Why Every Gardener Needs to be Aware of
Persistent Herbicides

Written by BCEMC Master Composter, Mary Green, November 2021, Revised April 2023

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The purpose of this document is to share information with fellow gardeners to make them aware of a gardening issue that has haunted me over the last four gardening seasons. Note: I definitely try to garden organically wherever reasonable.

In the spring of 2020, I transplanted tomato seedlings into two of my raised garden beds. In each planting hole, I mixed the existing soil with compost, worm castings, and 1-year-old aged horse manure, then set the tomato seedling in place. (I later ran out of time and ended up just sticking the rest of my tomato plants into the other raised beds without any soil prep.)

After about 2 weeks, the tomato plants in the beds with soil prep looked stunted. Their stems and newer leaves were all thin and twisted and most of them eventually died. The plants without soil prep looked and proceeded to grow normally.

After researching every known tomato disease in gardening literature, I found nothing comparable to what I was seeing.

In the spring of 2021, I went through this same scenario again. This time I did an online search for “twisting, misshapen tomato plants” and I learned something new: what I was seeing was chemical damage and Dan Humbles, Manager of the ABQ Greenhouses, identified and confirmed this for me.

 

What I learned: There is a somewhat-newer type of weed killer being used when growing animal feedstock (e.g., hay, grains, and straw), referred to as persistent herbicides, which can cause this kind of reaction in tomato plants.

 

I have enclosed a summary of the advantages and problems currently being experienced all over the world, as well as in all 50 states of the U.S., using persistent herbicides.

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This is the Good News

 

Over the last 20+ years, chemical manufacturers have developed a totally new type of herbicide (i.e., weed killer). For many farmers, ranchers, land managers, and landscapers, these new weed killers have shown substantial improvements over older herbicides.

When sprayed on fields of animal feedstocks or straw,

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• These new products often show residual sustainability, in that the user may not have to reapply these weed killers the following year or even longer; hence the name persistent herbicides.

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• Only very small amounts of these products are needed to treat pastures, fields, rangelands, roadsides, railroads, golf courses, ditch banks, wildlife management areas, trails, and permanent grasslands – as little as 7 to 20 ounces (concentrate) per acre/per year.

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• Scientific studies and evaluations have shown that these persistent herbicides have a much lower “toxicity level” compared with older herbicides, so do not impact the health of humans, livestock, or other animals.

 

And Here is the Bad News

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• While older types of herbicides would break down fairly easily after less than a year or when put into a composting process, these newer persistent herbicides do not.

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• There are a number of plants/trees that are sensitive to these persistent herbicides, in that their growth will be twisted, with curled leaves, misshapen fruit, reduced yields, and may eventually lead to the death of the plant.

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• Plants identified in the literature to be sensitive to these newer herbicides include:

  • Vegetables: Tomatoes, Potatoes, Carrots, Peppers, Cucumbers, Squash, Melons, Lettuce, Spinach, Eggplant, Peas, Beans, Other Legumes, Parsley, Mushrooms

  • Fruits: Strawberries, Grapes

  • Flowers: Asters, Daisies, Sunflowers, Dahlias, Marigolds, Some roses, Petunias, Pansies

  • Other crops: Cotton, Tobacco, Cannabis

  • Trees: Pine, Spruce, Some types of fir trees

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• If you use MANURES in your garden/farm: When an animal consumes feedstock that has been sprayed with a persistent herbicide, the herbicide does not break down within the animal’s digestive system. The vast majority (>95%) of the herbicide travels through the animal’s digestive system, exiting their system -- mixed in their manure and urine. The manure can possibly retain this herbicide for up to several years; some resources cited sustainability as long as 6 years. Unfortunately, this herbicide cannot be simply leached out of this manure or out of the soil on which the herbicide was originally sprayed.

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• The EPA has not outlawed persistent herbicides because they have been shown to not harm humans or animals. But given there are non-lethal side effects that need to be taken into account, each U.S. state has the power to choose how they wish to direct/mandate the use of persistent herbicides in their state. All states handle these products differently, but most states do require that the manufacturer put a notice within the product’s “Directions for Use” that the manure from animals that consume the affected feedstock NOT BE USED FOR COMPOST. However, given that most printed directions included with herbicides can be many pages long and in very fine print, it is doubtful that many end users will be aware of this restriction.

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• It is extremely difficult for an grower/gardener to verify that either (1) the hay/straw they use in their garden (say, for mulch) or (2) the manure they will be applying as compost, has or has not been exposed to a persistent herbicide.

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• Laboratory testing for these persistent herbicides in feedstocks, manures, or compost using these feedstocks is available, but can require weeks for turnaround as well as cost in the hundreds of dollars per sample.

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• The plants that are sensitive to these persistent herbicides can be negatively impacted by concentrations of these herbicides in the soil as minuscule as 8 parts per billion!

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• In the spring of 2023, with the help of 500+ seedlings of 47 tomato varieties, I have again encountered these persistent herbicides in bags of ORGANIC vegetable garden planting mix that were purchased at a national and international chain hardware store.

 

What You Can Do to Protect your Garden

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• Order hay/grains/straw only from a farm or resource where you know that no persistent pesticides have been used. (This can be very difficult to determine.)

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• Use manure only from animals that are fed feedstocks not treated with persistent herbicides. (This can be very difficult to determine.)

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• Test all compost, mulch, or manure before applying to your garden. While not particularly scientific, this can be accomplished by growing bean or pea seeds in samples of the material under scrutiny. Test enough samples to adequately represent the material you are concerned about. See Implementing a Plant Growth Testing Program, the U.S. Composting Council, 2015.

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References

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• Herbicide Residues in Compost, State of Oregon, Department of Environmental Quality, February 10, 2021.

• Persistent Herbicide Information for Horse and Livestock Owners, University of Vermont Extension, September 23, 2015.

• Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings, North Carolina State Extension, February 19, 2020.

• Understanding Persistent Herbicides, U.S. Composting Council, 2015.

• Implementing a Plant Growth Testing Program, U.S. Composting Council, 2015.