It's Stiltgrass Time Again

As summer is peaking so is the time to focus on removing Japanese stiltgrass, Microstegium vimineum.  At this time (beginning of August) in the Mid-Atlantic region this invasive grass is putting on a growth spurt, but has not yet begun to develop viable flowers. In one of my previous blog posts on this subject I prepared this table to help keep track what to do to combat stiltgrass at each point in the season.

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.

While it is too late to count on herbicides cutting close to the ground level and pulling at very effective methods to employ at this time in order to prevent Japanese stiltgrass from reproducing.  Because this is an annual grass, keeping it from reseeding goes a long way to eliminating from your environment.  In wooded areas and existing beds where there are many desirable plants intermixed with the stiltgrass I opt for pulling as my preferred approach.  Since it has grown fairly tall by this time, I usually pull it by the handfuls then leave it out in the sun to dry.  If you are weed whacking it you could leave the cut grass in place to dry; however, I’ve read that doing this can generate a thick layer of thatch that can be difficult for other plants to grow through.  Whenever possible I will rake out the cut grass and leave it in the sun.  If I’m confident that there is no viable seed in the stiltgrass I will mow over the dried grass to help return that organic material to the soil.

For several years I have been removing stiltgrass manually from this
shady mossy lawn.  This year I treated it with a preemergent herbicide
 in late April and am seeing only a few instances of stiltgrass.  In addition
to the moss and some fine fescue and sedges there are patches of selfheal,
Virginia jumpseed and white avens. 

I’ve noticed that this year that the stiltgrass is more brittle and harder to pull cleanly out of the ground.  This may be due to the drier weather that I am experiencing at this time.  I’m afraid that I may be leaving more rooted fragments that may be able to regenerate.  I will try to time my next pulling campaign to be after a significant rainfall to soften the soil. 

Another thing I’ve noticed this year is that the plants that I am pulling up have multiple rooting points along the stem.  I’m thinking that as I am reducing the number of stiltgrass plants, year by year, there is more room for them to spread laterally.  So a partial success, but it means that there is still more work to be done.

I've treated this woodland path for two consecutive years with a
preemergent as indicated by the white line.  You can see the
bright green stiltgrass to the back and on the right. In the treated area
are a variety of grasses and sedges.  Deer tongue grass is the
largest species here.  In late March this area is covered with spring beauties.

Looking over my property I see that the most significant reduction in Japanese stiltgrass is where I have applied a preemergent herbicide in early spring.  This year I applied Dimension™ (dithiopyr) at the higher recommended rate.  This is expected to be effective for 3-4 months.  It was applied at the time recommended for crabgrass control (380 GDD(32°F)), or about the time when the forsythia were coming into full bloom.  I am seeing next to no stiltgrass in many parts of the lawn as well as in the treated parts of my ‘mown meadows’.  Better results are seen where I have treated for at least two consecutive years.  In these meadows and in the more natural areas I try to avoid treating the same area with a preemergent two years in a row.  This is to allow the seeds of the many desirable plants to germinate on the off-years. 

As far as native plants that compete well against Japanese stiltgrass, golden ragwort, Packera aurea, seems to actually suppress the growth of stiltgrass.  Virginia wild oats, Elymus virginicus, and bottlebrush grass, E. hystrix,  compete well.  What I believe to be fowl manna grass, Glyceria striata, has been able to out compete Japanese stiltgrass in shady moist areas.  This grass develops early in the year with tall, thick growth that shades the ground.  The downside of this grass is that it dies back to the ground after going to seed in late June.  

This photo, taken at the end of May, shows thick growth of
a grass that I believe to be fowl manna grass.  The area inside
the white circle has been free of stiltgrass for the past two years.

 I have been posting blogs about my battle with Japanese stiltgrass since 2014, probably 3-4 times a year.  Following are links to several of the more informative posts.  Getting Ready for Stiltgrass 2021, July 2020 What to do about Stiltgrass Now, The Forsythia are in bloom, Now is the Time, and Making a Plan to manage Japanese Stiltgrass.

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.  

Stiltgrass Fall 2019

I'm continuing my multipronged attack on Japanese stiltgrass, Microstegium vineum, that I have been waging for about 5 years now.  Depending on where the stiltgrass is growing different techniques are more effective.

Here are links to some earlier posts on my campaign against stiltgrass: Spring 2019, Spring 2018, Fall 2017, Original Plan 2014, and Use of Pre-emergents.

