Monday, September 24, 2018

Japanese Stiltgrass Sitrep

For the past 5 years, since moving to rural central Maryland, I have been trying to figure out how to eliminate Japanese stiltgrass, Mircostegium vimineum, from my property.  This has been the subject of multiple blogs over the years and, unfortunately, will be so for a few more years.  Here's a link to my previous post on stiltgrass.

This area in the woodlands took about 30 min to clear by hand
and filled about half of the basket.  Rosy sedge is growing well underneath.
In shady woodland areas pulling stiltgrass out by hand, or string trimming and removal of the cut grass, seems to be pretty effective.  Removing the grass in late summer, prevents a new crop of seed from being produced.  The shady conditions seems to limit germination and flowering of the stiltgrass so that the sheer  volume is more manageable.  Annual pulling has reduced the amount of stiltgrass significantly.  As the stiltgrass has been going down I am seeing more native species, like Virginia jumpseed, Polygonum virginianum.

In part to full sun areas pulling the grass alone has not been as successful.  In some beds, with lots of competition the amount of stiltgrass has been reduced significantly.  In more meadow-like settings I have not seen great results, despite removing large quantities of stiltgrass each year in mid-summer.  One possibility here is that there is enough seed produced in the cleistogamous flowers trapped at ground level by the undergrowth to regenerate the seed bank. 

Getting stiltgrass out of the lawn is a more recent area of focus. There are two methods that have been effective for me.  One is to use a garden torch to burn the stiltgrass to the ground in late summer-early fall.  This eliminates the cleistogamous seeds in the lower stems and, in late summer, it is too late in the year for new plants to mature.  The burned areas are immediately ready for reseeding and most perennial grasses present will regrow from the roots, as long as you don't overdo it with the torch.

Another method that I tried last year that has proven very effective in the lawn is to use a pre-emergent herbicide in early spring.  Most products that work on crabgrass should work on stiltgrass.  See this earlier post for more details.  I used a product containing 'Dimension' (Dithiopyr) and had areas that were free of stiltgrass through the entire season.  To battle stiltgrass, these herbicides should be applied a couple of weeks earlier than you would for crabgrass.  When the forsythia are in early to mid-bloom should be about right. 

In the treated area I applied 'Dimension' pre-emergent herbicide the 3rd week of March. 
Now, at the end of summer, it is relatively free of Japanese stiltgrass.  In the untreated areas you can see
the brownish color of stiltgrass as it is beginning to die at the end of the season.

One problem with pre-emergents is that they are pretty much non-selective and will inhibit the growth and/or development of most seeds in the soil.  (Dithiopyr also inhibits the growth of new roots, not so good for really young plants.)  They are not appropriate to use at the time of reseeding or on lawns that are not well established.  For this reason I am reseeding/over seeding as early as possible this fall so that the new turf can get established before I treat again next spring.  In 2018, I applied Dimension at the end of March, this year I will wait until early-April (mid-to-late in the Forsythia bloom) to give the new lawn some extra time to grow.

This is an area where I raked out the stiltgrass and thatch to prepare for overseeding.
Most of this debris came from the 'untreated' area.
This year I am overseeding most areas with a sun-shade blend containing tall fescue, red fescue and perennial blue grass.  I have also put down the 'No Mow' blend of red and creeping fescues in areas of open shade where there is less competition from other turf grasses.  I'm located on the southern edge of the 'cool season' grass zone.  If I were a few hundred miles further south where warm season grasses dominate, I would be trying the native buffalo grass/blue gamma grass mixtures.  Raking out stiltgrass from the lawn alone has not proven effective.  While it removes a large mass of stiltgrass, it likely leaves enough seed behind to continue the infestation.  Raking is still necessary if you are going to reseed, in order to provide access to the soil for the newly applied grass seed.

You can see how poor this unmown area looks after the stiltgrass has been pulled out. 
This area has since been overseeded with 'No Mow' fescue.  If the fescue germinates
well this fall, I will probably treat  this area with a pre-emergent come springtime.
One suggestion that I read about was to leave stiltgrass alone until late summer, then mow it down short.  The idea is that by not mowing throughout the summer, fewer cleistogamous seeds are produced and the late timing of this mowing does not give the plants an opportunity to produce new flowers.  I have had a hard time getting this to work for me.  If you leave the stiltgrass to grow for too long, all of the shorter vegetation is starved of sunlight.  I tried this technique in one area, and we will see how quickly the perennial grasses bounce back.

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