The Forsythia are in bloom, now it's time to...

The blooming of Forsythia is a signal to begin a number of outdoor tasks.  It signals a good time to apply preemergent herbicides for control of summer annual weeds.  I'm giving this a go this year as another tool to get rid of Japanese stiltgrass, Mircostegium vimineum, from my property.  I've been making progress in the woods where I have been pulling it out in July and August as it is getting tall.  Getting it out of the lawn is another matter.  

It's the end of March, and although the Forsythia started blooming
at first in January, I think they are doing it for real now.

There, repeated mowings keep it short and actually induce early seed formation in the lower stems.  I have been raking/combing it out of the lawn in a few areas but that does not seem very effective.  It seems that while I remove a lot of the weed, any remaining stiltgrass just expands to fill in the gaps.  So while I prefer to avoid the used of synthetics, I've tried going it alone for a couple of years and now I need some help.  

Many of the common preemergent herbicides used for crab grass control have been found to be effective on stiltgrass when applied prior to seed germination.  Preemergents that have been shown to be effective include Dithiopyr ('Dimension'), Pendamethalin ('Pendulum'), Prodiamine ('Halts') and Trifluralin ('Preen Weed Preventer').  One of the challenges is finding a preemergent that does not come with added fertilizer.  In my case I do fertilization in the fall, since I am growing mostly cool season grasses, particularly fine fescues, Festuca rubra cultivars.  Fine fescues have low fertilizer demand and I see no good reason to apply fertilizer at a time when weeds are about to take off. The product I found contained only Dithopyr, no excess fertilizer.

Hairy Cress is a winter annual weed that has been taking over here.  Last year I mowed
it early before much of it started producing seed.  This year I'm hoping
 to get some added help from the preemergent herbicide.

As an added bonus this may help control the hairy bittercress, Cardamine hirsuta, that is growing in the lawn.  This weed is a winter annual, sprouting in fall and again in early spring.  While it dies back by mid-spring, this creates gaps in the turf that provides space for the stiltgrass to fill in. 

Dithopyr works by interfering with development of new roots, after seeds have germinated.  Perhaps I should have waited a little longer to put this down in order to allow the existing grasses to get further along, but I didn't want to forget. We'll see how all this works out later in the summer when I can compare treated and non-treat areas for amount of stiltgrass.  Check out this earlier post for some things I tried last fall.