Late last winter I began testing the effectiveness of
spraying garlic mustard, Alliaria
petiolata, rosettes with glyphosphate.
Spraying in late fall through winter has the advantage that most native
species are dormant at that time. While
I did not have early satisfaction of find a lot of dead garlic mustard plants
it did seem as though there were many fewer blooming stalks by mid-spring. You can read about that in my blog post ‘Fighting
Garlic Mustard with Fire? Or Something Else…’Here's a typical patch of garlic mustard as it looks in November. It is accompanied here
by Japanese honeysuckle, another invasive that can treated at this time.
This fall I am repeating the spraying. I had to wait until after some sub-freezing temperatures to make sure that the native vegetation was dormant and hence unaffected by the glyphosate spray. The advantage of spraying in late fall is that there is less risk to the native vegetation, particularly the spring ephemerals, like Dutchman’s breeches and spring beauties. Another reason to wait until fall is that a large number of garlic mustard seedlings (80-90%) do not survive the first year, as reported by The Nature Conservancy. So by waiting there will be fewer plants to treat and less herbicide used. The drawback to fall spraying is that the fresher leaf cover on the ground can hide more of the rosettes. Since I spray the individual rosettes rather than blanket spray I can move the leaves out of the way as I work and can use a lot less herbicide. You can find a report on the effectiveness of winter spraying in this paper by Frey, et al.
Another invasive that can be treated in fall is Japanese honeysuckle, Lonicera japonica. Here on my zone 6-7 property Japanese honeysuckle does not go completely dormant and is still susceptible to glyphosate spray. I sprayed a plot that was fairly dense with the honeysuckle this past week. We’ll see in the spring if it made a difference.
There are a number of other invasive ground covers that are still green now, but according to the literature I found foliar glyphosate is not particularly effective at this time. Among these are vinca, Vinca minor, creeping Charlie, Glechoma hederacea, and mock strawberry, Potentilla indica. So despite my desire to be rid of these I did not waste any of my spray on these unwelcomed plants.
By target spraying I can avoid the native species that still have living foliage. These include some plants with rounded leaves similar to garlic mustard like white avens, violets, and golden ragwort. Other natives to avoid spraying include sedges and cool season grasses, ferns and any other early spring plants coming up early.
Winter rosettes of white avens have whitish veins. You can see how it compares to the deeply veined leaves of garlic mustard, marked with white*'s. |
In comparison to garlic mustard, violet leaves are smooth and somewhat glossy. |
I spotted this burning bush/winged euonymus because it still had foliage on it. Others with their namesake bright red foliage were even easier to see and pull. |
Summary of Garlic Mustard Control Measures
Method |
Timing |
Pros |
Cons |
Effectiveness |
Fire |
Late Fall to Early Spring |
Non-toxic. |
Difficult to achieve ideal conditions; Need to keep under control. |
A good moderately hot fire is effective, but difficult to achieve. |
Herbicide Spray |
Dormant season |
High kill rate; can be targeted; no soil disturbance. |
Spraying toxic materials; may affect non-target species. |
Dormant season spraying reduces non-target species effects. |
Cutting |
Summer, after flowering |
Non-toxic; cutting at ground level nearly 100% effective; minimal soil disturbance. |
Labor intensive; disposal of cut stems/flower stalks; use of weed whacker causes collateral damage. |
Very effective when done right. |
Pulling |
Anytime ground is soft |
Non-toxic; very effective as long as most of the root is removed. |
Labor intensive; disposal of pulled plants required once flowers are present; soil disturbance. |
Very effective. |