Killer Compost & Murderous Mulch

The Green Bean Herbicide Test

If you apply compost or mulch that has strong herbicide residues in it, you can damage your garden for a full season or more. And this is not just a problem with manufactured compost or mulch. If you gather local materials and make your own, persistent herbicides on your materials can survive the composting process.

This is not a brand new problem, but it is becoming more common. The reason is the approval of persistent herbicides used to control broadleaf weeds in more non-crop areas. Being persistent means they are created to last and keep working for multiple years after being applied. The generic name for these long-lasting herbicides is pyridine-carboxylic acids. They are typically used in horse pastures, golf courses, hayfields, street and highway right-of-ways, and on home lawns. These herbicides do not harm grasses, only broadleaf plants.

"The culprit can be one of any three herbicides which have been approved for use on pastures and forage crops," said David Trinklein, University of Missouri Extension horticulture specialist. Newer versions of herbicides with active ingredients such as clopyralid, picloram and aminopyralid can pass through the digestive systems of foraging animals and arrive, unchanged, in the manure.

As you can guess, this has created a stealth problem since many materials we once considered clean and safe for composting now could be contaminated. Manure may be coming from animals eating hay purchased form a farmer who sprayed his hayfields, or grass clippings may be coming from a home that sprayed their yard.


What garden plants are effected?
Here are the broadleaf garden plants most negatively effected.

  • peas, beans, lentils, and clover (Legumes)
  • tomatoes, potatoes, eggplant, peppers (Solanaceous)
  • carrots, lettuce, spinach, beets (Vegetables)
  • strawberries, grapes (Small Fruits)
  • sunflower, petunias, daisies, asters, roses (Flowers)


What are the warning signs?
Here are clues to look for to recognize herbicide damaged plants.

  • poor seed germination, or death of young plants
  • Stunted growth: the main growth tip stops growing
  • twisted, cupped, or elongated leaves
  • reduced fruit set, or misshapen fruit
  • secondary leaves don't grow after seed leaves emerge

The Green Bean Herbicide Test

The green bean test is a way to tell if compost or mulch has persistent herbicide contamination BEFORE you apply it to your garden. The upside is that it is a dirt cheap test. Commercial test cost around $300. The downside is that it does take 2 or 3 weeks to do the test. So you have to plan ahead a little.

Here is how the Green Bean Test it is done. First obtain a small bucket of the compost or mulch you are considering. Then follow the steps below for Compost testing.

  1. Obtain 4 clean flower pots, and some green bean seeds.
  2. Obtain some commercial potting mix that contains fertilizer but is compost-free.
  3. Plant 4 green bean seeds into potting mix in each of 2 pots. Label these as No Compost
  4. Mix up a blend of 2 scoops of compost for each 1 scoop of potting mix.
  5. Plant 4 green bean seeds into this blend in each of 2 pots. Label these as With Compost
  6. Use saucers under each pot to allow water to be absorbed back in. Avoid overwatering.
  7. Follow normal seed starting steps. Keep the pots warm. Use grow lights if necessary.
  8. Grow until three sets of leaves appear. This normally takes 14 to 21 days.
  9. Compare the plants. If they all look the same, the compost is clean and safe.

For Mulch testing, replace steps 4 and 5 above with these.

  1. Cut up a few handfuls of mulch into a fine texture.
  2. Place 4 green bean seeds on top of commercial potting mix in each of 2 pots. Sprinkle a 1" deep layer of fine mulch over the top of the seeds. Label these as With Mulch


It would be very frustrating to do all the right things, like using compost and mulch in your garden instead of chemical fertilizers, and then have your plants wither from hidden herbicides. I hope this helps you avoid that nightmare. Below are some links to other information about this problem.
How to - A hub for composting information


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