Pull! Pull! Pull!

 Yes Pull!  Now is the time to pull out Japanese stiltgrass, Microstegium vimineum.  This invasive species is rampant in the eastern U.S. from Georgia to Massachusetts and west to the Mississippi River.  It affects home landscapes and natural area alike.  Here in the Mid-Atlantic region the grass is putting on a growth spurt prior to going into bloom.  So the plant is expending a lot energy now to grow taller and produce flowers.  It also means that it is a lot easier to pull out without getting on you knees.  Since this grass in an annual, keeping it from going to seed can go along way to controlling its spread and reducing its numbers.  

Early in the growing season Japanese stiltgrass stays relatively low, rooting at several nodes along the stem.  In late July it starts growing upward to gets its flowers higher off the ground.  If it were only that simple.  Stiltgrass not only produces flowers at the top of the stem it also has flowers at most of the vertical leaf nodes buried within the stems.  These are referred to as cleistogamous flowers.  

  This image shows flowers at the top and at the
lowest nodes. Cleistogamous flowers can occur all
along the stem as well.  Roots can also form roots at each
node where they contact the ground. If the grass is not
cut early in the season most of the flowers are
concentrated toward the top of the stem.

These cleistogamous flowers are one of the reasons stiltgrass is so hard to eliminate.  If you cut or mow stiltgrass early in the season without removing it completely, these stem flowers will form even lower on the plant requiring even more careful pulling later on.  I've seen a recommendation to leave the stiltgrass grow until late summer so that most of the flowers are higher in the plant.  Then when you pull you are able to get most all of the flowers with the least effort.  (Sounds good to me.)

Here are some highlights of my nearly 10-year battle with stiltgrass here in Maryland:

Pre-emergent herbicides are very effective in existing lawns and smooth surfaces.  These chemicals interfere with the development of germinating seeds but do not have a strong effect on established plants.  These must be applied in early spring prior to the germination of the stiltgrass seeds.   I have been using a preemergent containing Dimension (dithiopyr) for several years.  I took two years of successive treatments to get nearly complete removal of stiltgrass from the lawn in treated areas.  Moss has not been effected. 

Late summer pulling of previously uncut stiltgrass, especially in shady areas, has reduced the amount of stiltgrass in subsequent years.  This does require a multiyear effort.  Since nasty things like poison ivy, multiflora rose and wineberry can hide in the tall stiltgrass, it is important to wear gloves and arm protection when pulling.

Here's the before photo.  I wonder why there is so much
stiltgrass just in this area, and why so close to the path. 
It could be from the lawn mower blowing seed from
 the other side of the path where there is a lot more stiltgrass

After about 30 minutes of labor you can see the
existing plants reappear. (Some of the freshly
pulled stiltgrass is piled in the foreground-left.)

Weed-whacking and raking of the cut grass before it begins to bloom is very effective, BUT you need to get really close to the ground to remove all the stiltgrass. Raking up the cut grass helps existing plants bounce back. 

Identify and plant native species that can compete with stiltgrass. Two exceptional plants that seem to outcompete stiltgrass are golden ragwort (Packera aurea), and mayapple (Podophyllum peltatum).  Other plants that are strong competitors include clearweed ( Pilea pumila) and grasses that grow well in shade: river oats (Chasmantheum latifolium), mannagrass (Glycera sp.), Virginia wildrye (Elymus virginicus), and  nimblewill (Muhlenbergia schreberi).  Also there are a number of sedges that will persist under cover of stiltgrass and can form a dense cover if given the chance.  Rosy sedge (Carex rosea) is one example that does quite well on my property.  I recently noticed that there was much less stiltgrass growing in an area where celandine poppy (Stylophorum diphyllum) and mannagrass have been spreading.  The exclusion of stiltgrass is not as great as with Packera, but it is noticeable.

This opening under the trees is free of both garlic mustard and stiltgrass now. 
In early spring it is fully covered with mayapples and a few woodland phlox.

The area just beyond the bench has benefited from annual late
summer pulling and by a dense crop of mannagrass that totally
shaded the area from late April to early June.  The stiltgrass that is
growing there now is only a few inches tall.  Compare that
to the 2 foot tall stiltgrass in the foreground-left.

It's not just pulling in late summer and fall.  There are things you can do earlier in the year to control stiltgrass.  Here is a table summarizing some removal strategies:

Japanese Stiltgrass Control

Season	Actions	Pros	Cons	Comments
Late Winter Early Spring	Pre-emergent treatment	Effective in lawns and smoother (even) surfaces with access to soil surface.  Allows lawns and perennials to get a head start.	Will affect all germinating seeds for several months.  Uneven coverage on rough surfaces.  Application needs to be at the right time (same time as for crabgrass).	Takes at least 2 years to knock down seed bank to see significant progress.  More time to complete elimination, if ever. Many pre-emergents for crabgrass control are also approved for Japanese stiltgrass (aka Mary's grass, on the label)
Late spring-Summer	Herbicide	Grass selective herbicide leaves broad leaf plants and some sedges intact. Try products containing Fluazifop-p-butyl	Difficult to control collateral damage, especially with non-selective herbicides	Targeted application and use of selective materials may limit side effects.
Late spring-Summer	Pulling	Non-toxic and selective.  Opens space for other species.	Labor intensive; left over stilt grass will expand to fill gaps.	If removal is not complete this will need to be repeated at the end of summer
Late summer/Early fall	Cutting/ pulling/ burning	All methods to reduce the amount of seed	Need to complete actions before seed begins to ripen.	Copious amounts of seed being produced, need to be thorough and avoid spreading any ripened seed.

