As a garden writer I get a lot of email from people warning me about pending catastrophes: blights, bugs, invasive plants. Some are accurate, some are not. In 2012 we were told that a disease that kills Impatiens (a lovely annual flower for shade) would make growing it impossible — ever again. But last year it did fine for many gardeners. Late blight on tomatoes is predicted every year, but my tomatoes have only been affected once. But I recently learned some disturbing news about an invasive weed, garlic mustard (Alliaria petiolata), that we should all pay attention to.
Invasive plants generally out-compete our native plants because they grow anywhere, often putting out leaves earlier in the spring than our natives and holding them longer in the fall. Some, like the Norway maple, have roots that suck up water and nutrients far from the mother plant. Others, like barberry or honeysuckle, shade out natives in the understory of the forest. But garlic mustard, a seemingly innocuous little weed with a root system that is not hard to pull, goes one step beyond the others: it produces a toxin that kills necessary root-coating fungi on our maples, oaks and beeches. Where it grows, some of our favorite trees are in danger.
Mycorrhizal fungi are beneficial fungi that coat the fine root hairs of many trees and perennial plants. They get sugars from the green plants and, as payback, share minerals that the green plants need but can’t extract from the minerals in their natural state. The mycorrhizal fungi produce acids that dissolve minerals in the soil and make them into a form that is readily taken up by the green plants. It’s a perfect symbiotic relationship: you scratch my back, I’ll scratch yours.
Here’s the bad news: Garlic mustard kills mycorrhizal fungi by producing chemicals and releasing them into the soil. Maples, oaks and white ash are all trees that depend on mycorrhizal fungi to succeed. Not only that, garlic mustard inhibits the germination of seeds of many species of native plants, including many spring wildflowers. And it has no natural predators here in the United States where it has invaded from Europe. In Europe it has at least 69 insect predators. Garlic mustard produces chemicals that make it uninteresting as food to herbivores like deer, as well as to insect predators.
So what can we do? First, understand its life cycle and learn to identify it. Garlic mustard is a biennial, meaning that it has a two-year life cycle. In the first year it produces a low rosette of rounded leaves. The second year it sends up 18- to 36-inch flower spikes with pointy, heart-shaped leaves with jagged edges. The small white flowers have four petals and bloom in clusters about an inch or more in diameter. One plant can produce about 4,000 seeds. And although about 70 percent of the seeds will germinate the next year, some will remain viable in the soil for up to 10 years. A website full of good information is invasivespeciesinfo.gov/plants/garlicmustard.shtml.
Garlic mustard leaves, when crushed, smell a bit like garlic. Not as strong, but it has a distinct odor. The Europeans that brought it here in the 1860s often grew it as an herb or a garlic substitute, and I have tasted pesto made from the leaves. But since it produces cyanide at a level much higher than other plants, I choose not to consume it. I figure that if the deer won’t eat it, I won’t either.
Here’s the good news: pulling up garlic mustard is easy. It has a white tap root that comes right out if you give it a tug. It is not like many pest weeds — it doesn’t spread by roots that easily break off and start new plants. Goutweed, Japanese knotweed and witch grass all spread by root, but garlic mustard does not. After pulling it, place garlic mustard in the household trash, not the compost pile. Or if you must, seal it in black plastic bags and let it rot in the sun until fully decomposed.
How can you help to prevent its spread? Pull it if you see it. Watch for first year plants — it sometimes arrives in hay used for erosion control. It will grow in full sun or full shade. A good close mowing of plants will help, though one report I read said that garlic mustard cut at 10 cm (roughly 4 inches) would survive 29 percent of the time.
Garlic mustard is blooming right now! So go look for it. Organize a neighborhood group for a “pulling party.” It behooves us all to look out for it and do our best to reduce its numbers and prevent its spread.
Henry Homeyer can be reached at firstname.lastname@example.org. His website is Gardening-guy.com.