Compost is an important component of organic gardens, but nowadays it can easily be contaminated with persistent herbicides, which may damage crops and poison soil. Learn what’s causing the contamination and how to keep it out of your compost bin even if you garden organically and don’t spray.
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What are persistent herbicides?
There are four known persistent herbicides: Picloram (Dow AgroSciences, 1957), Clopyralid (Dow AgroSciences, 1987), Aminopyralid (Dow AgroSciences, 2005), and Aminocyclopyrachlor (DuPont, 2010).
These chemicals, which can persist on vegetation, soil, and compost for months or even years, are the main ingredient in over 150 retail products available to homeowners, farmers, and professionals for use in hayfields, horse pastures, as well as on golf courses, roadways, grain crops, and lawns, to kill unwanted broadleaf weeds.
However, the chemicals can be difficult to spot in products, appearing on labels with a variety of spellings.
While persistent herbicides have been around since 1957, their presence in home gardens has ramped up in recent years with better marketing and more availability. Herbicide-contaminated organic matter and compost has been referred to as “Killer Compost”.
How do persistent herbicides get into your compost bin?
According to North Carolina State Extension experts, persistent herbicide contamination is most prevalent in hay, manure, straw, and grass clippings—organic materials that are commonly used to make compost.
*If you buy bulk compost soil or bagged soil products, avoid herbicide contamination by asking the producer these 5 questions before purchasing their product. Even soils approved for organic agriculture are not tested for herbicide contamination!
Because of this, persistent herbicides have made their way into the very fabric of our food-producing gardens and farms. This contamination can cause stunted growth and lackluster performance in young perennial crops, annual vegetable crops, and flowers.
Studies show that complete deactivation and breakdown of the contaminant can take up to three or more years.
As a dedicated organic gardener who has suffered the effects of herbicides in my garden from contaminated organic matter, I feel that this is a major breach of environmental stewardship and our most basic right to a clean environment.
And of course, no action exists in a vacuum. In a study of 1300 honeybee hives, for example, up to 31 pesticides were found in a single pollen sample. (“Pesticides” is a broad term that encompasses all -icides, including insecticides, fungicides, herbicides, and more.)
In my ideal world, the agrochemical companies would take their harmful products off the market and pick up the tab for remediating our environment and health.
Instead, they publish stewardship documents with cheerful colors and simple diagrams that entrust farmers, homeowners, and professionals to apply the chemicals appropriately and keep affected materials out of compost and agricultural production. To say this strategy isn’t working is an understatement.
Would you like to grow more food with less effort? Check out my mini guide, The Permaculture Inspired Vegetable Garden.
Don’t persistent herbicides break down in the composting process?
A common misconception about persistent herbicides is that they break down during the composting process, either due to the high temperature, the microorganisms that are involved in the process, or both.
In a cold or passive backyard compost bin, where organic materials are added intermittently, it rarely heats up above 90° F, which is not hot enough to degrade persistent herbicides. In fact, persistent herbicides often don’t biodegrade even in a commercial composting facility, where the temperature reaches up to 170° F.
What’s more, while microorganisms help some herbicides to rapidly decompose in composting, they can exacerbate the problem with regard to persistent herbicides. Rather than helping them to break down, microorganisms may cause them to concentrate in the finished compost.
The US Composting Council explains why persistent herbicides tend to concentrate in animal manure and compost:
Composting is digestion by microorganisms. Microorganisms are animals. Scientific studies show that persistent herbicides pass unaltered during animal digestion. In fact, animal digestion tends to concentrate these chemicals.
[That’s because] persistent herbicides break down much more slowly than the materials in compost, and therefore, most of the chemicals pass into the finished compost.”
To summarize the problem, free organic materials collected offsite may be contaminated with herbicides and pass on that contamination to your compost or garden soil. The herbicide may also become concentrated through the composting process.
Wondering if you have herbicide-contaminated crops or soil?
Herbicide-affected plants can present various symptoms, including leaf curling, distorted shoot tips, stunted growth, misshapen fruit, reduced yields, and more. See pictures of damaged crops in my article about the effects of persistent herbicide in manure.