Woodlands  In the  shady woodland settings pulling the stiltgrass through the summer and especially in late summer just before blooming has been pretty effective.  In several of these shady areas removing the stiltgrass before is sets seed has shown good results after two seasons.  It is important to continue to follow up year after year to prevent a re-infestation.  When I first started do this I used a weed whacker to cut very close to the ground and I left the cut grass in place.  In places where I removed the cut grass progress toward eliminating it seemed to go faster.  Also as other, hopefully native, species begin to fill in, weed whacking risks cutting out those species as well.  Now I've gotten to the point of hand pulling the stiltgrass and piling it up in segregated locations where any seed present will be less likely to spread.

Here are some clumps of Virginia wild rye growing under a young tupelo tree. 
This is a cool season grass, seen here in June.  By the end of summer
 it will be golden brown with arching seed heads.

Bottlebrush grass is also a cool season grass that like partly sunny sites. 
These early season grasses have a chance to shade the ground before
the stiltgrass begins maturing in July

In these locations I have been trying to have more native species fill in, by either encouraging the existing natives of adding new ones.  Virginia wild rye, Elymus virginicus, is a native grass that grows pretty well in the shade, as is bottle brush grass, E. hystrix.  Other species that are filling in on their own are Virginia jumpseed, Persicaria virginiana, and clearweed, Pilea pumila.   A variety of smartweeds are also increasing, but I fear many of those are non-native species.

Clearweed is a native annual that has had a banner year here in 2019. 
It grows well in shady locations.  

Meadows  I've been having more trouble in the open meadow sites.  Some of this is my reluctance to do anything but hand pull.  I am trying not to disturb the more desirable species that co-inhabit the meadows.  In one area that I have been hand pulling for 3-4 years I'm seeing a reduction, but it still has a long way to go.  This past spring I put some pre-emergent  in several meadow areas and saw a further reduction in stiltgrass density, but not an elimination.  I also tried burning a patch of ground in early spring but that had no noticeable effect on the amount of stiltgrass.  (That is consistent with what the Forest Service reports on the use of fire.) 

By virtue of how stiltgrass grows, sending out shallowly rooted runners to fill all available space, by the end of summer you will have a similar density of flowering stalks whether you have 10 or 50 seedlings per square foot.  (These are my impressions, not proven facts).  But having fewer rooted individuals means it gets easier to clear an area by pulling because so many of the stems are interconnected.  I will try the pre-emergent again this spring, but do the application earlier (read on below).  One of my concerns in the meadow is that the pre-emergent will inhibit some native species from growing/reproducing.  I did not go in and inventory what was or was not growing in the treated and untreated areas.  I did see smartweeds and tick trefoils, Desmodium panniculata,  growing in all areas.  Where stiltgrass was growing thickly the ground plane was essentially barren after the grass was removed.

In May stiltgrass seedlings are less than an inch tall.  The right side of the area
was treated with Dimension pre-emergent herbicide in the first week of April. 
The untreated side has a greater density of stiltgrass seedlings.  I expect that
 applying the herbicide earlier in the spring will further reduce the amount
 of sprouted stiltgrass.

This is the same area, photograph from behind.  The treated side (left)
 has a slightly lower density of stilt grass, but still
 most of the available space is filled with stiltgrass stems.

Removing stiltgrass from the lawn.  Pre-emergent herbicide is still showing very good results.  2 years ago I applied herbicide (Dimension^tm) around the 3rd week of March.  While in the spring of 2019 I waited until the first week of April.  Or using the forsythia bloom as an indicator of spring weather, mid-bloom in 2018 and late bloom in 2019.  Despite the presence of the herbicide I did see more stiltgrass seedlings in 2019, so I think applying the pre-emergent earlier may be more effective.  For folks in the Northeast and Mid-West  there is a website that helps with the timing of pre-emergent application based on degree days.   Another problem with getting stiltgrass out of the lawn is that repeated mowing can stimulate earlier formation of flowers lower in the plant.  These are much harder to remove than the terminal flowers that form on undisturbed plants.

Here is some stiltgrass pulled from the lawn after mowing.  You can see
 how low in the plant the stem flowers (cleistogamous) occur. These flowers
 need to be cut off for mowing to be really effective at eliminating stiltgrass.

