John Elliott wrote:As a professional chemist and amateur mycologist, let me expand on Paul's comment. It's true that composting is not going to remove herbicides in the short term. Composting is a bacterial process and bacteria don't see the herbicide as a food source. What will metabolize the herbicide (eat it for lunch) is fungi, specifically a white-rot fungus. The easy way to remember the two main classes of saprophytic fungi is "brown-rot eats the cellulose and leaves the lignin; white-rot eats the lignin and leaves the cellulose". Most herbicides are aromatic molecules, meaning that they look like lignin to any fungal mycelium that is looking for a meal. They will be able to attack it and break it down into smaller components, rendering it "recycled".
Now, how do you do this on your lawn? Here is a reasonable plan of attack: (1) get a truckload of mulch that contains lots of tannins. Oak leaves would be perfect, what you want to avoid is actual structural wood like pine logs or lumber or wood shavings or newspaper, those things are all cellulose. (2) dump the pile where you can water it daily. (3) Go on a mushroom hunt for some white-rot fungi. You can start here with Phanerochaete chrysosporium, which is pretty easy to collect from the underside of a well rotted log. (4) Take your sample of white-rot fungi and whiz it in the blender, say a quarter cup of scrapings in a liter of water. (5) Sprinkle this over the pile of mulch and water it in good. You don't need to bother to cover it, the sun may dry out the top inch or so, but you want to water the pile every evening. Remember that advice when NOT to water your garden because it might cause fungal problems? Now is the time to break that rule -- you want the fungus to take off. After a few weeks, depending on the temperature, you should be able to dig into the pile a few inches and see gobs and gobs of white mycelia. This is good. Now the fungus is ready to do its work detoxifying your lawn. (6) Wait for a day when there is heavy rain forecast and get out before (or during if you don't mind getting wet) and spread your pile of prepared mulch out over your lawn. Not more than an inch thick, you want the heavy rain to beat down on the mulch and drench the fungal mycelia and spores into the soil.
The whole point of this exercise is to really spike the population of white-rot fungus in the top layer of your lawn and start this ecologically dead zone (from all the pesticide and herbicide abuse) back to being a living soil. While the white-rot fungus is breaking down the oak leaves, they will also eat any herbicide and pesticide residues. Dandelion seeds that blow in will now be able to grow. With all that fungal mycelium there, other soil critters will see the "Buffet Open" sign and move in for a free meal. If you mix the grass clippings with this specially prepared fungal mulch, they will also be recycled. But you don't want to do regular bacterial composting; you want slow, ambient temperature fungal growth conditions.
This concludes your mycoremediation class. Student questions are welcome.
I think this topic of of [soil] bioremediation to
safely utilize potentially herbicide-contaminated grass clippings is a "really big deal" and might merit further detailed discussion of practical research methods/experimentation ... we should remember that for many or most folks, the alternative to the
likely safe food from the garden [if the plants survived, grew and somehow produced something without chemotherapy] is known UNSAFE food from the supermarket.
There ARE reports in the scientific literature of it being done successfully with white rot fungi. When it comes to breaking down pesticides,
the most studied white rot fungi is indeed P. chrysosporium/ (as John Elliott mentions) which has shown ability to degrade a wide range of herbicides under different conditions ... when it comes to breaking things down large doses of UV radiation are going to be helpful, as are other stressors like large temperature swings ... but the "canary in the coal mine" will be the living things, eg. weeds, reptiles, amphibians, insects.
