is also true of plants growing rapidly (after a rain or irrigation and fertilization). Plants with a thinner protective cuticle are more susceptible to treatment with vinegar herbicides. To maximize your success, water and fertilize the weeds before treatment to encourage rapid, unprotected growth.

Vinegar is not translocated from leaf to root in plants, so it will only burn the top from perennial weeds with a well-developed system of roots or rhizomes below the soil. These plants will quickly re-grow after treatment. Frequent re-treatment may allow you to manage these perennial weeds by eventually depleting the reserve of food stored in the roots of these plants.

Can the herbicide vinegar be used in the kitchen? ABSOLUTELY NOT! These products are labeled for agricultural use, not food preparation. They are much more concentrated and even with dilution are not to be used in the kitchen.

While there are other products that may also be used for weed control in the garden, vinegar is very appealing to organic gardeners. Many of these products are manufactured in a manner to maintain their status as "organic". Read the label to be sure; however, remember that they can also be dangerous if used improperly. This danger applies to you as well as to non-target plants. These products contain acetic ACID, and it behaves as an acid. It is corrosive. As with any agricultural chemical, read, understand, and follow the directions on the label.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!