Garlic Mustard Removal Study

by Carl Strang

 

One of the challenges facing people trying to restore biodiversity to native woodlands is invasive plants. Earlier  I outlined the general problem in the context of shrubs. The herbaceous plant causing the most trouble in our woodlands is garlic mustard. Garlic mustard is a biennial. Seeds sprout in spring, grow into rosettes of leaves that survive the winter, then the plants grow up, flower and produce seeds in their second spring. At this point in the season the rosettes look like this.

 

gm-rosette-b

 

When I was at Willowbrook I undertook to remove garlic mustard from the fenced area that contains the outdoor animal exhibit. When I was transferred to Fullersburg I did the same in the Wildflower Trail area. Both were high quality areas in terms of the native plants that were present, but garlic mustard was expanding, and suppressing the native wildflowers. I started pulling out the garlic mustard each spring, and was gratified by the quick recovery by diverse native species. Then I began trying fall and winter pulling, and got good results with that, too.

 

My annual review of the scientific literature last December turned up a study* describing success in controlling garlic mustard by clipping the plants rather than uprooting them. I decided to try a study of my own, that would compare results of uprooting plants versus breaking them off, early and late in the season. Yes, a study had been done. But replication is important in science. I wanted to see for myself that new plants can’t grow up from the decapitated roots. Also, the earliest the other researchers had clipped their plants was late April. I wanted to try it earlier. My move to Mayslake was timely, because the restoration program is relatively advanced there, and I can focus on research rather than rescue.

 

I have set up 3 study plots, each 3 by 3 meters. I used large nails to mark the corners.

 

garlic-mustard-plot-corner-b

 

When the time came to treat the plots, I temporarily outlined the plot with bright orange string.

 

gm-plot-1-pre-treatment-b

 

Each of the 9 square meters in each plot gets one of 3 treatments: uprooting, breaking off below the lowest leaves in mid-March, and a control that will be left until the plants are about to flower. At that point I will cut them off below the lowest leaves. This last treatment follows a practice recommended by some experienced restoration specialists, who discourage uprooting because it may stimulate germination of garlic mustard seeds in the soil. I used a random number generator (easy to find on the Internet) to determine which squares got each treatment.

 

Here is an experimental square from which plants were removed.

 

gm-plot-3-post-treatment-clipped-3b

 

And here is a control square with the plants still in place.

 

gm-plot-3-post-treatment-control-2

 

I counted the plants in each square as I treated them. In the three study plots combined the total number of pulled plants was 1395, pinched off total 1617, and control plants 1176. The overall average density was 155 plants per square meter. Now, I wait and see what will happen. I’ll provide an update later.

 

(It should be obvious, but I’ll state it anyway, that I could do this kind of manipulation on public land as a forest preserve district employee, but still had to get clearance to do so.)

 

*Here is the reference for the study I mentioned: 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.

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2 Comments

  1. greentangle said,

    March 20, 2009 at 1:05 pm

    Have been enjoying your posts for a couple months. Nothing to say about garlic mustard, but in preparation for a move I was going through a pile of old “Environmental Ethics” last night and spotted your comment re wildlife rehab. Small world.

    • natureinquiries said,

      March 21, 2009 at 11:16 am

      Good luck in your move!


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