Contaminated Herbicide-Compost and Soil Mix. “Eye-Opening”




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I have learned so much more over the years about where to source my compost soil for my garden Here I have gathered a little knowledge along the way so I hope any little tips or tricks can help you in your own garden. In the article Contaminated Herbicide-Compost and Soil Mix. “Eye-Opening” you will find lots of information.

The Problem with Persistent Herbicides in Compost Soil

Persistent herbicides have become quite pervasive in the organic matter that is collected and created in bulk compost at commercial facilities.  Lots of the materials such as hay, manure, straw, and bulk grass clippings, are probably contaminated.

This is then likely passed on to the garden companies that sell compost or bagged soil products that we buy. Compost soil was the foundation for all my garden beds until persistent herbicide contamination became a concern for me from these commercial facilities.

The complete deactivation and breakdown of the contaminant in our gardens can take up to three or more years, causing stunted growth and lackluster performance in young perennial crops, annual vegetable crops, and flowers.

In the past, I have used compost from a bulk mulch supplier as well as horse manure from local stables to build gardens and improve the soil in the past. Of all the manures, horse manure has one of the highest chances of being contaminated with herbicides. 

I was completely unaware of this risk, and I believe that my gardens demonstrated lackluster performance because of it, despite my commitment to ecologically sound gardening practices.

Although I’m creating compost in my own compost bins, it’s not quite enough for many beginning phases. So here are some tips I have found along the way.

Learn 5 Essential Questions to Ask a Soil Producer Before Buying Compost Soil for Your Garden

1: What are the primary ingredients of the compost soil?

2: Does the soil company or its products have any accreditations or certifications?

3: Is the compost soil tested? If so, what tests are conducted?

4: Does the producer demonstrate knowledge of, or an interest in, soil ecology?

5: What considerations does the producer give to sustainability or environmental issues?

Avoiding Herbicide Contamination in Purchased Compost Soil ~ Additional Information 

Compost and soil mix products sometimes contain herbicide residue. This residue can damage certain vegetables, fruit, and flower crops.

The residue likely comes from contaminated hay, grass clippings, or manure. These materials may make their way into the regional composting system.

Clopyralid and aminopyralid are examples of herbicides that persist through the composting process.

Gardeners and landscapers should be aware of symptoms of plant injury from herbicides. Watch for stunted growth and distorted, curled leaves.

Be wary of getting mushroom compost, animal manures, and bedding to make your own compost. 

5 Keys to Success

  1. Before you purchase a bulk load of compost or soil mix, contact the vendor. Ask how they handle herbicide contamination risk. Have they reduced potential sources of contamination? Bioassay tests use plants such as peas grown in compost or soil mix to check for abnormal growth caused by herbicide contamination. Chemical lab testing might not indicate herbicide contamination.
  2. Perform your own bioassay test. Ideally, test the material before you apply a compost or soil mix product to your garden or landscape.
  3. Watch your plants for herbicide injuries such as distorted growth and cupped leaves.
  4. If you observe these symptoms, rule out other sources of herbicide damage. Were herbicides applied nearby? Could that have damaged your plants?
  5. Contact the vendor if you determine that contaminated compost or soil mix is the cause. Letting them know about the situation helps others avoid the problem.

Identify Herbicide Damage

Herbicide damage from contaminated compost or soil mix is caused by growth regulator herbicides. These herbicides more often affect broadleaf plants. Look for distorted or cupped leaves. The damage occurs in new growth, including the ends of the shoots.

Crops prone to damage from this group of herbicides include (grouped by plant family): 

  • Beans and peas
  • Tomatoes, peppers, and potatoes
  • Squash (summer and winter)
  • Lettuce, sunflowers, and dahlias
  • Spinach, chard, and beets
  • Carrots and parsley
  • Roses
  • Grapes
  • Hemp (Cannabis sativa)
  • Other broadleaf plants

How to Remediate Herbicide-Contaminated Soil

If you observe distorted growth or cupped leaves on your plants, determine the cause of the damage. 

If you have applied compost or soil mix and ruled out other sources of herbicide damage, you may have herbicide-contaminated compost or planting mix.

What Damage Does Herbicide-Contaminated Compost or Soil Mix Cause?

Herbicide-contaminated compost affects plants prone to injury from growth regulator herbicides. It causes distorted or cupped leaves and growing points. 

Do I Need to Do Anything? 

Yes. Don’t eat produce from plants showing damage from herbicide-contaminated compost or planting mix. If the herbicide-contaminated compost or soil mix has already been applied, don’t plant crops prone to damage.

Take steps to remediate the soil.


Persistent herbicides are in our environment and have contaminated even the most ecologically sound gardens and farms. So, although composting soil used to be a great way to boost garden success, it now requires close scrutiny. Great tips on the HMR gardening website for soil tips.

By asking the right questions, you can select a responsible producer and source clean, herbicide-free compost soil for your garden.

Xo Joanne

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