How Do I Know If My Compost or Mulch Is Contaminated With Herbicides?

While compost and mulch offer many benefits, applying herbicide-contaminated materials can wreak havoc on your garden. Join Briana Yablonski as she covers how to determine if your compost and mulch contain harmful herbicides.

Close-up of a man's hand holding compost with herbicide contamination, on a blurred green background. The compost is similar in color to soil, slightly lumpy.


Adding compost or mulch to your garden seems harmless enough. After all, people tout the myriad of benefits these materials provide. So you do what any good gardener would: grab some composted horse bedding from a local farm or a bale of straw from the garden center and sprinkle it over your garden. But a few days later, you notice some of your plants’ leaves curling. And within a week, the plants’ stems are twisting. The cause? Herbicide-contaminated compost or mulch.

While not all types of compost and mulch contain herbicides, this is an issue every gardener should be aware of. I’ll cover signs of herbicide damage to look out for, how to source herbicide-free materials, and how to tell if your compost and mulch contain herbicides.

The Short Answer

Unfortunately, you can’t tell if compost and mulch contain herbicides just by looking at them. One way to test for herbicides is to plant a few bean seeds in the material. If the quick-germinating seedlings show signs of herbicide contamination, like curled leaves and twisted stems, the material contains herbicides. If they look healthy, your purchased compost or mulch is likely safe to use.

The Long Answer

Herbicides are complicated, so I won’t cover everything you need to know about them in this article. However, I will explain how they relate to compost and mulch and the dangers you should know.

What Causes Herbicide Contamination?

Close-up of a man in a protective suit spraying pesticides on a potato plantation in the garden. The man is wearing a white jumpsuit and blue gloves. He sprays pesticides from an orange plastic can with a long spray nozzle. Potato plants bloom with white, star-shaped flowers.
Avoid materials treated with persistent herbicides like Clopyralid.

Herbicides, of course! But before you say I provided the obvious answer, let’s clarify: not all herbicides threaten compost and mulch. While these materials can technically contain any herbicide, persistent herbicides are the greatest concern.

These materials remain active in the soil for up to three years and can withstand composting and animal digestion. If grass is sprayed with one of these products and then eaten by animals, the animals’ manure can still contain the herbicides. If manure is later composted, the finished compost can also contain the herbicide.

The four known persistent herbicides are Clopyralid, Aminopyralid, Aminocyclopyrachlor, and Picloram. Remember that these are the active ingredients, not the product name or brand, so you can find hundreds of products containing one or more of these herbicides.

These herbicides kill broadleaf plants without harming grasses. That means people often spray them on fields of rye, wheat, and barley, on pastures of grasses like ryegrass and timothy, and on golf courses.

What Types of Garden Amendments Are Susceptible to Herbicide Contamination?

Close-up of young garlic growing in a garden bed with hay mulch. The plant produces long, slender, green leaves that arise from the base of the plant.
Mulch and compost, especially hay or straw, can contain herbicides.

Technically, any type of compost and mulch can contain herbicides, but some materials pose bigger threats than others.

First, let’s think about the materials used to make compost and mulch. When it comes to mulch, you can use organic materials such as wood chips, straw, hay, and leaves. You can compost just about any organic material, including yard waste, wood chips, food scraps, and animal bedding.

If you didn’t grow these materials yourself, there’s always a chance someone sprayed them with herbicides. However, you should be the most concerned about hay or straw mulch since these materials are the most likely to have been sprayed with one of the persistent herbicides.

One type of compost that is more likely to contain persistent herbicides is compost made from animal bedding or animal manure. Remember, persistent herbicides withstand digestion and composting! Mushroom compost may also contain persistent herbicides since mushroom producers often use hay or straw as a mushroom-growing substrate.

What Are the Signs of Herbicide-Contaminated Plants?

Close-up of a young potato plant with damaged leaves and stems due to Herbicide exposure. The plant produces upright green stems covered with oval, wilted, curled, dark green leaves.
Persistent herbicide-contaminated mulch affects dicot plants, causing stunted growth, leaf distortion, and potential death.

Not all plants will respond equally when you apply herbicide-contaminated mulch or compost. Since these persistent herbicides only impact broadleaf plants, grasses like corn and sod will remain unchanged. However, you’ll notice changes in dicot plants, including tomatoes, peppers, lettuce, beans, peas, beets, squash, cucumbers, and sunflowers.

You can expect to see signs of herbicide contamination a few days after applying the contaminated material to your garden. If you add contaminated mulch on top of your soil, you may not notice the effects until after the mulch becomes wet. It’s also important to note that the severity of the symptoms depends on the amount of herbicides in the material.

