Herbicide Contamination in Compost and Mulch: How to Fix It

Most organic gardeners and farmers are big proponents of compost and mulch. These natural materials add organic matter to the soil, foster a healthy environment for beneficial soil microbes, and limit weeds and water evaporation. When used correctly, they make it easier to grow robust plants without pesticides and herbicides. However, herbicide contamination is also a risk, depending on your sourcing.

Since both materials come from decomposed sources, herbicides sometimes travel through organic waste streams and end up in them. If you’re unaware you’re dealing with contaminated materials, you may wonder why your plants look deformed and stunted.

Fortunately, there are ways you can manage these unwanted materials. This article covers how to determine if herbicides are present, what to do if they are, and how to fix your garden if you’ve already applied contaminated products.

Where Does Herbicide Contamination Come From?

You can probably figure out that herbicides are responsible for herbicide contamination. However, not all of these chemicals lead to tainted garden amendments. Only products that persist in the environment for a long time will end up in your compost pile or straw mulch.

The products Clopyralid, Aminopyralid, Aminocyclopyrachlor, and Picloram have a long half-life, meaning they exist in the environment for an extended period. The breakdown time varies by product and environmental conditions, but these chemicals can exist in the soil for multiple months to a few years. These products are unaffected by the microbial activity and heat of composting systems and animals’ digestive tracts. Therefore, you can’t do much to speed up their breakdown.

Hay and straw are the main culprits of contaminated amendments. That’s because all four persistent herbicides kill broadleaf weeds but leave monocots like wheat, ryegrass, and barley unharmed. Therefore, you should be especially wary of straw mulch and compost made from animal bedding or manure.

How to Test Compost and Mulch for Herbicides

Although you can’t simply look at these materials and determine if they’re contaminated, it’s easy to test for the presence of herbicides. You could send a sample of the material to a lab for testing, but you can also rely on plants to do the testing and talking.

Since these chemicals affect broadleaf plants, you can grow a seedling in the material and look for defects. Growers often use beans because they are fast-growing and develop obvious deformities. All you have to do is plant a bean seed in the material and then care for it as you normally would.

If herbicides are present, the seedling will show symptoms as early as germination. Some seeds may form cotyledons (the first set of leaves) but not true leaves. Other seedlings may develop true leaves, but they’ll appear curled, twisted, or stunted.

If multiple seedlings look normal, your materials are likely free from contamination.

What Should I Do If My Compost or Mulch Are Contaminated?

If you’ve tested your materials and found that herbicides are present, the next steps depend on the situation. Maybe you’ve just tested a sample before purchasing a few yards. Or perhaps you have bales and bales of contaminated straw mulch on our property. Regardless of the circumstances, here are the next steps you should take.

You Haven’t Brought Material Home

If you haven’t yet brought the contaminated material home, don’t! There are plenty of sources of herbicide-free compost and mulch for you to use instead. And remember that most trusted suppliers will allow you to purchase a small sample before buying bags or scoops.

The material suppliers may be unaware of the herbicide contamination, so you can always start a conversation about it. The more people who know about this issue, the better!

The Material is on Your Property

If you’ve already brought home a load of compost or a few bales of straw mulch, don’t panic! Knowing which materials are potentially contaminated is the first step in maintaining a healthy garden.

If the materials are in a spot away from your gardens, you can leave them be. Although the herbicides will take weeks or months to fully break down, it will eventually happen. Then, you can use them in your garden without fear of them killing your plants.

However, if you keep the amendments around, be aware of rain. Water can cause the chemicals to leach into the surrounding soil and harm your plants. Therefore, it’s a good idea to cover the materials with a tarp or place them in a sheltered area out of the rain.

Another option is to remove the mulch and/or compost from your property. This is often a preferable option if you live in a small area where it’s hard to keep the contaminated materials away from your garden.

You can offer the materials to other gardeners, but make sure to let them know about the herbicides. Some people will have no problem accepting a few yards of compost if they have the time and space for the chemicals to break down. You can also dispose of the materials in the trash.

The Material is Near Your Garden

If you brought your amendment home and set it next to your garden, move it ASAP. Water can quickly cause the chemicals to leach out of the material and into the surrounding soil. You can move it to a covered area away from your garden as I mentioned above.

What Should I Do If I Applied Contaminated Mulch or Compost in My Garden?

