We have loads of invasive Scotch broom (Cytisus scoparius) on our property and we've been trying to figure out how to best make use of all that organic matter without having to burn it in an attempt to bring it under a semblance of control. We finally got a whole whack-a-do chipped with the idea of adding it to compost as woody matter to increase the fungal content, and also as garden mulch around plants. Then someone went and said that it secretes a tar that is toxic to other plants. I have never heard of this. From my research I've only found reference to this once, here: https://www.arrowlakesnews.com/news/the-drive-to-sweep-away-scotch-broom/ and I'm not sure if this is reliable, or if they're maybe mistaking Scotch broom for something else. I turn to you, Knowledgeable Ones, for any insights you might have before I make a terrible mistake and ruin both my compost and garden with potentially harmful plant matter.
For questions like this, I usually turn to google scholar. Better than an ordinary web search, it is limited to scientific papers. The results can be hard to understand if one is outside the field. Without delving into the actual research, I did run a search with the term “toxic secretions” along with the Latin name. You can fiddle with the search terms and narrow the search down to better meet your needs.
Just to follow up, it occurred to me (eventually) to write to the organization quoted in the article I posted about the 'toxic tar'. The response seems wooly to me: "I believe that the tar that they are referring to is the oil in the stems and leaves, but I don't have a reference for that. Perhaps that was an anecdotal comment." Unless anyone else has any insights which they choose to post here, I will continue with my experiment and apply the chips. I'll try to remember to report back, just in case anyone else comes across this kind of question. Thank you!
Well geeze Anne, now that you've invoked Google Scholar my answer feels about as lightweight as it actually is! That's a good reminder...
So nothing scholarly here, just observation. We've had a bunch of scotch broom in one pasture and along edges of others. We've just been mowing & pulling the stuff and am winning the war as scotch broom hasn't overcome the power of the brush mower. Even where the broom was really thick, it had lots of grass underneath. After mowing (and pulling...) there is no noticeable decline in the grass in that area. Conclusion - nothing strong going on.
Thus, my observation suggests that although the scotch broom might have an unpleasant substance it hasn't been enough to bother other plants. If you concentrate it all into compost ... well, yeah you might have a problem. But its probably a problem of concentration, not presence, so spread thinly or otherwise dilute the amount of broom in your piles.
Eliot Mason wrote:Thus, my observation suggests that although the scotch broom might have an unpleasant substance it hasn't been enough to bother other plants. If you concentrate it all into compost ... well, yeah you might have a problem. But its probably a problem of concentration, not presence, so spread thinly or otherwise dilute the amount of broom in your piles.
Hmmm... Thanks for the insights. I think this is why I'd like to get the full story on what this mysterious substance is. In a cold compost, the compound would likely survive (or not), but would it survive a thermal compost pile? Would all those super active microbes gobble it up and digest it into something benign?
Ok, my search continues; is there a toxic substance and, if there is, what is it?
The research I've found so far are about broom toxicity and mammals. I'm still looking for effects on plants. Oyster mushrooms are next on my list of leads.
I use scotch broom a lot in my hot compost, particularly in the winter when green stuff is harder to come by. It definitely helps heat the pile up, can still get it to 130-140 degrees in December. Never noticed any toxicity problems, and a lot goes into my annual garden.
With regards to cold composting, not sure how long the chemicals stick around. I vermicompost some similarly to a study I read (attached.) The study is pretty cool because they accurately measured phytotoxicity and nutrients from start to finish.
Sorry, Elliot Mason! A longstanding habit after way too much grad school. But observation trumps random articles on the internet, as does actual research. Maybe there is no tar. Maybe it's just high in nitrogen and someone noticed somehow it burned their seedlings!
I referenced an idea from a Mother Earth News article on another thread. Unlikely that it was peer reviewed, in the traditional sense! So, to me, the idea was worth trying, but not worth betting the farm on it.
Broom roots change the biology of the soil, reducing acid and adding nitrogen - this is the opposite of what many native species like in my part of the world. I haven't heard of them being overly toxic when decomposing.
The plants themselves are high in nicotine which can be toxic when burnt.
The best solution I've found for them so far is goats.
And I was going to suggest the thread, but you already beat me to it.