Saturday, April 6, 2019

Stiltgrass actions for Spring 2019

I have been battling Japanese stiltgrass, Microstegium vimineum, for the past 5 years or so.   Pulling stiltgrass out, particularly in late summer, in the shaded woodlands has been effective in reducing the amount of stiltgrass over several years.  Getting it out of the lawn is another matter.  I've tried raking it out in late summer, and over planting with more desirable grasses (particularly fine and tall fescues).  This has helped, but stiltgrass as a warm season annual, is very adept at filling any gaps in the lawn starting in mid-spring.  While I really like the look of the fine fescues,  particularly the no or low-mow fescue blends, these are slow to establish.  The tall fescues are more aggressive than the fine fescues.  When over-seeded in late summer, these do a better job of getting established and filling in the gaps. 

The area outlined in black has been burned with a garden torch in late summer for the past two years. 
The control patch in the middle was not burned; it has slightly less coverage with perennial grasses.

More recently I've used a garden torch in late summer to eliminate it before it can set seed.  This has been somewhat effective but that is not so practical for a large area.  

Last year I used a pre-emergent herbicide, like those you would use for crabgrass, for the first time.  It was very effective at suppressing the stiltgrass for an entire season.  (I should say that the chemical's effect is probably gone after 2-3 months, but by that time the stiltgrass germination rate has dropped.)  Other perennial grasses were able to start filling in.  I will expand the area of application this year and also test it in some of the meadow areas, as well.

The treated areas have a little more coverage with perennial, mostly cool season, grasses than the untreated areas. 
The brownish spots are mostly zoysia or nimblewill, both warm season species.

While Japanese stiltgrass has a very high rate of germination, I expect that there may be some that waits a season of two to germinate, so these pre-emergent herbicide treatments may need to be repeated for several years.  I will try to leave a small area that I treated last year  untreated this year to see how strongly the stiltgrass comes back. 

In a blog post last year I mentioned several pre-emergents that were effective.  This year I will be using the same as last year, dithiopyr, sold as Dimension.  It has the advantage that I could find it alone, without added fertilizers.  Also, it was less expensive than Preen garden weed preventer (trifluralin).  Timing-wise, the general recommendation is to put the pre-emergent herbicide down when the forsythia are in bloom.  Last year I did this on the early side, just before peak bloom.  This year I will wait just a little later.  My thought is that this will give the existing grasses a little more time to spread before I put down the herbicide, which works by inhibiting new root growth near the soil surface.  I recently found a website that give guidance on when to apply pre-emergent herbicides and other turf related IPM activities called GDDTracker.  A nice feature is that it compares the current and previous year's temperature histories.

I will also try the pre-emergent herbicide in an area that I mow only once a year then leave to grow as a meadow.  In one section I burned off the surface debris.  Half of this I will treat with the pre-emergent, the other half will be untreated (Burn Only).  I will also treat an unburned area with the herbicide to see if burning makes a difference.  The rest of the meadow will serve as the 'control' group.  I am a little concerned about using the pre-emergent herbicide in' wilder' areas.  It could have a negative effect on some of the native annuals there like sweet everlasting, Pseudognaphalium obtussifolium, and yellow fumewort, Corydalis flavula.  But, then again, the stiltgrass has a definite negative effect.

Here is a section of my 'Mown Meadow' where I will do a Stiltgrass/Pre-emergent experiment.  This will give me an idea of how much prep work is needing in this wilder area.

Another action that I will be doing is to plant some bottlebrush grass seedlings, Elymus hystrix, into areas that are infected with Japanese stiltgrass (some herbicide treated, others not).  Bottlebrush grass does particularly well in open shade, conditions that Japanese stiltgrass excels in.  Just removing the stiltgrass alone is not enough, I need to establish some desirable native species to keep the stiltgrass out.