If you learned about the dangers of herbicide contamination after you applied tainted amendments to your garden, don’t panic! Even though these materials can harm some of your plants, it doesn’t assure the demise of your garden. I’ve listed a few of your options below.

Remove the Material

If you want to return your garden to its unadulterated state, your best bet is to remove the material that contains the herbicide. However, if the compost or mulch has been sitting on your soil for more than a few days, it’s possible that the herbicides have made their way into the soil. Even if you remove the amendments, the chemicals will remain in your garden. With that said, removing the material will certainly decrease the amount of herbicides in your garden.

The best removal method depends on which material you used and how you incorporated it into your beds. If you spread straw or hay mulch around your plants, you can pile up the material with your hands or a rake. Once the mulch is away from your plants, move it out of the garden.

If you spread compost on top of the soil surface, you can shovel the top inch of soil into a trash can or other large container. Although this can be difficult and tedious work, it’s the best way to remove as much as possible. You can also try to rake it off the surface, but this will often just mix it into the top of the bed.

Removal is much more difficult if you’ve tilled the compost into the ground. The best way to remove the herbicides is to remove as much soil as the compost was incorporated into. For example, if you tilled the top three inches of soil, you should remove these three inches. Since this is a lot of material—especially if you have a large landscape—you may prefer other methods.

Grow Unaffected Plants

Remember that not all plants are affected by persistent herbicides. Since all of these products target broadleaf plants and leave monocots unharmed, you can still grow some plants successfully.

One option is to grow some of the plants in the affected area for one season. At this point, you can retest the soil for the presence of herbicides. If they are still present, you can grow another round of these plants. I recommend choosing a different plant to avoid the buildup of pests and diseases; for example, grow a round of onions followed by a round of corn. If the soil seems healthy, you can plant any type of crop.

You can also choose to plant the area with cover crops like annual rye, wheat, or oats. These plants don’t require much care, but they keep the soil covered with living matter. They help feed soil life and add organic matter to the ground. When the season is over, you can mow or crimp the cover crop and retest the soil. By the time the herbicides have broken down, your soil will be healthier, thanks to the cover crops.

Since different cover crops grow best in various seasons, it’s possible to plant multiple rounds of these soil-boosting crops in a single year. Oats, ryegrass, barley, and annual rye thrive in the cooler weather, while millet and sudangrass have no problem growing through even hot southern summers. Keeping the soil planted at all times will keep it healthy while you wait for the herbicides to break down.

Leave the Area Alone

Another option is to leave the area alone. This may be the most desirable option if you have a large area where you need to fix herbicide contamination. Of course, you won’t get anything out of your beds if you don’t plant them.

Rather than leaving the soil bare, consider covering it with a silage tarp. This will limit erosion and prevent weed seeds from germinating. A better alternative is to plant cover crops, as mentioned above.

If you leave the area untouched, you can retest the soil every few months. After a few beans grow without any issues, your garden is ready for whatever plants you want to add.

Tips for Avoiding and Fixing Herbicide Contamination

If you want to avoid dealing with these issues, keep these tips in mind.

Source Materials from a Trusted Source

The best way to avoid bringing tainted material home is to always purchase it from a source you trust. Since synthetic herbicides are banned in certified organic production, you can look for a supplier with an organic certification. This certification proves the producer never used synthetic materials to create their products.

However, mulch suppliers rarely apply for organic certification. Therefore, it’s up to you to do your due diligence. If you’re buying straw or hay mulch, always ask whether the fields have been sprayed with herbicides. You should also inquire about neighboring fields since herbicides can drift with the wind.

If you want to skip the trouble, simply source mulch from a trusted brand like GardenStraw. This is what Kevin uses on the Epic Homestead, and it is reliably chemical-free.

Always Test Before Applying

If you found a great deal on mushroom compost or compost made from horse bedding, it’s hard to pass it up! However, before you apply this new amendment, test it to see if herbicides are present. Remember the planting and waiting method above. Wait a week to see how the beans look, as this can save you hours of work in the future

Consider Making Your Own Compost

If you have a small garden, you may be able to make enough compost to supply your plants with the organic matter and nutrients they need. Even if you need to purchase compost for your garden, there’s no harm in turning your kitchen waste, prunings, leaves, cardboard, and food scraps into compost.

Although compost may seem intimidating, learning some basic information will help get you started. If you find you enjoy the process, you can ramp up production to produce a larger amount of herbicide-free compost for your own yard.