Selective Herbicides: Sneaky Organic Garden Killers

Broad spectrum selective herbicides can ruin your garden. It’s easy to think “oh, well… no problem! I never use herbicides!” I thought the very same thing until I learned how easily they can sneak into the garden through very commonly used garden helpers like manure and straw. Here’s what they are, what they do, and how you can most likely avoid them.

Discovering The Potential Problems

I use a lot of straw. It’s the “brown” (carbon) in my compost and the mulch in my garden beds. It’s the charming fall decoration in my yard before I compost it or top off my garden beds. It looks clean and “farmy”, it’s beautiful and I get it locally.

My straw habits abruptly changed when I stumbled upon this post “Killer Compost- It Happened To Us” recently by Joe Lamp’l of PBS’s gardening show Growing a Greener World.

Joe used composted manure from ruminants grazing on hay & grass sprayed with a broad spectrum herbicide that kills the weeds but not the grass. Unfortunately, they are extremely long-lived and stunt food crop growth. They are VERY common herbicides and it never occurred to me that they might be lurking in the straw I put on my garden beds. So I launched a full-blown research project into these broad-spectrum selective herbicides. What I learned horrified me. I mean… these things are crazy!

What Are Selective Herbicides?

These herbicides, called “pyridine carboxylic acids”,  are a class commonly used in growing hay and forage grasses. They’re selective, meaning they will kill a broad spectrum of weeds, but let the grasses grow. Farmers use them because they’re profitable! Fewer weeds mean more crop to sell/feed, etc… Some of the most common brand names are Grazon (DOW Chemical), Roundup Lawns, and 2-4D. They have an incredibly long residual effect. If you use something treated with these herbicides like straw from fields where they use these herbicides, it can affect the production of your garden for years! Manure from animals grazed on or fed hay from pastures sprayed with them have an even more dramatic effect because of the concentration levels. There’s a fantastic in-depth report on persistent herbicides done by the US Composting Council available for download at the end of the post, along with some further reading links.

How Do I Know If There Are Herbicides In My Straw?

I get all my garden manure from my worm bin and my chicken coop so I’m safe on that front. I get my straw from a local (mostly organic) nursery. It’s a very reputable urban nursery. The owner is extremely knowledgeable, they give organic gardening classes and everything I’ve ever seen there is non-GMO. It never occurred to me prior to reading Joe’s article to ask what herbicides were used in growing the locally sourced straw they sell.

Enlightened by all my research on selective herbicides, I called to ask the owner about which (if any) herbicides were used in the straw. To my extreme surprise, he said “I don’t know. I get it from a local guy in Ramona. I’m sure he doesn’t use anything, ‘cause grass grows from the bales of straw if they get moisture in them.” I mean… I didn’t want to argue with the man but that’s EXACTLY the point of selective herbicides. Grass grows, weeds (and peppers and tomatoes and legumes, etc…) don’t.

Since learning about these pyridine carboxylic acids I’m hesitant to even collect and compost neighborhood leaves! Such a bummer! The good news is that I’ve learned to ask questions, even with trusted suppliers. I’m building a list of “safe” neighbors to collect leaves and tree branches from to make my own mulch

Making My Own Mulch

I’ve attempted to find an organic supplier of straw to no avail. Every nursery I’ve called said it was just too expensive to order and there’s not enough demand. Well, poop! This left me with growing and making my own, which is the silver lining in all of this new knowledge. I’ve since learned to make my own bio-dynamic mulch which is practically free, gives me a great use for all of my tree and vine trimmings, and in my opinion, makes me at LEAST 10% more crunchy!

So next time you go pick up a truck full of manure for your garden or a bale of straw/hay that isn’t organic, ask questions about the herbicides used in its production. If the answer is “I don’t know” or “I’m not sure” stay away!

Happy Growing!

Further reading:

US Composting Council on Persistent Pesticides: http://compostingcouncil.org/wp/wp-content/plugins/wp-pdfupload/pdf/9199/USCC-Position-Statement-on-Persistent-Herbicides-FINAL.pdf

NC State University: Herbicide Carryover in Manure and Hay- Caution to Organic Farmers and Home Gardeners you can download here: https://www.pwswcd.org/images/HM_Herbicide%20Carryover%20in%20Manure.pdf