Garlic Mustard Results

by Carl Strang

Last week I returned to the garlic mustard plots I had treated in the spring, to assess the results of my mass cutting experiment. This was the finale of a series of experiments to compare alternative manual removal methods for this invasive biennial. In one plot of approximately 10 square meters I used a grass whip to cut the plants at a height of 10-12 inches, removing the fruiting tops but leaving some foliage. I found that 58% of these plants subsequently had flowered and produced a few seeds, though there was very little reproductive output, especially compared to intact plants.

This is the typical paltry fruit production on one of the cut plants.

With only a little more effort and time, cutting the plants closer to the ground and removing practically all the leaves is feasible. In the other study plot, about the same size as the first, I had cut the plants at a height of 4-6 inches. There, only 1% of the plants produced a tiny number of seeds. The difference was stark, but I calculated the chi-squared test statistic anyway, using the taller-cut plot results (representing a practice I have seen used in restoration work) as the basis for expected values in the shorter-cut plot. The value of 390 is, of course, highly significant.

The short-cut garlic mustard plants on July 6 were almost all naked dead stems. The green leaves all belong to other plant species.

Circumstances had prevented my doing this assessment as early as I would have liked. The stems all were dead, and some had been trampled to the ground (the shorter ones more so than the taller ones), so that the total numbers of stems in the two plots were different: 472 in the higher-cut plot, 297 in the shorter-cut plot. To the eye they had been identical at treatment time. Nevertheless, I feel confident in recommending the practice of mass cutting garlic mustard plants at a height of 4-6 inches as they are completing their blooming period. The cut tops need to be hauled away, as they potentially could release some seeds as they dry. As established in the early experiments, such late cutting of the plants kills them. The common practice of uprooting garlic mustard, though effective, is less efficient. It should be reserved for situations where mass cutting would cause undesirable harm to other species (for instance, in high quality areas where the garlic mustard is just becoming established), or where the removal must be done early in the season.

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