Some folks have reported success with cutting the lawn really low in August, the beginning of flowering season.   Looking at some of the stiltgrass growing in the lawn, it looks like you would need to cut down to about an inch to reach those lower flowers.  I tried that in one section of my lawn that I have not otherwise treated and is mostly stiltgrass.  Next spring we'll see how that area compares to normally mown areas. 

After you manage to suppress the stiltgrass you need to get something else growing in its place.  The easiest option in the lawn is to overseed with a well adapted turf grass.  In the Mid-Atlantic the commercially available options are tall fescue blends for sun and fine fescues for shade.  In my rural setting I also have many other grasses growing in and around the lawn.  Some of the native species that are mixed in include path rush (Juncus tenuis), purple top (Tridens flavens), nimblewill (Muhlenbergia schreberi) and a variety of sedges.  While I encourage these to grow, they would be much happier if they were not mown on a regular basis.  Nimblewill, though, does seem to tolerate mowing pretty well.  

This is what nimblewill looks like in the lawn.  This warm season grass is
 not as 'pretty' as commercial turf grasses, but is is native and is well adapted to
 the climate here, even in this unseasonable hot and dry year.

I was planning on overseeding a large area in early fall, but here in Maryland it has been unseasonably hot and dry so I have been holding off seeding until the weather cools.  Getting the new seed established in the fall is critical to using most pre-emergents because they will suppress growth of most weeds planted in the spring. 

Another option to turf grasses is to convert from lawn to meadow or other types of plants.  Based on my meadow experience you should get rid of the stiltgrass before just adding other plants.  This can be done chemically, by solarizing (covering with plastic for  enough time to kill the stiltgrass seeds), or covering with cardboard or many layers of paper followed by mulch or clean soil and then planting on top.

In one shady area of lawn I have using timed mowing as the primary means of stiltgrass control.  I leave the area unmown from mid-June until mid-August.  At that time I'll pull out and remove the tallest grass and follow up with a low mowing.  After 3 years of this I am getting less stiltgrass, but it is still pretty thick.  One explanation is that there is a lot of stiltgrass in the surrounding area and the seeds may be coming in from there, either blown in or carried by the rain.  I did put down some pre-emergent in a small section of this area and there was no stiltgrass present there.  Mosses and perennial grasses seemed unaffected. 

The vegetable garden  In the garden one of the places stiltgrass flourishes is along the fence line.  It interlaces with the chicken wire and can be hard to remove completely.  This year I have been using a flame weeder to clear those areas.  This method is more effective early in the season when plants are smaller and more tender.  As weeds mature they have more moisture in their stems and leaves and are more difficult to kill with heat.  One caution with using a garden torch is not to expose soaker hoses or drip irrigation lines to the direct flame.  Unlike some weeds a soaker hose will not recover after being melted. 

Next steps: 

• Overseed with turf grass.  For next season  I am going after a large open area that is mostly mown stiltgrass.  I will use pre-emergent on this area in early spring assuming I can get the new seed established.
• Continue use of pre-emergent in lawn areas where stiltgrass is depleted.  While I have not been targeting it, the amount of hair cress, Cardamine hirsuta, a winter annual weed, seems to be decreasing in the treated areas as well.
• Continue testing pre-emergent in meadow, this time apply earlier in spring
• Expand areas in the woods where stiltgrass has been pulled.  Try and remember that the best strategy for removing invasive species is to clear a small area really well and then expand that area each year.  Also start in a less infested area and then move to higher density areas.  

Progress on Japanese Stiltgrass Removal - 2017

My 4 year long battle with Japanese stiltgrass, Microstegium vimineum, is continuing and it must be a continuing fight to remove this invasive grass from my property.  Since this is an annual grass the goals are to keep it from producing a new crop of seed and to eventually deplete the seed bank so that it won't crop up again from older seed.  Since this grass is all around I will need to take the fight to the neighborhood and beyond.  This will be easier if I can demonstrate to  other that this is a battle that is worth fighting.

One approach to Japanese stiltgrass removal is to nuke the site with round-up or a grass selective herbicide.  Despite it being effective on the current year's growth, this would still be a multiyear approach to deplete the seed bank.  I'd rather not kill everything because I have a number of interesting plants that I want to keep and I am generally opposed to the indiscriminate use of herbicides or pesticides. 

