Organic options

Many gardeners turn to natural herbicides to wipe out pesky plants

Tony Bertauski
The Post and Courier

Two hours after being applied to weeds, the natural herbicides (from left) Scythe, soap and vinegar have caused the weeds to collapse and become discolored, compared to the control sample on the right.

'Natural' does not always mean safer.

When I was 14, I spent the summer working in soybean fields. We were given hooks fastened to the end of wooden poles. The hooks were rusty with a lethally sharpened edge.

Supervised by two 20-something farmhands, each kid walked several rows hooking out pigweed, smartweed and any other thing not a soybean. And what we hooked, hopefully, did not bleed. This was natural weed control.

We started at 6 a.m., stomped along the rows with 15 pounds of mud on each shoe and got soaked from the waist down from morning dew. We were bored by 6:05 a.m.

By 7:30 a.m., we were groundhogging through the rows on our hands and knees, popping up to drill each other with dirt clods. By 9 a.m., we were swinging at butterflies.

Occasionally, a hook slipped from someone's grip and helicoptered over our heads.

The 20-something farmhands took the hooks away the following summer and handed us squirt bottles of Roundup herbicide. They said, "Don't squirt each other." But we were 15. And bored.

The hooks came back the next summer.

Natural vs. synthetic

Synthetic herbicides, as a group, are more hazardous than natural weed killers. Many organic gardeners want weed-control alternatives to synthetic pesticides. Research indicates natural herbicides are significantly more expensive than synthetic, which limits large commercial use. However, they are useful for gardeners.

Most natural herbicides are nonselective, meaning they kill or damage everything sprayed. Many products dehydrate weeds by stripping the waxy cuticle from the leaf or disrupting cell membranes. In 70-degree weather, plants wilt in a couple hours. In hot, dry weather, effects are noticeable in 15 minutes. Because they kill only foliage and not roots, natural herbicides mostly are effective on annual weeds. Perennial weeds, such as dandelion and dollarweed, will grow back.

Natural products can cause mild reactions when exposed to them, but they break down in the environment rapidly, reducing the negative impacts associated with synthetics.

Refer to the Organic Materials Review Institute (www.omri.org) to see which products are approved for organic gardening. Web sites such as www.biconet.com and www.planetnatural.com carry many organic natural products.

Soaps

Soaps used to control weeds are potassium salts of fatty acids. The fatty acids are derived from vegetable sources, such as palms or coconuts. Potassium salts of fatty acids also are approved by the U.S. Food and Drug Administration as food additives.

You might be most familiar with insecticidal soaps that dehydrate insects by stripping their cuticle. Herbicidal soaps work the same way. Some soap products list control of algae, moss and lichens as well.

Although soaps are synthetic, brands such as Safer and M-pede are approved by the institute for organic use.

Essential oils

Clove oil and cinnamon oil contain eugenol, which kills weeds by disrupting cell membranes, leaking out contents. Matran and EcoExempt are commercially available products and are approved by the institute. Weed Zap is a combination of clove and cinnamon oil.

Citrus oil is another institute-approved herbicide. The active ingredient is limonene, a popular hand cleaner and degreaser that has been used as a food additive. Its high volatility gives products a citric scent. However, the volatility limited its use as an herbicide. Products such as Nature's Avenger have overcome the volatility issue to be effective herbicides.

Vinegar

Acetic acid is the active ingredient in vinegar that kills weeds, similar to essential oils. However, this is not your household vinegar. Tests indicate weed control is effective only when concentrations of acetic acid are boosted above 20 percent. Household vinegar contains 5 percent acetic acid.

Some vinegar products containing concentrations greater than 20 percent acetic acid do not list "herbicide" among their uses, even though they can be used as such. Acetic acid does have other uses, such as a cleaning agent and lowering water pH. Products that don't list it as an herbicide are able to avoid the costly process of Environmental Protection Agency registration. Be aware that products with concentrations over 11 percent can burn your skin and severely injure the eyes. Labels on products that are not listed as an herbicide will not be required to tell you that.

Products such as AllDown and Burnout II contain vinegar concentrate in addition to other essential oils. Bradfords produces a 20 percent acetic acid product. AllDown boasts organic certification from other agents but not by the Organic Materials Review Institute. The others did not appear on the institute database.

Pelargonic acid

Pelargonic acid is a short-chain fatty acid naturally found in plants and approved by the FDA as a food additive. However, it is synthetically produced as an herbicide commonly sold as Scythe. It is a very effective, fast-acting, nonselective herbicide. It is not organic, but, rather, considered a biochemical because of its natural origin.

When comparing vinegar, soap and Scythe in our pesticides class at Trident Technical College, Scythe was the winner although its use is still most effective on annual weeds.

Corn gluten

One of the most exciting organic herbicides is corn gluten. It is a pre-emergent herbicide that prevents weed seeds from growing. It is not a weed killer (post-emergent). Pre-emergents have to be applied before weed seeds germinate, so timing is critical. In the Lowcountry, applications are made March 1, Oct. 1 and Dec. 1. However, corn gluten needs to be applied four to six weeks before target dates, so, for the October application, corn gluten should be applied in late August.

Corn gluten contains 10 percent nitrogen. March is too early for fertilizer, and December is too early, unless overseeded. That means the only practical use for corn gluten is the August application that can be used to fertilize and pre-emerge. It can also be used May 1 as the first summer application and extend summer weed control.

One last caveat concerning corn gluten. The first year of use, expect only 50 percent weed control. The second or third year you can expect better than 90 percent, the equivalent of synthetic products.

Learning curves

I was never injured walking beans, and I earned a whopping $3.35 an hour. In a way, it was like summer camp except there was no swimming or games or campfires.

However, I learned a lot from the 20-something farmhands. They knew a lot about birds and bees. At least, they thought they did.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback or request specific column topics, e-mail Tony at tony.bertauski@tridenttech.edu.