You can read more information about my on-going battle with Japanese stiltgrass in previous blog-posts by entering 'Microstegium' in the 'Search this Blog' box at the top of this page.

Good luck and I would like to hear about your experiences battling this invasive species.

Getting Ready for Stiltgrass 2021

My battle against Japanese stiltgrass, Microstegium vimineum, is a nearly year round effort.  To help me focus my efforts I’ve prepared the following table to remind me of where I will get the most for each hour of work in each season.

Japanese Stiltgrass Control

Season	Actions	Pros	Cons	Comments
Late Winter Early Spring	Pre-emergent treatment	Effective in lawns and smoother (even) surfaces with access to soil surface.  Allows lawns and perennials to get a head start.	Will affect all germinating seeds for several months.  Uneven coverage on rough surfaces.  Application needs to be at the right time.	Takes at least 2 years to knock down seed bank to see significant progress.  More time to complete elimination, if ever.
Late spring-Summer	Herbicide	Grass selective herbicide leaves broad leaf plants and some sedges intact. Try products containing Fluazifop-p-butyl	Difficult to control collateral damage, especially with non-selective herbicides	Targeted application and use of selective materials may limit side effects.
Late spring-Summer	Pulling	Non-toxic and selective.  Opens space for other species.	Labor intensive; left over stilt grass will expand to fill gaps.	If removal is not complete this will need to be repeated at the end of summer
Late summer/Early fall	Cutting/ pulling/ burning	All methods to reduce the amount of seed	Need to complete actions before seed begins to ripen.	Copious amounts of seed being produced, need to be thorough and avoid spreading any ripened seed.

 Because Japanese stiltgrass is an annual there are two points of vulnerability based on its life cycle.  As an annual all plants start new from seed each spring.  If you can disrupt germination you can make a significant dent in how much stiltgrass you will have to deal with.  This is where I have found preemergent herbicides to be very useful.  Products labeled for control of crabgrass have proven effective.  The other weak spot is seed production in early fall.  More on that below.

On the left you can see the effect of a single treatment with
a preemergent crabgrass herbicide.  The right side is thick with
bright green Japanese stiltgrass.  Photo taken in early July.

Springtime

An important part of using a preemergent is getting it applied at the proper time, neither too early or too late in the spring.  There are tools on the internet that track growing degree days, such as GDD Tracker 4.0.  The idea is that plants will develop at a rate controlled by how much heat they receive over time.  For germination, the soil temperature is particularly important.  It is important to realize with a preemergent it is critical to have the material well distributed on the soil prior to the onset of germination.  That’s why I like a really like the calculator, it builds in the time when you should actually apply the product.  Another way to time the application is to watch for when forsythia are in bloom.  In my limited experience just before peak bloom is a good time to do the preemergent application.

I have found that preemergent use on the lawn is very effective.  There was a significant reduction in stiltgrass in the lawn after treatment for two consecutive years.  Also the red fescue that I overseeded is really starting to knit together.  I should do a test this year to see if I can skip treating a portion of the lawn for a year without a resurgence of stiltgrass?  Maybe for a small area.  I'd hate to lose the progress I've made so far.

I have also experimented with preemergent treatments in a meadow area.  Here the ground is much coarser and there is a lot of debris on the ground.  It is very difficult to get an even application of the granular herbicide.  I have seen a decrease in the amount of stiltgrass seedlings in areas where the preemergent was applied.  But there are also patches where the stiltgrass still comes in densely.  I assume that these are due to uneven application.    

Another concern I have with this meadow area is that the preemergent treatments will inhibit the germination of other desirable plants.  The meadow area I have is dominated by wingstem (Verbesina alternifolia), wild blackberry, smartweed and a variety of sedges.  In addition there are the usual invasives: Wineberry (Rubus phoenicolasius), Oriental bittersweet, Japanese honeysuckle and multiflora rose.  Besides reducing the amount of Japanese stiltgrass I have also noticed a significant reduction in the amount of bull thistle, Cirsium vulgare, a non-native species.  (Of course having a thick layer of JSG will also inhibit or kill anything that is not well established, so one needs to balance the effects of the herbicide with not doing anything.)  Last spring I transplanted in a number of black- and brown-eyed Susans (Rudbeckia hirta and R. triloba) and some wild bergamot (Monarda fistulosa) in the gaps that opened up.  It is particularly important for the ‘Susans’ that they be allowed to reseed.  For this year (2021) I will not do the preemergent treatment in the areas with these reseeding perennials.  Instead I will focus on some areas that have not been treated before.

