As we are coming to the end of stiltgrass
season for 2020 I wanted to review what I’ve learned about controlling this
invasive species on my Mid-Atlantic property over this year.
Pulling
Pulling Japanese stiltgrass, Microstegium vimineum, is an effective means for
controlling this annual grass, but it is extremely labor intensive. While its primary germination period is
mid-spring it does continue to germinate into summer, particularly if more
openings appear in a previously shaded area.
So you may find that you need to redo areas in late summer that you
thought had been taken care of earlier.
This was the case in a shaded woodland where I thought I did a pretty
good job clearing the stiltgrass at the end of July. When I returned to that place in
mid-September it was in need of more pulling.
Apparently I left enough scraps around that they were able to
regrow. So if you only want to pull
stiltgrass once in a season get every bit out the first time, or wait until
just before it goes to seed, maybe early September (depending on local
conditions) and then pull out all that you can.
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This was the initial condition, before any clearing in 2020.
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This was after clearing in late July, by September this area was overgrown again with stiltgrass. This time it only took about 10 minutes to clear the area. The underlying Rosey sedge, Carex rosea, still looked good.
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While searching for more information about the cleistogamous seeds in stiltgrass (those seeds hidden in the stem), I found a Master’s thesis
by Samantha Nestory of the University of Delaware. There it was noted that Japanese stiltgrass grown in
sunnier locations had more cleistogamous
seed than in shade, 47% vs 28%. It also pointed out that, at maturity, there is
a cleistogamous
flower stalk at nearly every joint along the stem. After reading this I
checked out some of the stiltgrass that was ripening. Sure enough, nearly every joint had a flower
stalk hidden or nearly hidden within. In
the woodlands I did not find as many of these hidden flower stalks. Another observation about JSG is that it is
able to put out roots wherever a leaf node, or joint, touches the ground. By this means a single stiltgrass seedling
can cover a large area. Also residual
fragments can quickly reestablish. I’m
not sure what this means for stiltgrass that is pulled then dropped back on the
ground. Will it reroot? |
Next to my finger you can see the flower stalk that was hidden under the leaf sheath. In sunny locations by early fall there can be one of these at each joint.
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This brings up the topic of how to allocate our
most precious resource, time. While I
often dive into some of the most thickly infested areas and rip out the biggest
plants, it is actually more effective to begin in less densely infested areas
and clear them completely. After those
areas are clear, move on to thicker areas.
The idea is that if an area is totally clear you won’t need to come back
to redo it as much. Whereas while you
are battling a thick infestation, the lightly infested area is getting worse
and then you end up with twice as much heavily infested space.
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Here's a small scale example of complete removal of Japanese stiltgrass. Ideally, I won't have to come back again this year and can spend my time elsewhere.
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Lawn
This year I was late on reseeding the lawn. I didn't get out there until early October. I typically use a bow rake to tear our
residual stiltgrass, then overseed with an appropriate cool season turf grass
in mid-September. (We are near the
southern limits of where cool season grasses are preferred.) Overseeding helps to fill in the gaps in the lawn that would otherwise be filled by more stiltgrass. By seeding in fall these cool season grasses
can get established and not be affected when I apply a pre-emergent herbicide
in the spring to kill the stiltgrass. I should have raked out the stiltgrass much earlier, before the seed was ripening.
Herbicides
I’ve had
great success controlling stiltgrass in the lawn using pre-emergent herbicides in
early spring. Most products labelled for pre-emergent use to control crabgrass are effective. I have noticed that the
amount of hairy cress, Cardamine hirsuta, has also been reduced (this is due in
part to mowing at least once in mid-spring to cut off the flowers before they
can set seed.) While the pre-emergent works very well in the lawn, it is not as effective in the
rougher meadow areas.
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In this mini-meadow I have started to use the pre-emergent herbicide Dimension™ to augment pulling of Japanese stiltgrass. Naturally occurring species that are flourishing here include deer tongue and purpletop grasses, wingstem (Verbesina alternafolia) and wild blackberry. I have also planted in some panic grass, wild bergamot and brown-eyed Susan.
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After two years of treatment in small meadow
I have opened up enough space to get some more desirable plants established
like wild beebalm, Monarda fistulosa, black-eyed
and brown-eyed Susans, Rudbeckia sp.,and
grasses like Virginia wild rye, Elymus virginicus,
and panic grass, Panicum virgatum. I noted this year that, in addition
to a decrease in stiltgrass, the nonnative invasive bull thistle, Cirsium vulgare, was largely absent this
year. This thistle is a short lived
annual or biennial species so is susceptible to control by pre-emergent
herbicides. I have been watching for
other changes in the species mix because of the pre-emergent treatments. Since I have put in some black-eyed Susans,
which depend on reseeding to survive, I will not use pre-emergent in that area
next spring to see if I can get them to come back on their own. Ideally I would like to build up a strong
network of native species that can exclude the stiltgrass on their own.
