Kill it With Fire, and Other Methods of Invasive Species Management : Garlic Mustard - Alliaria petiolata - Herbe à ail

Though you may not have thought much about it, I can guarantee that you've encountered this extremely invasive species. Alliaria petiolata (garlic mustard) is native to Europe and was introduced in North America as a flavouring herb [1]. Since its introduction, this plant has spread broadly, and, because of some of its growth and reproductive characteristics, poses a serious threat to the biodiversity of forest understory habitats [2].

Alliaria petiolata in a park in Montreal

A North American range map for the introduced species can be found here. The plant is listed as invasive, prohibited, prohibited invasive, prohibited noxious, banned, and Class A or B invasive in various parts of the US [3]. The plant is prohibited in Alberta and Saskatchewan [4]. It is a member of the Brassicaceae (mustard family) and is biennial; the first year, it produces a basal rosette of leaves, and the second, a flowering stalk [2].

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 [5], 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 [5].

Dense population of Alliaria petiolata

What do we do about these kinds of plants? Well, it depends a lot on the particular environments in which we are trying to control the species, and on the species and its life cycles and reproductive ecology. In the case of A. petiolata, for example, pulling is a poor method of control which can actually make the situation worse, as the plant thrives in disturbed habitats and pulling creates lots of disturbed soil [5]. With other plants, pulling might be a better solution.

Flowers and seed pods of Alliaria petiolata

Burning has been attempted as well, but unfortunately not very successfully; when the fire doesn't get every last seed, the burning produces a habitat that is very favourable to the spread and reestablishment of the population [6]. Chemical control is similarly patchily effective at best.

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 [5]. 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 [5]. A the population level, it is recommended to clip the smaller spreading populations first and then work on controlling the source populations [5].

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.