Wednesday, May 16, 2018

Stiltgrass progress - May 2018

It would be nice if the growth of Japanese stiltgrass, Microstegium vimineum, would wait until we were done dealing with garlic mustard for just a little while, but I have seen signs of it germinating here (Central Maryland) in early May.  So now is a good time to take a look at how some of the steps I've been taking to get rid of this invasive grass are doing.
A snapshot of one of the lawn areas.  In the foreground is
Zoysia grass, still in the process of greening up. 
This grass grows thickly and excludes many weeds.  Further
on are areas where Japanese stiltgrass has infested the lawn.

This is what a shady, unmanaged area looks like.  The arrows are
pointing to areas with a high density of stiltgrass seedlings. 
Also present are plantain, dandelions and violets.
In areas where I have not been actively managing Japanese stiltgrass I am seeing dense patches of seedlings, mostly in the gaps between clumps of grass and other lawn plants.  This area has been overseeded with a 'no-mow' fescue, but otherwise gets no special treatment.

Vigorous raking does seem to have reduced the number of stiltgrass seedlings in this early period of germination.  This practice will be continued here to see if the lawn can grow back together to exclude future invasions.  This method takes a lot of work, especially if I were to expand beyond the 100 sf test area.  Left alone even these few seedlings would expand to fill in the gaps in the lawn and produce a significant crop of seeds.

In this area I have been vigorously raking the lawn in mid-
 to late-summer for the past 2 years to remove established stilgrass plants
to try to reduce the number of seed produced.  The arrows (3) point
 to small clusters of stiltgrass.

In the areas treated with the pre-emergent herbicide, Dithiopyr ('Dimension'), I've noticed little to no seedlings of stiltgrass, or other plants for that matter.  Reflecting back on the timing, I should have waited until mid-April to apply this material so as to let some of the cool season grasses get started.  But it's hard to predict when a warm snap will come along and mess everything up. 

This area was treated with a pre-emergent herbicide, dithiopyr ('Dimension') at the
end of March.  I was not able to spot any stiltgrass seedlings in this area. 
Other plants here are clover, Indian strawberry, and purple lamium.

One of the most dramatic results was where I used a lawn torch to burn off all the surface vegetation in late summer last year when the stiltgrass was in flower.  A few perennial grass plants bounced back after a week, but it was necessary to reseed the area to provide cover for the soil.  As I mentioned awhile back burning seem to be most effective in the fall prior to the seed ripening/dispersal.  Burning also eliminates the cleistogamous seeds found low on the stem.  Burning after seed is dispersed is too late and spring burning just opens the group to germination from the existing seed bank (see this report from the Forest Service.)  The burned area seems completely free of stiltgass, as well as some other weeds while the control area is dense with stiltgrass seedlings.

Here is a comparison of an area that I burned with a yard torch in late summer
last year (right side) with an control patch where nothing was done. 
The burned are was overseeded with a tall fescue blend, nothing for the control. 
Arrows indicate dense patches of stiltgrass seedlings.
While it is too early to be certain, it appears that there is progress being made against stiltgrass in my lawn.  The pre-emergent seems to be working at this early date.  The downside is that it also affects desirable seedlings.  As indicated on the label, it is best used on established lawns.  There is a possibility that stiltgrass will reemerge in mid-summer after the pre-emergent breaks down (about 3 months after application).  Burning seems to be an effective non-chemical approach.  It looks bad for a little while, but there are not residual effects to worry about.  Since stiltgrass seed can remain viable for 5 years in the soil I will need to stay with this for some time to come.