Why You Should Be Aware Of Persistent Herbicides, Even If You Are An Organic Gardener
Bernalillo County Extension Master Composters
Written by BCEMC Master Composter, Mary Green, November 2021
I am writing to you, my gardening friends and colleagues, to share a bit of information that may help some of you to avoid a gardening issue that I have been haunted by over the last two gardening seasons.
In the spring of 2020, I transplanted 8-week-old tomato seedlings into one of my garden beds. In each hole I dug, I added compost, worm castings, and aged horse manure to the bottom of the hole, mixed regular soil with it, then set the tomato seedling in place.
After about 10 days, these tomato plants looked TERRIBLE. Their stems and newer leaves were all thin and twisted and did not look at all healthy.
After researching every tomato disease I had ever heard of, I found nothing comparable to what I was seeing in my garden. I replaced all those plants with new tomato seedlings, but also pulled out all of the added soil amendments, replacing it with just soil from other garden beds.
These new tomatoes grew fine and I went on to have a good crop. I chalked the problem up to the plethora of grubs I was dealing with at the time.
In the spring of 2021, I went through this same scenario again (despite having already treated the soil for grubs). This time I did an online search for "twisting, misshapen tomato plants" and I learned something new: the damage I was seeing was chemical (and Dan Humbles, the Manager of the ABQ Greenhouses, confirmed this for me). There was a newer type of weed killer being used on animal feedstock (e.g., hay, grains, and straw), referred to as persistent herbicides, that could cause this kind of reaction in tomato plants. But I reasoned that wasn’t possible because I always used organic everything! Well, did I have a lot to learn ....
From my research, I have enclosed a summary of the advantages as well as problems experienced in many countries using persistent herbicides. Gardeners and composters in all 50 U.S. states have experienced problems with persistent herbicides for a number of years.
This is the Good News
Over the last few years, several chemical manufacturers have been able to develop new types of herbicides (i.e., weed killers). For many farmers, ranchers, land managers, and landscapers, these new weed killers have shown a substantial improvements over previous products.
When sprayed on fields of straw or feedstocks such as hay:
- 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.
- 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.
- 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
- While older herbicides will break down fairly easily in the composting process, these newer persistent herbicides do not.
- 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.
Plants known to be sensitive to these newer herbicides include:
- Vegetables: Tomatoes, Potatoes, Carrots, Peppers, 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
- IF YOU USE MANURES IN YOUR GARDEN/FARM: When an animal, such as a horse or cow, 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 4 to 6 years. Unfortunately, this herbicide cannot be simply leached out of the manure or out of the soil on which the herbicide was originally sprayed.
- The EPA has not outlawed persistent herbicides because they have not been shown to harm humans or animals. But due to the 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 for Use" included with herbicides can be many pages long in very fine print, it may be doubtful that a significant number of end users will be aware of this restriction.
- It is extremely difficult for an organic 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.
- Laboratory testing for these persistent herbicides in feedstocks, compost material using these feedstocks or manures is available, but can require weeks for turnaround as well as cost in the hundreds of dollars per sample.
- The plants that are sensitive to these persistent herbicides can be negatively impacted by concentrations of these herbicides as minuscule as two parts per billion!
What You Can Do to Protect your Garden
- Order hay/grains/straw only from a farm or resource where you know that no persistent pesticides have been used.
- Use manure from animals that are fed feedstocks not treated with persistent herbicides.
- Test all compost, mulch, or manure before applying to your garden. This can be accomplished by growing bean or pea seeds in samples of the material under scrutiny. Test enough samples to adequately represent the materials you are concerned about. See Implementing a Plant Growth Testing Program, U.S. Composting Council, 2015.
- 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.