Now, let’s cover what symptoms to look out for. If you’re growing plants from seed, you may notice that seeds will germinate and produce cotyledons but not true leaves. Seedlings may also form a set of true leaves but then stop growing.

As far as transplants go, contamination often causes plants to stop growing by stalling growth in plant tips. Leaves become stunted, curled, twisted, or otherwise distorted—unique changes that are easy to recognize once you see them. Severely affected plants eventually die.

Herbicide-Contaminated Materials Aren’t the Only Risk

Close-up of compost in a composter in the garden. The composter is a tall, cylindrical plastic structure, black in color, reminiscent of a trash can. Compost consists of organic waste, garden debris, leaves, grass and so on.
Plants may suffer herbicide injury from drift, not just compost or mulch contamination.

If your plants show signs of herbicide injury, compost or mulch aren’t necessarily to blame. While it doesn’t hurt to test these materials for herbicides, recognize that herbicides can enter your garden in other ways.

Even if you don’t spray herbicides on your property, these chemicals can travel with the wind and land on your plants. Farmers refer to this unwelcome phenomenon as herbicide drift

How Do I Test for Herbicide Contamination?

Fortunately, there’s an easy and inexpensive way to determine whether or not your mulch or compost contains one of these broadleaf herbicides. Just plant a seed from a legume like a bean or pea, let it grow, and observe.

Why legumes? First, legumes are broadleaf plants, so they’re affected by these persistent herbicides. And since they grow quickly, they show signs of contamination within a few days. Plus, it doesn’t hurt that these seeds are inexpensive. You can use another type of broadleaf plant to test your material, but legumes are the gold standard.

How to Test Compost for Herbicide Contamination

Close-up of two peat pots with young pea seedlings on a wooden table. Pea seedlings are characterized by their delicate and slender appearance. The leaves are arranged alternately along the stem and have a tendril at the end, which aids in the plant's climbing habit.
Test compost by planting legume seeds in pots with potting mix and compost, then comparing the growth.

Once you’ve obtained your legume seed, follow these steps to test compost for herbicide contamination.

  1. Obtain four similar pots and seeds. Four-inch pots work well.
  2. Fill two pots with potting soil and two pots with one part compost and one part potting mix, then label each container accordingly. The pots with just potting soil will act as the control group.
  3. Place three legume seeds in each pot, cover with soil, and water well.
  4. Wait for the seeds to germinate, then place the seedlings somewhere sunny and warm.
  5. Once the seedlings have three sets of true leaves, compare their growth.
  6. If the plants grown in compost show signs of herbicide contamination that are not present in the control group, the compost is likely contaminated with herbicides.

How to Test Mulch for Herbicide Contamination

Close-up of Fava bean seedling among hay mulch. The seedling features compound leaves with multiple leaflets arranged in pairs along a central stem. The leaves are a blue-green color and have a rounded shape.
Test hay or straw mulch for herbicides by watering plants with water-soaked mulch and comparing growth.

If you have hay or straw you’d like to use as mulch, you can conduct the following experiment to test for herbicides.

  1. Fill a five-gallon bucket halfway with mulch and then cover the material with water. Let the mixture sit for at least 24 hours.
  2. Obtain four four-inch pots and fill the containers with potting soil.
  3. Plant three legume seeds in each pot and cover with soil.
  4. Label two pots “mulch” and two pots “control.”
  5. Water the control pots with regular water, and water the mulch pots with the water from the five-gallon container. Continue to use this water throughout the course of the experiment.
  6. Once the plants have developed three sets of true leaves, compare their growth.
  7. If the plants watered with the water-soaked mulch exhibit signs of herbicide contamination not present in the control group, avoid applying the mulch to your garden.

What Should I Do if I’ve Applied Herbicide-Contaminated Compost or Mulch?

If you’re reading this article and thinking you wish you knew this information before you applied your compost or mulch, you’re not alone. Unfortunately, I know both home gardeners and large-scale farmers who have unknowingly added herbicide-contaminated materials to their plants. They only recognized their mistake when their plants started struggling.

There’s not one right way to act after applying herbicide-contaminated material. You can attempt to remove the contaminated material, let things go and hope for the best, or replant your garden in a new area. I’ll go into a bit more detail about each method so you can make the choice that’s right for you.