The green bean herbicide test, also called a bioassay, uses legume crops such as beans, peas, or clover as a sort of canary in the coal mine. This test, which is easy for home gardeners to conduct, utilizes these herbicide-sensitive crops to analyze finished compost, manure, or bagged soils before you add it to your garden.
If you determine that you have contaminated organic matter, report your findings to your local extension office and to the source of the material, if known. Don’t put herbicide-contaminated compost in your garden. Furthermore, do not eat produce from plants showing damage.
How to Rehabilitate Herbicide-Contaminated Compost or Garden Soil
In time, persistent herbicides eventually break down, but not on a time scale that organic gardeners would appreciate. However, with some patience and special care, you can nudge the contaminants toward a speedier decomposition process.
Here are some strategies that can speed up the process:
- Distribute activated charcoal or biochar throughout the compost bin or contaminated area to neutralize the chemical while it gradually breaks down. This can help to reduce runoff of the contaminant.
- Spread the compost over a small area and plant it with a cover crop of annual grasses. Grasses aren’t sensitive to these herbicides that target broadleaf plants, and can help absorb and bind to the chemical. I prefer to sow a mix of foxtail millet and sorghum sudangrass. At the end of the season, cut and discard the plant matter in the trash.
- Higher heat and oxygen can speed up the decomposition of these contaminants. While I typically promote no-till garden practices, tilling several times during the hottest months of the year solarizes and oxygenates the soil. Again, spread the compost over a small area in order to till it.
Whichever rehabilitation strategy you use, conduct the bioassay method to test regularly. Then repeat your preferred rehabilitation until the compost tests clean.
Tips for Making Herbicide-Free Compost
Building and maintaining a no-till, organic garden takes a lot of organic materials and compost. Sourcing herbicide-free materials for making clean compost and mulch takes special consideration, but it can be done.
The following are some ideas for avoiding or reducing herbicide contamination when sourcing materials for your compost.
Hay, manure, and straw are some of the best organic materials for creating great compost, and also some of the most likely to contain persistent herbicides.
Can you develop these materials organically on your own site? It would mean not spraying hay or straw fields. With regard to livestock, use certified Organic feed, and use no-spray materials for bedding.
Not enough space to grow your own farm inputs? Search your area for No Spray or certified Organic farmers. I drive an hour and a half to get to an Organic farmer who sells herbicide-free hay, manure, and straw. Know your farmer and ask lots of questions.
Food waste is far less likely than manure, straw, hay, and grass clippings to contain persistent herbicides. So get started with fruit and vegetable scraps from your own kitchen. Are there other sources of food waste in your community that you can incorporate into your compost bin?
Create your own “manure” by starting a DIY worm bin for turning food scraps into nutrient-rich worm castings.
You’re not spraying herbicides on your lawn, right? Good, then you’ve got grass clippings, leaves, and other yard waste!
Picking up bags of leaves and yard waste from the curb used to be a great way to gather free, surplus organic materials from around your neighborhood. However, herbicides have become common in residential landscapes.
While farmers and professionals are trained and certified by the federal EPA to properly apply herbicides, homeowners are not. I believe there is a lot of misuse and overuse of herbicides in residential neighborhoods because of it.
If you’re picking up free yard waste bags, here’s how I would think about it:
Fall leaves are an incredibly rich source of nutrients, and they break down to become an amazing mulch and soil conditioner. And it’s unlikely that ornamental trees have been sprayed with herbicides. So, grab those leaf bags!
However, skip the miscellaneous yard waste bags containing assorted grass clippings and weeds, since they have a higher chance of being contaminated. Connect with your family, friends, and neighbors who for sure don’t spray!
I used to recommend picking up coffee grounds from local coffee shops. I’ve done this often. But coffee is one of the most intensively sprayed crops with pesticides. Interestingly, the EPA classifies herbicides as pesticides.
Does the pesticide residue make it into the cup and then remain in the coffee grounds? There’s been no testing to prove or disprove this hypothesis.
Moral of the story: Use your own (certified Organic) coffee grounds and seek out those local coffee shops that source responsibly grown coffee.