The key to non-toxic approach to stiltgrass removal is good timing.  Pulling the grass in late summer as the flowers are forming (late July to early September in Maryland) is the best time to concentrate your efforts.  Early season pulling helps, but if some plants remain they will expand to fill the void and you will need to return to pull again.  Stiltgrass pulled before seed has formed can be composted, though I usually segregate it from the rest of my garden scraps just in case.   Any grass pulled after flowering should be bagged or at least segregated and covered so that seed can not escape.

Here's a woodland edge area in 2013 when I started pulling/whacking stiltgrass

In 2014 coverage is not as dense, but still dominant.

In 2015 other plants are showing up.  Also I had seeded in some Virginia rye in 2014.

Now in 2017 stiltgrass is no longer dominant.  In the foreground is
Virginia rye, Elymus virginicus, drying out after seeds have matured.  

 The photos above show the progress that has been made over 4 years.  For the first two years this site was weed whacked to near ground level and then raked out.  As the stiltgrass density dropped I switched over to hand raking and pulling for the past two years.  These methods were more selective and allowed me to avoid the more desirable plants that were coming back in.

Mature Japanese stiltgrass, note the
location of the cleistogamous flowers.

In lawns, mowing stiltgrass is done to control its spread.  Unfortunately stilt grass has the ability to form cleistogamous flowers near the base of the stem.  These flowers are enclosed in a leaf sheath about an inch off the ground and can survive most mowings.  It has been reported that these flowers mature earlier in stiltgrass in regularly mown lawns.  To address this Russ Anderson, a West Virginia forester has suggested allowing stiltgrass to grow freely through July and then mow it closely to the ground in August as the flowers begin to develop.  The expectation is that there will be fewer cleistogamous flowers and by cutting it back late in the season there is not time enough for the grass to regrow.  I have been doing a variation of this in an isolated area where I am growing fine fescue.  Instead of mowing, I use a manual grass whip.  This rips out the stiltgrass and leaves more of the fine straight blades of the fescue intact.  After doing this for two years the amount of stiltgrass in the area has decreased, but I still have a few years to go.  

Another technique that I have been using  to remove stiltgrass from the lawn is to briskly rake the lawn after a rain to try and catch the lateral stems (stolons) of the stiltgrass.  Using a spring or bamboo rake does tear out a lot of stiltgrass, but it also tears up the soil and can damage the bunch and perennial grasses as well.  I also found that after doing this in early July, even though I removed a great deal of stiltgrass, much of it had regrown to fill the gaps in the lawn.  Inspection of the stiltgrass at this time did not show evidence of the cleistigomous flowers. 

This area was raked over with a steel garden rake.  Nearly all this debris is stiltgrass.

What appears to have worked better is a stiff steel garden or bow rake that is pulled along the surface of the ground.  It catches and pulls out the stolons of the stiltgrass and since the action is not so vigorous, most of the other bunching grass are left intact.  I did this in mid-September so there is not enough time for the stiltgrass to regrow.  Even though the soil was dry, the stiltgrass pulled out easily.  Inspection of the stems showed that there were some cleistogamous flowers forming at this time.

Areas that are blackened were burned with the torch (next to watering can). 
I kept a full watering can nearby just in case the fire got out of hand (it didn't).  I left a small area
in the middle un-burned as a 'control group'.

These mechanical methods are helping reduce the number of stiltgrass plants in the lawn but there are still a number that escape the raking and pulling.  A new approach that I am trying out this year is fire.  Burning stands of stiltgrass is not effective early in the season while new plants are still germinating, or in the off season when plants are already dead (stiltgrass seed is relatively fire resistant).  However, burning in late summer as the stiltgrass is beginning to flower may be a way to successfully battle a stiltgrass invasion.  This method was suggested to me by Joene Hendry, who writes the blog  Joene's Garden.  The process I am trying is to first mow the area close to the ground (about an inch), then allow a day or two for the grass to dry out.  Next use a garden torch to burn the remaining stems, this should get at those hidden flowers.  While this may seem extreme, any perennial  grasses with good root systems will bounce back in a week or two, but the stiltgass is not able to recover.   I did try burning without mowing first, but stiltgrass holds a lot of moisture and it takes a long time to completely burn up such a moist plant.  Mown plants, without a lot of leaves, were quickly burned up.

