Invasive species alert: Garlic mustard is invading Manitoulin forests

Flowering stems of garlic mustard (foreground in front of the Forget-me-nots) with clusters of white flowers with four petals. Stem leaves are spade-shaped with scalloped edges.

by Judith Jones

MANITOULIN – Another invasive species has crept onto Manitoulin Island and has now spread enough to become worrisome—garlic mustard. 

Garlic mustard (Alliaria petiolata) is a herbaceous (not woody) plant in the mustard family. The upper leaves are shaped like a spade with a long point, while leaves on the ground are like a rounded heart. Both have scalloped edges and a wrinkly, veiny surface texture. They look a little like violet leaves but smell of garlic when crushed. Garlic mustard plants have a two-year life cycle. In the first year, they are just a cluster of leaves growing on the ground. In the second year, they make a stalk up to one metre (three feet) tall and a bunch of small white flowers with four petals. 

Garlic mustard is capable of wiping out native forest plants. It is a danger to the trilliums, fiddleheads, leeks, adder’s tongues and other spring wildflowers. It has some very scary weapons in its tool box. The roots can put chemicals into the soil that stop other plants from growing. Also, one large plant can produce over 3,000 seeds and the seeds can last up to 30 years in the soil. With these weapons, a stand of garlic mustard can become the dominant forest ground cover in just five to seven years.

This week, garlic mustard is beginning to flower and is very noticeable. Now is the time to check whether this invader is on your property and to make plans to do something about it before it destroys our forests. Getting garlic mustard under control is going to be a multi-year process, so the important thing is to get started and to keep going, even if it’s just a little bit at a time. Remember it’s going to take a while. Here are some things you can do.

Hand pulling: This is a good strategy for small patches if it’s done before the seed pods are forming. You’ll spot the tall flowering plants, and they pull out easily. Grasp the plants at the base and pull gently to try to get the whole root without breaking it or the stem. Once you’ve got an eye for the tall plants, you’ll also probably notice a ton of the first-year plants on the ground. Do what you can to get them, too, but make a mental note to come back again in a week and see if some have grown up and also to get back again to this spot next year.

One downside of hand-pulling is that it opens up the soil and allows new garlic mustard seeds to sprout. Remember, control will take a few years. However, you can cover up the newly exposed ground with leaves or some other type of mulch or plant something else there, but you really don’t want to introduce anything that might become troublesome, so stick to native plants (see link at the bottom). 

Cutting or mowing at the bottom: This works if you do it before the plants produce seeds. The plants don’t all flower at once and some  stems may re-sprout, so cutting may need to be done a few times over a couple weeks. Cutting at the bottom is better than just cutting off the flowers because you won’t leave a stem where new flowers can sprout. It also doesn’t stir up the soil to expose more seeds.

Cutting off the flowers: This does prevent the plants from going to seed and is a little less work than No. 2, but you have to keep at it until the end of the summer since new flowers can still sprout from the stems.

Careful disposal of the material: Don’t compost or pile the stems because the flowers can still produce seeds even after the stems are cut. Seal the material in paper yard waste bags or black plastic garbage bags and leave the bags in the sun. After at least a week, the bags can be burned or taken to the dump.

Garlic mustard was intentionally brought from Europe in the 1800s to be grown for edible greens. Smaller plants can be eaten if you like the flavour. 

There is a lot more to know about garlic mustard including how to restore the bush and the soil after removing a lot of plants, and what native species can be planted that will out-compete this invader. Ontario Best Management Practices (the official document that tells you what works and what doesn’t work and why) and a lot more info can be found on the website about Ontario’s Invading Species at InvadingSpecies.com/garlic-mustard/

Judith Jones is an environmental consultant based in the Bidwell area. Since 2016 she has co-ordinated the Manitoulin Phragmites Project, a program to eliminate invasive phragmites from shores and wetlands on Manitoulin Island and surrounding islands.