Growing Your Own Organic Compost Materials
If you’re accustomed to making compost using a surplus of inputs from local farms as well as local yard waste, then you might be wondering how to collect enough bulk materials that are herbicide-free.
The good news is that by changing a few everyday gardening practices, you can grow your own materials for roughly the same amount of time it takes to collect these materials from around your community.
Having a diversity of plantings in your own garden is a key component of a resilient permaculture garden, which makes use of resources within the system.
Learn more about improving the quality of your soil in your permaculture garden:
You’ll find loads of information just like this in my award-winning book, The Suburban Micro-Farm.
Green Materials for the Compost Bin
“Biomass” plants like borage, comfrey, and rhubarb make oodles of green matter that can be chopped regularly and added to the compost bin. I chop my comfrey plants back about once a week throughout the growing season.
Here’s a unique idea that I tried in my new yard: Grow a clover lawn as a green manure.
Seriously, try mowing a clover lawn in the spring or summer and piling up the clippings. The high-nitrogen material will heat up quickly and smell exactly like manure! (Ask me how I know!) To avoid bad smells, practice good composting by layering green and brown materials in the compost bin.
You can also grow cover crops in the garden. When you chop them back, the roots enrich the soil while giving you green matter for the compost bin.
Brown Materials for the Compost Bin
One of my favorite ways to make a straw-like mulch and generate a “brown” carbon material for the compost bin is to grow ornamental grasses. They create a lot of organic matter in a relatively small amount of space.
Simply chop ornamental grasses back at the end of the season when the grass stalks have dried, and then into smaller pieces to make mulch and a compost material. Seek out a clumping species/variety that is not invasive in your locale.
Ideally, you’ll have deciduous trees that drop leaves for you to collect.
Alternatively, turn a backyard orchard into a food forest that generates organic material in addition to food, by layering together fruit trees, berry bushes, herbs, flowers, and more. Cut back the undergrowth of the fruit trees to use as a nutrient-rich mulch and compost material.
Finally, you might be able to sneak in a mixed hedgerow along a property line, which can provide deciduous organic matter, undergrowth cuttings, and smaller woody materials from pruning for the compost bin.
In short, you have more control over what goes into your compost by growing your own organic materials. Doing this makes your garden more resilient and less susceptible to outside risks like herbicide contamination.
Can you think of other sources of clean organic compost materials?
As we’ve seen, persistent herbicides are becoming more widespread in the organic materials that enrich our compost and gardens. Therefore, starting with clean raw materials is important. However, with effort you can grow and source clean ingredients for your compost bin and maintain a healthy garden ecosystem.
4 FAQs About Herbicides in Compost
#1: How do you test for herbicides in compost?
Herbicide-affected plants can present various symptoms, including leaf curling, distorted shoot tips, stunted growth, misshapen fruit, reduced yields, and more.
A bioassay is a test that uses herbicide-sensitive legume crops, such as beans, peas, or clover, to analyze finished compost soil, manure, or bagged soils before you add it to your garden. It’s a simple test that any home gardener can conduct.
Simply plant some pea or green bean seeds in the questionable compost soil, and care for them under grow lights if necessary. After 14 days check for symptoms of contamination.
#2: How do you get herbicide out of compost and soil?
Persistent herbicides in compost soil eventually break down, but not on a time scale that organic gardeners would appreciate. However, you can nudge the contaminants toward a speedier decomposition process.
Distribute activated charcoal or biochar in the compost to neutralize the contaminant. Or plant the compost with grasses like millet and sorghum, which will bind to the herbicide. Cut and discard at the end of the season.
#3: How do you prevent herbicide from getting in compost?
Persistent herbicide contamination is most prevalent in hay, manure, straw, and grass clippings—organic materials that are commonly used to make compost. Source these materials from no spray, organic farms or grow/raise/mow your own chemical free compost materials.
Food scraps are a great material for the compost bin, as they’re less likely to contain herbicide residue.
#4: Do herbicides break down in compost?
While some herbicides can break down in the composting process, there are a few persistent herbicides that do not biodegrade in composting, and can remain active in the finished compost. This contaminated compost can damage crops and garden soil.
Have you experienced damage from persistent herbicides in your garden or compost bin? If so, how did you handle it?