Preschoolers Making Compost and Possible Herbicide Residues

by Jan
(Encinitas, CA)

We are in the process of making our own compost. We have a straw bale donated to us. I'm concerned about our compost becoming a "killer compost" from possible herbicide. We are in southern CA. Should I be concerned?

Herbicide Residues In the Pre School Compost? What to Do


Liz, I am so happy to see that you are planning to compost with your pre-school kids. It will be a great and important experience for them. But...

Unfortunately, you do need to be concerned about the possibility of herbicide contamination, but there is a good chance this straw is fine.

Ask the Donor First


Try to find out how the straw was grown. If the donor was also the person who grew the crop the straw is from ask them about the types of herbicides they might have used. Many fields either do not use any herbicides at all or use only herbicides that will break down quite easily in the composting process.

If the donor did not grow the straw ask where they got it and try to trace where it is from.

  • Use the straw in the compost if
    • The grower is sure they did not use any herbicides
    • The grower used only something like round-up which breaks down effectively in compost.

  • Do not use the straw in the compost if
    • You find out the grower used one of the persistent herbicides.
    • You find out the straw was harvested from a ditch or from under a utility right of way. If harvested it from either a ditch or a utility right of way then there is a good chance you'll have a problem. These herbicides are often used on roadsides and rights of way precisely because they are persistent and don't need to be applied very often.

The Donor Doesn't Know.. or You Already Started Using the Straw


If you can't determine where this straw came from then it is a bit of a gamble. The thing to remember is that eventually, even these persistent herbicides will break down, and until they do you can grow the crops that aren't affected too much by the herbicides.

If you've already started using the straw in the compost carry on until you have your compost finished. Then, before actually using the compost in the garden do the bioassay test. This will be a brilliant learning opportunity for the kids and will stay with them forever.

Bioassay Test


Because peas are very sensitive to these herbicides they are an excellent test crop. Happily, they are also a crop kids like to grow. At the bottom of this page there are detailed step by step instructions for testing either the compost or the straw for picloram, clopyralid or aminopyralid herbicide residues in the compost and your straw.

Make sure the kids know they are being scientists when doing this. If the leaves of your test plants are cupped or twisted then you have contamination.

What to Do with Contaminated Compost or Straw


If you are still at the straw stage of your compost project you can return the straw to your donor. If you have already started to compost the straw you could make one smaller section of your garden the place where this contaminated compost is used.

Grow grass family crops here. You can grow corn, wheat and rice and these plants will not be affected. Every year do a test with sensitive plants and see which ones start to do okay. Record your results with words and pictures.

Plants that are sensitive include:
  • Cucumber - shows damage at 100 ppb
  • Lettuce - shows damage at 10 ppb
  • Tomato - shows damage at 3 ppb
  • Peas - shows damage at 1-10 ppb
  • Sunflowers - shows damage at 1 ppb


Good Luck and let us know what happens.

Kindest Regards
Leslie

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