by Carl Strang
We have entered the time of year when removing garlic mustard is a popular pastime of people wishing to improve woodland plant communities. Garlic mustard is an Old World import that has the unpleasant characteristic of poisoning the plants around it (including trees), and spreading to dominate the forest floor.
In early spring it’s nice and green, and has arrays of small white flowers, but it quickly goes to seed and dies, leaving the area barren except for its dead brown stalks. One of its weaknesses is that it is a biennial, which limits its rate of spread when it first invades an area. In its second, flowering year it can be uprooted and removed.
As I have outlined in this blog through descriptions of various experiments over the past several years, the plants can be clipped or pulled, depending on the point of the season. That study was inspired by a paper (Pardini, Eleanor A., Brittany J. Teller, and Tiffany M. Knight. 2008. Consequences of density dependence for management of a stage-structured invasive plant (Alliaria petiolata). Am. Midl. Nat. 160:310-322) claiming that uprooting is unnecessary, and sharing experimental results in proof.
My own research has confirmed that conclusion, though timing plays a role. As long as the second year plants are rosettes (through March into some point in April), uprooting is more effective than cutting. Once the stems elongate and begin to bloom, clipping at ground level kills the plants as effectively as uprooting them.
In last year’s final experiment I applied a grass whip to cut green fruiting plants at two heights, 10 inches or so above the ground, and below the lowest leaves (2-3 inches). Both were effective, but the lower level cutting (which was no more difficult) was extremely so, with only about 1% of the plants producing minimal abortive looking fruits.
This week I looked at those plots again. The higher cut area had a few scattered seedlings. The close cut area had practically none. In contrast, surrounding untreated areas were dense with seedlings.
I’ll close with a reference to another paper (Chapman, Julia I., Philip D. Cantino, and Brian C. McCarthy. 2012. Seed production in garlic mustard (Alliaria petiolata) prevented by some methods of manual removal. Natural Areas Journal 32: 305-315). They conducted experiments to see whether uprooted and cut garlic mustard tops can still produce viable seed. They likewise found support for killing the roots by cutting off the tops. A few produced tiny sprouts that died without reproducing. No seeds were produced by plants uprooted when flowering. Cutting off the roots did not affect seed production in the tops. Early fruiting plants produced more viable seeds after cutting than did late fruiting plants. Plants that were hung or piled produced fewer seeds than those scattered on the ground. They did not measure seed production in undisturbed plants. They mention that cutting plants close to the ground with a string-trimmer should be effective. Pulling or cutting before the plants are fruiting should require nothing more, but fruiting plants should be removed from the site. In remote places where hauling away the mass of entire cut plants is impractical, it should be enough to remove the fruiting parts and take them away for disposal.