|Alliaria petiolata in a park in Montreal|
So what's the big deal with this plant?
Well, it's a collection of things. This introduced plant has none of the natural predators here that are present in its native range , so unlike in its native range it spreads here essentially unchecked. It is known to damage native plant and tree populations [5,6]. There is some evidence that it inhibits the growth of other plants around it, as well [5,6]. This plant is also very difficult to control, as it produces large amounts of seed, is a perennial, and prospers in disturbed areas .
|Dense population of Alliaria petiolata|
|Flowers and seed pods of Alliaria petiolata|
Simply clipping the plant back also isn't enough; if it's clipped back before it has produced flowers, it'll just keep on coming back . But, cutting after the flowers have started but before the seeds have set is an effective strategy if repeated over time, especially if the plants are cut as low as possible (ground level) [5,6] -- but note that the clipped parts must be removed and properly disposed of or they can still set the seed - the Nature Conservancy of Canada recommends cutting the plant repeatedly from top to bottom . A the population level, it is recommended to clip the smaller spreading populations first and then work on controlling the source populations .
Basically if you're in North America and you've got this plant somewhere, be ruthless. Wait until it flowers, then cut it up from top to bottom. Then do it again in the next years until the thing stops coming back up.