Over the last few decades conversationists have been concerned about the aggressive spread of garlic mustard along streams and in the understory of woodlands, where it appears to crowd out native plants.
However, recent research has complicated how horticulturalists understand the spread of garlic mustard. Cathy McMullen, adjunct associate professor in the Department of Natural Resource Ecology and Management at Iowa State University, joined Garden Variety to explain.
“Originally, it was thought that garlic mustard was a really great competitor and was displacing native species, particularly non-woody species growing in the understory,” McMullen said.
Researchers believed it did so by producing allelo compounds, which may be toxic to competing plants.
However, new research found that native plants and their mycorrhizal fungi appear to be developing a resistance to the garlic mustard’s allelo compounds over time as they have been exposed to them.
“In addition, it appears that garlic mustard tissue — or concentration of allelo compounds in the tissue — may diminish as the population gets older, and this is likely due to the cost of producing these allelochemicals,” McMullen said. “They take a lot of carbon, a lot of nutrients to produce them, and if the yields are diminishing, then selection kind of suggests it's not worth producing these anymore.”
The research also suggests that deer and earthworms have played a key role in helping garlic mustard thrive by turning up the ground and exposing bare soil that creates a seed bed for garlic mustard to spread over.
Unfortunately, the common practice for removing garlic mustard — pulling the plant out of the landscape by its roots — does the same thing.
“I know that's a kind of a controversial line of thinking, that we shouldn't be pulling the garlic mustard,” McMullen said. “I mean, I and many of my students and colleagues, we've spent hours pulling and bagging garlic mustard, so it's really quite a reversal, if that's the case.”
If you have a smaller, newer patch of garlic mustard you are trying to remove, McMullen suggests pulling them up from April through June, before seeds begin to fall. But if you have a larger population, you could just snip the heads off to prevent them from maturing and entering the seed bank.
Either way you go about it, remember to bag up any material you cut or pull and throw it away. If you let it fall to the ground, the plant can still find a way to mature.
To further grow your gardening knowledge, sign up for our Garden Variety newsletter. And check out all the episodes of Garden Variety, the horticulture podcast for all the things you’d like to grow or grow better.
