Ornamec is a grass killer and can be sprayed "over-the-top"of most ornamentals
without any damage.

As for the length of time it takes to kill it, you would probably notice a
wilting within 24 hours but it would take a few days for the stilt grass to
die.

Stilt grass does not set seed until September so you would have plenty of time
to retreat if you missed some.

For more info on Ornamec,
http://www.pbigordon.com/Product_Pag..._Herbicide.htm

Also on stilt grass,
ipm.ppws.vt.edu/weeds/Microstegium.pdf

Marcy