You've seen it. Probably hundreds of times already this spring. It lines the edges of trails, fills in the shady corners of yards, creeps along fence lines and ravine paths. Heart-shaped leaves with toothed edges, clusters of small white four-petaled flowers, and — if you crush a handful — a smell that's unmistakably garlicky.
That's garlic mustard. And if you live in Hamilton, it's everywhere.
Here's the thing: this isn't just a weedy nuisance you can ignore. Garlic mustard is one of the most ecologically damaging invasive plants in Southern Ontario — and it has a surprising connection to ticks that I think more people should know about. So let's get into it.
What is garlic mustard, exactly?
Garlic mustard (Alliaria petiolata) is a biennial herb originally from Europe. It was brought to North America by settlers in the late 1800s as a food and herbal medicine plant — which, honestly, tracks, because it does taste like garlic. The problem is it escaped cultivation and has been making itself extremely at home ever since.
It's been in Ontario since at least 1879, when it was first recorded in Toronto. It has had a long time to get comfortable.
Here's how the life cycle works:
- Year one: Garlic mustard grows as a low rosette of kidney-shaped leaves. It stays green under snow — giving it a head start on almost everything else in the garden.
- Year two: It shoots up a flowering stalk (up to a metre tall) in early to late May, sets seed, then dies.
The seed numbers are genuinely alarming. In ideal conditions, a single plant can produce up to 150 seed pods with up to 22 seeds per pod. One study found a dense population producing over 105,000 seeds per square metre. Seeds stay viable in the soil for up to five years.
One more thing worth knowing: garlic mustard can self-pollinate. A single plant is enough to start a new population.
Why Hamilton specifically should care
Garlic mustard is a Category 1 invasive species on the SERO 2025 list — the highest priority tier in Southern Ontario. It's in the same category as Japanese knotweed and phragmites. This isn't a "keep an eye on it" situation.
And Hamilton is a prime target. Our ravine system, trail networks, and woodland edges are exactly the kind of habitat garlic mustard loves. If you've walked the trails at the RBG lately, you've likely walked through it — and if you're anywhere near the alleys around Barton Street, you've definitely seen it creeping along fence lines and filling in every disturbed edge it can find. The RBG's natural areas have been dealing with this at a scale that's hard to overstate — over 300 Eurasian plant species have colonized their natural areas, with garlic mustard among the most aggressive. They have dedicated invasive species technicians working on this seasonally. It's that serious.
What it actually does to a forest floor
This is the part that gets me. Garlic mustard doesn't just compete with other plants for space and light — it chemically undermines them.
It's allelopathic, meaning its roots release chemicals (including glucosinates and cyanide compounds) that change soil chemistry and prevent other species from germinating nearby. This is a neat trick from an evolutionary standpoint and a disaster from an ecological one.
The plants it displaces most aggressively are spring ephemerals — the native wildflowers that carpet forest floors before the tree canopy leafs out. Trilliums, bloodroot, wild ginger, Wood Poppy, Drooping Trillium. These are slow-growing, low-competition plants that have no defences against something this aggressive.
There's one more layer to this: garlic mustard's chemicals also deter deer. So deer avoid it and instead eat the native wildflowers, accelerating the loss of exactly the plants that need protecting. The deer make the problem worse without even meaning to.
One documented site with high plant diversity saw garlic mustard appear about 12 years ago — and within five years, it had nearly completely dominated the area. The effects can be long-lasting and may permanently alter forests, even after the garlic mustard is removed.
Wait — garlic mustard and ticks?
This one surprised me when I first came across it. I'd heard the claim that "garlic mustard is Mother Nature's way of getting rid of ticks" — the idea being that its chemicals might repel ticks or make the habitat inhospitable to them.
Spoiler: all research points the other way. Garlic mustard actually increases deer tick populations. Here's the chain:
- Garlic mustard's allelopathic takeover collapses plant biodiversity on the forest floor.
- Fewer plant species means fewer animal species — insects, birds, small mammals — that depend on that variety.
- When animal diversity drops, one species tends to fill the gap: the white-footed mouse.
- White-footed mice are the primary reservoir for the Lyme disease bacterium. They infect between 40% and 90% of tick larvae that feed on them.
- In diverse habitats, ticks feed on lots of different animals — most of which are poor Lyme reservoirs. In a mouse-dominated habitat, almost every tick meal comes from the most infectious animal available.
The result: more garlic mustard → less biodiversity → more mice → more infected ticks. It's a cascade that starts with a plant most people wouldn't think twice about pulling.
Preliminary research has drawn a direct link between invasive plants like garlic mustard and increased Lyme disease risk. This is an area where more research is still needed, but the directional evidence is clear. (If you want to go down the rabbit hole on this one, the Nantucket Conservation Foundation did a thorough breakdown — link in resources below.)
The pesto question
Yes, you can eat it. And if pulling a bunch of garlic mustard with the goal of making pesto gets you out there pulling, genuinely — great. Gardenstead has a solid Ontario-framed recipe linked in the resources below if you want to try it.
Just know that foraging won't solve the problem. The seed bank in your soil stays viable for up to five years, and if you're not careful about how you harvest and dispose of the rest of the plant, you can spread seeds in transit. Pull it, use what you want, bag everything else. Don't compost it. Don't put it in yard waste. Garbage only.
What you can actually do
Here's what actually works — and none of it requires a perfect yard or a full weekend.
Pull it by hand in spring, before it sets seed. This is the window. Once the seed pods form, you've lost that round. Aim to get it while it's flowering but before pods develop — roughly May in Hamilton.
Pull the whole plant, roots and all. Leaving the root behind gives it a chance to resprout.
Bag it for the garbage. Not compost, not yard waste — garbage. Seeds can survive both. Some people leave pulled plants to desiccate in the sun for a few days before bagging, which is fine as long as they haven't set seed yet.
Think in years, not seasons. Because the seed bank persists for up to five years, you're committing to a multi-year pull. That sounds daunting but here's the reframe: every plant you remove before seed set is hundreds of thousands of seeds that don't go into your soil. You are making a real difference even if you can't see it immediately.
Start at the edges of your infestation, not the middle. The OIPC recommends targeting smaller satellite populations first — the new patches spreading outward — rather than trying to tackle the dense core. Stop the spread before you dig in.
One more thing worth knowing: garlic mustard grows in an advance-retreat pattern because of its biennial cycle. You might see a lot of flowering plants one year and mostly rosettes the next. This can make it look like the population is shrinking on its own. It isn't. The infestation is expanding — just doing it in a two-year rhythm. Don't let a quiet year fool you into thinking it's handled.
Take what works, leave the rest — and give yourself credit for every plant you pull before it sets seed. You're doing more than you think.
Learning resources
- RBG — Identifying and Removing Invasive Garlic Mustard — good visual guide for spotting it in both years, including the first-year rosette.
- RBG — Invasive Species: Garlic Mustard — a closer look at the plant's ecology and what you can do to help, with RBG Herbarium Intern Nina.
- OIPC Best Management Practices for Garlic Mustard (Ontario) — the most thorough Ontario-specific guide to identification, lifecycle, and control methods.
- Nantucket Conservation Foundation — Garlic Mustard: Friend or Foe in the Battle Against Deer Ticks? — the best plain-language breakdown of the tick connection and the research behind it.
- Gardenstead — How to Make Garlic Mustard Pesto — Ontario-framed recipe, if you want to eat what you pull.
- SERO 2025 Invasive Exotic Plant Species Ranking for Southern Ontario — full provincial ranking list, for context on where garlic mustard sits relative to other invasives.