Treating Garlic Mustard in the Offseason

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.

Golden ragwort leaves are palmately veined
and are regularly toothed on the margins.

A fall trip through the woods also turns up other invasives with distinctive foliage or berries like winged euonymus, barberry, and Oriental bittersweet.  Small specimens of these can be pulled from the moister fall soil. These can also be treated with 20% solutions of glyphosate using the 'cut and paint method.'

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.

To control garlic mustard one needs to use a combination of tactics appropriate to the situation and season.  Winter spraying with glyphosate will be easier and more effective than fire.  In the spring, pulling or targeted spraying would be most effective.  And for those plants remaining in the late summer, cutting close to the ground or continued pulling will be in order.

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.

 

It looked like a mild winter until...

We've been having a pretty mild winter this year.  The average temperature in the U.S. in February 2017 was 7.3 F above the 20th century average.  Comparing photos I'd taken in 2014 and 2015 with this year indicates that this year we are 2-4 weeks ahead, based on the blooming of the crocus and forsythia.  On March 14th we finally got a good dose of snow in our neck of the woods (west-central Maryland).

This late snow is out of place with the forsythia that has been blooming since late-February this year.

About a week before the snow storm I took a walk around the woods to see what was starting to come up.  The first thing I checked on was the spicebush.  It usually begins blooming shortly after the forsythia.  This year, while the forsythia had been in bloom for a couple of weeks, the spicebush was just getting started.

March 10th and the flower buds on the spicebush were just opening.

The next plant I checked was the pussy willow, Salix discolor.  This native tree/shrub is one of the earliest blooming native plants and is an important source of pollen to early season native bees.  Since this species is dioecious, only the male plants are sources of pollen; however both male and female flowers have nectar.

The buds of this pussy willow are just opening.  When fully in
bloom the flower buds of this male plant will be covered
with yellow pollen-bearing anthers.

Looking down on the ground in the leaf litter I found a number of Virginia bluebells, Mertensia virginica, that had just come up.  Sometimes the new leaves have a purple tinge to them, but that color quickly fades to green.  The spikes of flower buds follow quickly after this first flush of leaves.

These Virginia bluebells have just come up.

Also showing up on the ground was white avens, Geum canadense, which is pretty common in this area.  While its not particularly beautiful in bloom, it does fill in gaps in the shady understory and its wispy white flowers break up the sea of green leaves.  In the early spring it is by the light colored veins on the deeply divided leaves.

The leaf markings on this white avens rival those of some Heucheras;
however, as it matures the dominant leaves will be smaller and the veination less noticeable.

The plant to the left in this photo is purple deadnettle, Lamium purpureum.  This introduced species is scattered throughout the shady areas.  While weedy, it does not appear to be causing too much trouble with the other plants.

These violets look a little like garlic mustard, ...

Also noted among the fallen leaves were fresh leaves of some native violets, probably woolly blue violet, V. sororia.  These nearly round leaves have finely serrated margins (crenate) and fairly smooth leaves.

They can be distinguished from the over-wintering garlic mustard rosettes that have longer, slender petiole and leaves that are deeply veined and more deeply toothed serrate leaf margins.

Garlic mustard has deeply veined leaves that
look tired, having been out all winter.

Garlic mustard is not the only invasive species that is evident right now.  In fact late winter is a good time to spot some invasive species since they tend to come to life a little before most of the native species.  Multiflora rose and barberry are both leafing out now making them stand out in the woods.  Since the soil was soft I was able to pull a number of these bad guys out of the ground.  This is also a good time to spot Japanese honeysuckle growing up in the trees.  Now is a good time to  cut these climbing vines and deny the roots an early burst of energy.

Most native honeysuckles have the two terminal leaves fused together
like this just below the flower bud.

While not growing with the same vigor as the Japanese honelysuckle at this time, the native coral or trumpet honeysuckle is also leafing out. Flower buds are beginning to form, though the normal bloom season is closer to mid-spring.