Information on Invasive Species in the Great Lakes area

Garlic Mustard
Fred Nix
April 11/08

“Controlling garlic mustard,” according to Franca De Angelis, “is an urgent matter for the Bruce Trail Conservancy.”

Franca, who has hiked the Bruce Trail twice, was speaking at the annual general meeting of the Caledon Hills Bruce Trail Club on Sunday, April 6th. According to her, garlic mustard is threatening native woodland flowers and may even be retarding the development of Ontario hardwood forests.

Given that hikers are likely spreading the garlic mustard seeds, this gives Bruce Trail members a particular interest in the subject

Franca and I (her husband) first became aware of garlic mustard on a geological hike with Beth Kümmling in 2004. We were tramping through the Pretty River Valley, listening to Beth describe a long since drained glacial lake, when one of the budding geologists on the hike said: “Oh, look at all the garlic mustard.” Not knowing much about it, but being told that it out-competes native wild flowers like trilliums, we happily started helping pull the plants from the forest floor. So much for Beth's glacial lake!

We didn't think too much more about garlic mustard until, about three years ago, it first started appearing in the Hockley Valley, our home turf. Interestingly, its first appearance was along the edges of the Bruce Trail or at places where hikers parked their cars. Does this give a clue as to how it is spread? It appears that the seeds attach themselves to hiker's boots or possibly their clothes – and, perhaps, the fur of the dogs that hiker's sometimes have with them – and then fall off at some other location.

However garlic mustard is spread, when it first appeared in the Hockley Valley, Franca became concerned. One of the first things she did was attend a Bruce Trail land stewardship meeting in Owen Sound where she listened to Hamish Duthie talk about garlic mustard and ways to control it. Hamish is on the Bruce Trail's Environmental Committee. She also contacted the Royal Botanical Gardens in Hamilton, one of the few organizations that have a garlic mustard management program in place – right where the Bruce Trail traverses its property.

Garlic mustard is an invasive species, originally brought to North American from Europe. The first record of it in North America is in 1868 in Long Island, New York. It now is common in the forests of many states and, unfortunately, Ontario. The problem with it is that it has no natural enemies in North America – insects, animals or parasites – so it grows unchecked. And, as it grows, it out-competes native forest plants by monopolizing light, moisture, nutrients, soil and space. Anyone who enjoys a hike in the woods in the spring, admiring the trilliums, spring beauties and Dutchman's breeches, should be worried. Garlic mustard will replace these flowers. And if you doubt that, take a hike along the Bruce Trail through St. Catharines to see what a menace garlic mustard can become.

Garlic mustard reproduces only by seed. Most seeds germinate within the first or second year after being produced but can remain viable in the soil for up to five years. Seeds germinate in the spring and form low growing rosettes of dark purple to green, kidney-shaped leaves with scalloped edges. Young leaves smell of garlic or onion when crushed, although the odour becomes less intense as plants grow older.

Research completed in 2006 suggests that garlic mustard harms fungi that native forest trees need for proper growth; hence the worry that the spread of garlic mustard may retard the growth of native tree species. Here is how Lisa Gross describes the problem:
“Many forest trees and other vascular plants form mutually beneficial relationships with arbuscular mycorrhizal fungi (AMF). The fungus has long filaments that penetrate the roots of plants (forming branched structures called arbuscules) and snake through the soil in an intricate interwoven network of mycelium, which effectively extends the plant's root system. AMF depend on the plant for energy, and the plant depends on the fungus for nutrients. Many non-native plants, including garlic mustard, do not depend on native AMF and often take root in landscapes altered by development or logging, where AMF networks are disturbed. When these non-mycotrophic invasives propagate, they may diminish AMF densities even further.

Biologists are especially concerned about what might happen if a non-mycorrhizal invasive plant turns up in a mature, intact forest with an established mycelial network—which is just what garlic mustard has started to do. In the North American forests it has recently invaded, the plant inhibits the growth of understory plants, including the seedlings of canopy trees. Stinson et al. suspected the invader might somehow be thwarting the symbiotic relationship between fungus and tree.”1

Management strategies for controlling garlic mustard are varied and diverse – a lot depends on the location and the characteristics of a particular patch.

For two sections of the Bruce Trail in the Hockley Valley – the section immediately south of Dunby Road and the section on the south side of the valley between the 3rd and 4th lines of Mono – Franca and Fred are leading teams of volunteers on May 13th who will pull the plants and bag them in black garbage bags. To make sure the seeds are killed, these bags will be sealed and left under the sun for several weeks – effectively ‘stewing' the vegitation inside. A better strategy would be to burn the plants but this requires a burn location and plenty of combustable material, something that is not always available.