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.  

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.