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Organic Weed Control Just Became Difficult Again

Loss of an organic herbicide sets organic growers back to where they used to be
by Mark Greenspan
April 06, 2021


Whether an organic grower or not, weed control using herbicides has been quickly falling out of fashion. In the earliest part of this century, foul-smelling pre-emergence herbicides were being eschewed for their potential destruction of soil-borne life and potential danger to higher animals despite their long-lasting ability to control unwanted vegetation. Growers were proudly proclaiming “Roundup-only” weed control, with the belief that the softer chemical glyphosate would be an acceptable way to control weeds without long-term degradation to the soil and to humans and higher animals. And frankly, glyphosate was a wonder drug for weed control, as it translocates throughout the plant and not only kills the foliage, but also the roots, providing long-term control for weeds, theoretically without danger to animals, humans or the environment.

But since the Roundup-only movement started, glyphosate has been given a couple of dark, bloody and swollen black eyes, whether deserved or not. The public simply does not want growers to use glyphosate, feeling it to be unsafe and potentially carcinogenic to humans. While glyphosate’s true hazard to humans remains in question, the general public has taken a firm stand against it and numerous wineries are steadfast in their stance against growers using any of that herbicide. While glyphosate may or may not be as dangerous as the general public seems to be convinced of, there is reason to be concerned about its impact on soil health. Glyphosate blocks the shikimic acid biochemical pathway in plants. That’s convenient since humans and higher animals do not have such a process and, hence, the chemical can kill plants without harming humans. But bacteria do have a shikimic acid pathway and soil bacteria, including the countless beneficial ones, can potentially be damaged by glyphosate.

So, if pre-emergent herbicides and glyphosate were off the table, the only alternatives for conventional growers were and are contact herbicides, which kill the foliage they come in contact with, but do not translocate within the plant. This means that, if the root system were strong enough, the weed could re-grow and require either re-application of herbicide or mechanical removal.

The Holy Grail?

It seemed like we finally got what we needed for with a product called Weed Slayer. Introduced late in 2017 its active ingredients were eugenol (clove oil) and molasses. It was packaged with a companion product called Agro Gold® WS, which was an activator-like product required to be applied in the same tank as the herbicide. After its introduction, it was pulled from the market for a brief period of time because the Agro Gold product was proprietary and its contents needed to be disclosed before it could be listed as organic. The secret ingredient was reportedly Bacillus thuringensis, a bacteria commonly used as an organic insecticide. The activator differentiated Weed Slayer from other eugenol products in that it allowed for entry into the plant by the herbicidal ingredient.

No, Just a Beacon

It was what we all had been desperately seeking: an organic herbicide that not only worked but worked well. Visually, it behaved very similarly to glyphosate. That is, it took a couple of weeks before any symptoms of kill occurred. After that, weeds would turn chlorotic and die. It wasn’t perfect and it was finicky as to selection of spray adjuvants. We used it in our farming company and found that it worked very well. Excited by those results, I heartily recommended the product to our clients who farm organically, and it seemed like most of my colleagues in the industry were also on the bandwagon.
But, if something seems too good to be true, it often is just that.

Continue reading "Organic Weed Control Just Became Difficult Again," in the April 2021 Wine Business Monthly 


by Mark Greenspan  


Dr. Mark Greenspan has more than 30 years of scientific viticulture research and viticultural field experience. He specializes in irrigation and nutrition management, yield and canopy management, vineyard climate and microclimate, vineyard design and vineyard technologies. He is the founder of Advanced Viticulture, Inc. based in Windsor, California (, providing consulting, technology and automation, vineyard management and vineyard development for wineries, winemakers and wine growers devoted to producing premium wines. 

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