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.