Deer Control & Garlic Mustard

Garlic Mustard (Alliaria) is listed by the Indiana Native Plant and Wildflower Society as one of the ten most destructive invasive species in Indiana. Beverly Shores residents have first-hand experience with the scourge. Each spring, residents pull large numbers of garlic mustard plants from Beverly Shores roadsides, attempting to prevent the plants from taking over the forest understory.

Control isn't easy. As the Aldo Leopold Foundation's Woodland School notes,

Garlic mustard is a very vigorous biennial plant. If control efforts are being carried out when garlic mustard is nearing maturity (when the plant is in or beyond the flowering stage), it will retain the ability to produce seeds and therefore the only control option is to hand‐pull, bag, and dispose of the plants properly.

Garlic mustard can be sprayed with glyphosate, but that will kill much more than the garlic mustard. Pulling is only effective if the activity doesn't spread the seeds widely. It requires long-term efforts as the plant is a biennial and the seeds can survive for a number of years.

In Beverly Shores, garlic mustard is less pervasive than it once was. Why? Part of the answer is aggressive removal by residents, but ERG believes that our deer control efforts have played an important role. There is science to support this belief. A study carried out at the University of Pittsburgh compared garlic mustard growth in local woodlands browsed by deer with the plant's success in plots from which deer were excluded. The results?

The presence of deer ensures Alliaria's high population growth rate and high density. … Overall, where deer are present, Alliaria populations are increasing ~30% annually, but in plots protected from deer they are declining ~12% annually and if conditions remain the same will eventually go extinct …

Or, as a press release on the study characterizes the results, “Too much garlic mustard in your neighborhood forest? Actually, the problem may be too many deer.”

See In a long-term experimental demography study, excluding ungulates reversed invader's explosive population growth rate and restored natives, published in the Proceedings of the National Academy of Sciences, vol. 111, no. 12.

Where deer are present, garlic mustard takes over. What loses out? Trillium and other flowers. The above study found that trillium populations were decimated by deer, but recovered where deer were excluded. The results from the Pittsburgh area repeat those of a study done in the Indiana Dunes National Lakeshore (Negative Effects of White-tailed Deer on Native Wildflowers at Indiana Dunes National Lakeshore (INDU)). Deer browsing not only cut trillium, which deer love to eat, but also Jack-In-The-Pulpit, which they find unpalatable. The impact on the latter is supposedly "due to the great decrease in soil quality caused by the removal of vegetation by overabundant deer, which increases compaction and decreases moisture." (See Blame Bambi for Bountiful Garlic Mustard)