I have not been using post-emergent herbicides on Japanese stiltgrass, but they do have their place in the arsenal. I found
research that indicated that the use of grass selective herbicide can be
effective, without causing damage to non-grass species. Fluazifop-p-butyl is a selective post-emergent
herbicide that can adequately control M.
vimineum with minimal effect on the non-graminoid native plant community (Judge et al. 2005b).
Fenoxaprop-p-ethyl, is a selective post-emergent herbicide that provides
excellent control of M. vimineum and
can maintain or even increase cover and richness of native species
post-treatment (Judge et al. 2005a, b, Judge
et al. 2008, Pomp et al. 2010, Ward
and Mervosh 2012). Fenoxaprop is not
effective on sedges or cool season perennial grasses, like red fescue, so that would
be a good thing, since there are many sedge species occupying my woods. However there are also many shade tolerant
grasses like bottlebrush grass, deer tongue grass and mannagrasses that could
be affected. So I would be hesitant to
use these useless it was in a very targeted fashion.
Fire
These past few years I’ve been using fire, primarily from a
garden torch, to control Japanese stiltgrass. I’ve found
that fire is good for clearing a space prior to planting, but not for clearing without a plan for back fill with desirable species. In some areas I’ve burned the stiltgrass
seedlings in early summer only to have the area recovered with more stiltgrass;
mostly from rooted stems coming in from nearby plants. Fire alone is more effective in late in season (August) when there is not
enough time for new JSG to germinate and reach maturity. It can work particularly well if there are
well rooted perennials in the area.
These perennials are able to resprout after their tops have been singed off. Cool
season perennial grasses are a good example of these.
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Here I used my garden torch to burn away the stiltgrass. Then I planted some plugs of switch grass, Panicum virgatum.
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This year I also used fire to dispose of late season
stiltgrass that was full of seed. After
realizing just how much seed is contained in a stiltgrass stem, I decided that
rather than moving piles of stiltgrass around I would burn what I had in a
central location. Fortunately, I live in
an area with plenty of space and that allows burning. The key to getting stiltgrass to burn well is
to allow it to dry out. I allowed my big
piles [photo] of stiltgrass to dry about 2 weeks to get it dry enough to burn
rapidly, In all I estimate I had nearly
2 cubic yards of stiltgrass stems plus thatch from where I reseeded. This was reduced to less than 2 cubic feet of
smoldering ash. [photo]. When burning
remember to follow all local regulations.
Don’t burn on windy days, keep the flames under control, and have water
on hand to put out any unintended fires and dowse the ashes when done.
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These piles of ripe stiltgrass, plus some additional thatch raked out of the lawn, were reduced to a couple of cubic feet of ash.
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Native Competitors
A more exciting aspect of
stiltgrass control is finding native plants to fill in or even resist Japanese switchgrass. Many people, myself included, have noted that
golden ragwort, Packera aurea, is very effective at excluding JSG.
In the woods nearby is a large dense patch of
mayapples, Podophylum peltatum, a spring ephemeral. I have
noticed that during garlic mustard season (April to June) there is no garlic
mustard growing there. This year I also realized that there was not any stiltgrass there either, even though the ground is
essentially bare save a few sedges and Virginia creeper, once the mayapple has retired for the summer. I recently saw a post on Facebook where there
was a patch of wild ginger, Asarum canadense, that was relatively free of
JSG. These two species spread
extensively by rhizomes. May there’s
something to that?
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In early through spring the area circled is covered with mayapples. This dense cover seems to have excluded both garlic mustard and Japanese stiltgrass.
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There are also grasses that maybe
useful. River oats, Chasmantheum
latifolium, grows in dense stands and I have found stiltgrass only on the outer
edges. In the woods the rosy sedge, Carex rosea, has done very well with just a little help from me. In a moist wooded area I noticed that I had
an early season grass that excluded the stiltgrass until it went to seed in June. I’m pretty sure it was a species of mannagrass, Glyceria sp. (I keyed it out as
American mannagrass, G. grandis, but that is a rare species in Maryland, so I
will double check when it blooms again next spring.) In late summer while pulling stiltgrass I
came across another patch of grass that had just a very few stiltgrass stems. Currently unidentified, it seems to block the
stiltgrass with a dense layer of thatch from a previous season’s growth. This is another one to try to identify come
spring.
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This small, yet unidentified grass seems to have repelled an invasion of stiltgrass.
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Next Year: - Continue with the pre-emergent treatments on the lawn and in limited areas in the meadows.
- Focus my efforts in areas to achieve 'complete' removal before moving on to new areas.
- When burning to clear an area have something ready to fill in.
- Identify those grasses and see what else is holding its ground.
If you have any additional ideas or know of other competitive
native species I'd love it if you could share that here!