Another element of stiltgrass removal is to fill in the gaps with desirable vegetation.  Since I am located in the cool season grass zone I have been using a fine fescue blend (like Eco-lawn) in the shadier areas and tall fescue blends in the sunnier spots.  I do have some zoysia grass growing in the sunnier spots.  This warm season grass grows thickly and excludes most of the stiltgrass.  It's unfortunate that there are not more native options for turf in this area (although red fescue, Festuca rubra, is technically a native species).  By avoiding the use of broad spectrum herbicides I do allow the native sedges, violets and other species a chance to repopulate.

I did get an early start over-seeding this fall and I hope to have the new plants established before it gets too cold.  If these desired grasses get established I will experiment with some pre-emergent herbicides.  These act by  interfering with root development after seed germination.  These are particularly effective on annual species such as Japanese stiltgrass and crabgrass.   I imagine a year without stiltgrass competition should help develop a thicker lawn. 

August - Prime time for battling Japanese Stiltgrass

Japanese Stiltgrass, Microstegium vimineum, is a major invasive grass species in the Mid-Atlantic and encroaching on New England.  It forms a dense layer of vegetation that shades out native plants and other desirable species.  In late July and through the rest of summer it puts on a growth spurt jumping from 6 inches to over 3 feet tall.  This is the prime time to take action to reduce its numbers for next year.  As an annual grass the main goal is to keep it from setting seed and, eventually, the seed bank will be depleted.  Unfortunately that can take 5 to 7 years, but if you don't start doing something today, waiting longer will just make it that much harder.

I've published posts in 2014 and 2015 on my approach to dealing with stiltgrass, and I've had to go back several times to remind myself of what to do and when.  So here goes...

Freshly pulled Japanese stiltgrass.  There is a lot of leafy growth
for the relatively small root system.

Letting the pulled stiltgrass sit in the sun for a day or two helps to kill it
and it reduces the weight of the debris.  Before pulling,
 the stiltgrass in the bed was about 3 feet tall.

In planting beds where the grass has gotten several feet high pull it out by hand at the end of July or early August.  Its shallowly rooted and easy to pull, especially after a little rain.  Perennials are more deeply rooted and are harder to pull.  I've found that by loosening my grip a little I can leave perennials like New England Aster in place, while pulling out the grass.  Also at this time the stiltgrass is taller than many of the desirable plants so by grabbing the tops I can leave the lower growing plants undisturbed.  One of the reasons for pull now is to let some light reach the soil and induce more stiltgrass to germinate.  This late germinating grass won't have time to mature and will be killed off when the temperature drops.




In areas to large for pulling I usually wait until blooming is just starting (end of August-early September) and then weed whack it close to the ground.  The idea here is to prevent another season of seed from maturing.  Unfortunately stiltgrass is very slow to decompose and can form a mat layer that also shades out the native seed bank.  Where the cut grass is particularly thick I try to rake it together in piles in the shade where any viable seed is less likely to germinate.

This is an area of lawn that has been seeded twice with a low-mow  blend of fine fescues.
It has not been mown for at least 4 weeks.  There is much less stiltgrass than in the unseeded area to the
 upper right. I'll wait until the end of August (ca. 2 weeks) to mow this lawn (I've already pulled out the stiltgrass from the bed).

For lawns that are full of stiltgrass the recommendation that I'm am trying to follow is to leave it unmown from late July to late August.  Then, just as the flowers begin to form, cut the lawn at a lower level.  The idea here is that constant mowing encourages the stiltgrass to form cleistogamus flowers low on the plant.  These flowers self pollinate and are difficult to remove.  While I'm not sure if it's true, it seems that letting the plant put its energy into upper open flowers will reduce the number of these lower flowers.  These upper flowers can then be sheared off and the plants may not have time to recover with a second bloom.

The final stage for the lawn is to rake out the remaining stiltgrass in mid-September and overseed with a desirable cool season turf grass.  The cool season grasses will germinate in the fall and fill the gaps previously inhabited by the stiltgrass.

This shady area was solid stilt grass 4 years ago.  Now, after 3 years of annual weed whacking
and some pulling there is more diversity.  The tall grass here is Virginia wild rye, Elymus virginicus,
which is now going into bloom

I am happy to report that following these practices is showing promise.  In the lawn overseeding with a low-mow blend and 'deferred' mowing has reduced the density of the stiltgrass.  Also in some of the natural areas I am seeing less stiltgrass and more diversity.  One species that is doing well in the shade is Virginia Wild Rye, that I seeded in two years ago.