Last year I did experiment with surface burning of stiltgrass seedlings.  While it did provide some temporary control, more seedlings appeared a week or two later.  For me burning with a torch is a maintenance activity and it’s easier than bending over to pick out seedlings.  Spring fire does not work as a one-time treatment in my hands.

Mid-Season

I have been reading more about grass specific herbicides and how they would fit into a plan for eliminating Japanese stiltgrass.  I can see using them as a tool in maintaining a perennial bed or in a well defined, designed landscape.  For me I don’t expect to be using them because most of my landscape is more like managed wildlands.  I generally welcome any native species that pops up annual, perennial, shrub or grass.  So I don't want to run the risk of killing something new and unexpected.

This patch of unmown grass has benefited from both regular pulling
of stiltgrass and a single treatment with a preemergent. 
As I've opened spaces I've planted in natives such as switch grass
and wild bergamot (taller plants to the back right).  The yellow flowers
are wingstem and goldenrods that have come in naturally.

Most of my mid-season activity is focused on pulling and back-filling with desirable, native species.  This year I will be moving many of my excess plants from the vegetable garden, Rudbeckias and Monarda, as well as some overgrown New England asters and goldenrods into spots where I will have weeded out the stiltgrass.  Native species that are competing well against stiltgrass include golden ragwort, mayapple, wild bergamot and grasses and sedges such as river oats, deertongue grass, Virginia wild oats, and rosey and sallow sedges (Carex rosea and C. lurida).

 

This patch of golden ragwort, seen here in mid-April, started from a
single clump planted 2 years earlier.  This species is very effective
at excluding both Japanese stiltgrass and garlic mustard.

6 years ago I seeded in Virginia wild rye.  It is particularly evident
in unmown, shady areas around trees.  This cool season grass gets
started early in the season, well before stiltgrass, and grows 2-4 feet tall.

If pulling is not complete, the remaining stiltgrass will refill newly created voids.  One thing I learned last year was that it can put down roots anywhere a node comes in contact with the soil.  I will try to limit my efforts on mid-season pulling and burning to when I have something to fill back into the open space or where there are new natives trying to get established.  I will store up my energy for late summer when there is less time for the stiltgrass to regenerate and have enough energy to produce flowers and seeds.

Late Summer and Fall

This is a time when you can make a significant dent in the amount of seed that is produced for next year with the least effort.  Pulling stiltgrass that is 3+ feet tall in late summer will remove a huge amount of potential seed.  It’s also easier on the back than pulling shorter plants.  Timing is important.  It should be done early enough that any seed present on the plants will not be able to mature and late enough that and remaining fragments of plants won’t have time to recover and put up additional flowers.  Here in the mid-Atlantic August is about the right time for that. 

If you find yourself pulling stiltgrass later in the season after seeds have started to form you will need to be more careful about disposing the pulled grass.  You don’t want to risk spreading any seed around.  While I knew that stiltgrass has flowers hidden within the stem, I had not realized until last year that there may be flower stalks buried in nearly every leaf node of a healthy stiltgrass stem.  That is a lot of potential seed!

By my index finger you can see one of the flower spikes
 that was hidden within the stem.  These are able to
self pollinate and produce seed without ever opening up.

Cutting or mowing close to ground level is most effective at this time, too.  Burning with a torch at ground level (when safe and where allowed) is also effective.  I’m not certain, but there is a possibility that some cuttings or unburned stem parts may reroot if they contact the soil under favorable conditions.  I will often rake up the cuttings and put them in a separate pile where they can decompose without mixing in with other materials.

In years past I have spent a lot of time in the late summer and early fall raking stiltgrass out of the lawn.  This was probably of little use since much of the stiltgrass had probably developed seed by then.  Last year I combined raking with overseeding with cool season turf grasses.  The hope was that the cool season grasses would germinate quickly and fill in the gaps left from the stiltgrass.  That should work, in theory, but I can't comment on the results.

So the plans are in place and it's almost time to get started!

2020 Stiltgrass Wrapup

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. 

This was the initial condition, before any clearing in 2020.

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.

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.

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. 

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. 

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.  

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.

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.

Here I used my garden torch to burn away the stiltgrass. 
Then I planted some plugs of switch grass, Panicum virgatum.

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.

These piles of ripe stiltgrass, plus some additional thatch raked out of the lawn,
were reduced to a couple of cubic feet of ash.

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?  

In early through spring the area circled is covered with mayapples. 
This dense cover seems to have excluded both garlic mustard and Japanese stiltgrass.

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.

This small, yet unidentified grass seems to have repelled an invasion of stiltgrass.

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!

 

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.