Remove the Contaminated Material

Hay mulch removal. Close-up of female hands in black gloves removing hay mulch from a bed of growing peppers in the garden. Hay mulch consists of dried grasses, usually cut and baled, creating a loose, straw-like material. Hay mulch serves as an effective insulator, helping retain soil moisture, suppress weeds, and regulate temperature.
To save a contaminated crop, remove affected material promptly.

If you’re desperate to save this year’s crop, the best action is removing the contaminated material. Of course, this isn’t always easy. If you’ve mixed contaminated compost into your soil, it will be practically impossible to remove the compost without removing the native soil and plants. However, if you’ve laid down contaminated mulch on top of your soil, removal is more feasible.

While working at a farm in Virginia, I heard a herbicide horror story from a neighboring farm. They had just mulched their entire tomato crop with hay. This was a common practice in this region and one they had enacted for years. However, that year was different. Within a week, they noticed the tomato plants had curled leaves. At first, they thought it was wind damage, but before long, they realized they had accidentally applied herbicide-laden hay.

Rather than lose their entire tomato crop and thousands of dollars of income, they decided to pull the hay mulch from the beds. But removing thousands of pounds of wet, heavy hay wasn’t an easy task. Fortunately, the community rallied around them and they were able to successfully remove the mulch from the field and spare the tomato crop.

If you decide to remove contaminated mulch from your garden, act as quickly as possible to prevent more herbicides from entering the soil. After you’ve removed the material, place it somewhere where it won’t contaminate more soil.

Leave the Material and Hope for the Best

Close-up of a tomato plant with curled leaves. A tomato plant exhibiting leaf curl is characterized by the abnormal upward or inward rolling of its leaves. The tomato plant features a central stem supporting a multitude of pinnately compound leaves with serrated edges, ranging in color from bright green to dark green.
Removing affected material is crucial, as even a small amount of contaminated compost may cause harm.

If you only added a few handfuls of contaminated compost, it’s possible plants will continue to grow without experiencing major harm. They may show slightly curled leaves but still grow and produce flowers and fruits. But they could also die.

If you continue gardening in the same area, you can also plant monocots like corn, lemongrass, onions, and leeks. These plants are unaffected by the persistent herbicides that attack broadleaf plants, so they’ll grow without any problems.

Plant a Garden in a New Area

Close-up of a gardener's hands planting malted tomato seedlings in the garden. The gardener's hands are dressed in blue gloves with multi-colored patterns. Tomato seedlings have short vertical stems with pinnately compound leaves with serrated edges. The leaves are dark green in color.
Relocate the garden if contaminated compost hinders broadleaf plant growth.

If you applied loads of contaminated compost to your garden, there’s a good chance you won’t be able to grow broadleaf plants there this season. Rather than continuing to plant new crops only to watch them die, you can move your garden to a new area. While this isn’t a feasible option for everyone, it’s a great choice if you can swing it. It is particularly important for organic growers who are concerned about potential health impacts of synthetic herbicides.

Remember that persistent herbicides can remain in the soil for up to three years, so your garden may remain infected for multiple growing seasons. It’s a good idea to plant a few test bean seeds in the affected area before planting an entire garden.

Tips for Sourcing Herbicide-Free Material

Close-up of a gardener's hands pouring kitchen scraps into a large wooden compost bin in the garden. A gardener pours organic waste out of a gray metal bowl. The compost bin contains a lot of different organic waste, garden debris, dry leaves, etc.
Avoid herbicide-contaminated products by making your own or sourcing OMRI-approved materials.

Now that you know all the challenges of herbicide contamination, you’re probably wondering how to avoid these harmful materials. The best way to ensure compost is free from these persistent herbicides is to make it yourself! And while you may not be able to grow straw or hay for mulch, you can use alternative materials from your property like leaves and wood chips.

If making your own is out of the question, your best choice is to source that you’re sure is free from herbicides. The Organic Materials Review Institute (OMRI) is the agency that reviews and approves materials for use in certified organic operations. Therefore, OMRI-approved products are free from synthetic herbicides.

You can also find herbicide-free versions through word of mouth. If a compost or mulch supplier in your area provides herbicide-contaminated materials, word will travel quickly. And that’s just one reason to connect with nearby gardeners and farmers!

You can always ask for a sample if you want to check and see if a material contains persistent herbicides. Any reputable suppliers will gladly give or sell you a small amount of material so that you can perform the bean seed test at home.

Final Thoughts

Now that you’re familiar with the presence and impacts of herbicides, you can work to avoid applying these materials to your garden. By selecting herbicide-free materials, you can continue to reap all the benefits of compost and mulch!

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