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.

 

July 2020 What to do about Japanese Stiltgrass NOW

In late July through early August Japanese stiltgrass, Mircostegium vimineum, puts on a major growth spurt prior to blooming.  This presents an opportunity for removing a large amount of this invasive annual grass before it blooms and begins to set seed for the following years.  There are several methods that can be used now, which is best depends on the particular situation. 

 

Pulling  This is my method of choice in areas of mixed species. In late summer stiltgrass can grow to 3-4 feet tall.  At that height it is above many desirable plants and you don’t need to bend over too far to pull it out.  By using a loose grip and grabbing higher up on the plant mass I can selectively pull out the stiltgass and leave most perennial and more deeply rooted plants in place. By wiggling the grass side to side as I’m pulling the relatively weak roots are broken and the stems remain intact.  

Here stiltgrass and the native grass nimblewill, Muhlenbergia schreberi, are growing together.

Pulling the stiltgrass with a light touch leaves the well rooted nimblewill in place.

I usually leave the piles of stiltgrass to dry out in the sun for a couple of days before disposing of it.  If the grass is not setting seed I’ll put it in a brush pile or in a segregated location where it can break down but will not accidentally be spread elsewhere.  If the grass has bloomed it will need to be bagged or put into an isolated location where the seed cannot escape.  This is the number one reason for pulling before bloom!

This is a woodland area where I have been pulling stiltgrass of 2-3 years. 
Though it looks dense there are not that many individual plants.

 

After 20 minutes of pulling I was able to clear this area of stiltgrass. 
What remains is a ground cover of mostly rosy sedge, Carex rosea.

Low Mowing  Mowing as close to the ground as possible in late summer (just before bloom) is a common recommendation for combating stiltgrass.  This prevents blooming and seed set in the upper stems.  Unfortunately stiltgass will also set seed in the stem at the base of the plant, 1-2 inches above ground level.  These cleistigamous flowers are difficult to remove without cutting really close to the ground.   If you wait too long to mow, after seeds begin to develop, mowing may only serve to spread seed unless you have a well fitting grass catcher on your mower.  ( In that case you should dispose to the clippings in a way that the seed will not escape into the environment.)  These extra flowers make stiltgrass very difficult to remove from turf areas.

Late summer mowing had been used in this area for 2-3 years.  Here it is in 2017. 
There is a lower density of stiltgrass, but still a lot.

I’ve been trying to remove stiltgrass solely by timed mowing in one area but have made only minor progress over 3-4 years.  What I have found to be most effective in lawns is to use a pre-emergent herbicide, marketed for crabgrass, in early spring.

Here is the area in 2020.  In addition to late mowing I treated this area with the
preemergent herbicide Dimension.  Above the line has been treated for
two consecutive years, below the line only once in spring 2020.

 

Weed whacking  Using a weed whacker can be effective where you can selectively cut down to ground level to remove those stem flowers. It can get tedious if you are trying to preserve desirable plants intermixed with stiltgrass.  You can leave the cut stems in place if the grass has not begun to bloom, but if it has you should consider raking out the debris.  (If plants are beginning to set seed weed whacking will likely spread more seed than doing nothing.)

I had good success in a woodland edge area using this method.  Starting out, the area was nearly a monoculture of stiltgrass so I did not have to be too careful.  In the second and third years I also raked out the cut stems.  I have also seeded the area with Virginia wild rye, Elymus virginicus, a shade tolerant native grass.  Now six years later I can manage that area with just a little pulling. 

 

Fire  Because stiltgrass is a weakly rooted annual burning at ground level can be an effective means for killing it.  I have been experimenting with burning stiltgrass at various times during the year.  Burning in early and mid spring reduces the amount of stiltgrass seedlings but does not eliminate its presence.  This is in part due to the extended time over which stiltgrass can germinate in the spring.  Burning in late spring seems to thin out the amount of stiltgrass but can also reduce the amount of desirable vegetation.  In one spot where I did a mid-spring burn the area was overgrown with stiltgrass by the end of July.  On removing that stiltgrass there were very few other plants growing there. 

This is a small area of lawn dense with stiltgrass that I burned with a garden torch
two days ago.  It's not necessary to consume the entire green plant with fire,
just burn the roots at the soil surface.  Perennial grasses should
bounce back in a week or two.

In this area I pulled most of the stiltgrass then used the torch to burn the surface
to kill any remaining vegetation.  I was then able to immediately plant several
plugs of switch grass, Panicum virgatum.

Where burning has proved very effective is in hard to reach areas where clear ground is desired, such as pathways and fencelines.  Burning stiltgrass is particularly effective in late summer. In lawns with cool season grasses you can burn areas infested with stiltgrass with a garden torch.  The cool season grasses are dormant because of the hot weather and dry conditions.  As the weather cools in September the clumps of cool season perennial grasses will resprout, without the presence of stiltgrass.

Here is the garden torch in action, burning the roots of some 3 foot tall stiltgrass. 
I can stick the torch through the openings in the fence the get to hard to reach areas.

It is interesting to note that heat is conducted better in slightly moist soil than in dry. While burning stiltgrass leaves no chemical residues you must use great caution to keep flames under control and not allow any fire to spread out of control.  There may also be local rules to follow.  

Herbicide   While I have had very good results from using preemergent herbicides in early spring to get stiltgrass out of the lawn, I have not had personal experience using postemergence herbicides to control Japanese stiltgrass.  Based on information provided by a number of university agricultural extensions there are several herbicides that can be used in summer time (up to the onset of flowering)  to control stiltgrass.  Low levels of the broad spectrum herbicide glyphosate, about half the normal concentration, are reported to be effective against Japanese stiltgrass.  Glufossinate is another broad spectrum herbicide that is effective against stiltgrass. 

I pulled this from the Rutgers Cooperative Extension (NJ) site:

Glyphosate and Glufossinate (various trade names) can be used to spot treat Japanese stiltgrass in gardens and planting beds. Both are broad spectrum herbicides that should be applied only to the unwanted plants. If applied to the foliage, stems, or woody portions of desirable plants, it could damage them as well.

Sethoxydim (tradename Bonide Grass Beater Over-the-Top Grass Killer®) and Fluazifop-P-Butyl (Ortho Grass B Gon Garden Grass Killer®) are selective herbicides that can be applied to growing stiltgrass in landscape beds. When used according to the label, these herbicides will not damage most non-grass ornamental plants. Be sure to follow the label closely and heed all precautions.

According to the labels on the Bionide and Ortho products these products are not effective on sedges.  That is a good feature for use in woodland edge habitats since native sedges are a major component of the ground covers growing there.  I not sure if that means that those, mostly desirable species would be unharmed, or, just not killed at a high rate.  I noticed that the label on the Bionide product indicated that it could be used safely on lawn with red and Chewings fescue.

Here is another example of the effectiveness of a preemergent herbicide. 
To the left of the line I applied dithiopyr (Dimension) in mid-March,
to the right is untreated.