I've been delinquent in regard to posting about the status of my "experiment" in using Tenacity to remove fine fescue from a norther mix, even though the experiment is ongoing and I have been taking some pictures along the way. I've also realized that I probably shouldn't have really called it an "experiment" as there's no scientific aspect to it -- no control, no separate patches with different application rates and intervals, and no careful measurements of relative success.
The quick summary is that the results so far have been not what I had expected. Nonetheless, I'll at least explain what I've done, and how it's going. I'll probably have to split my write-up into a couple different posts.
The area being "experimented" upon is approximately 1500sqft on the "other side" of the driveway. This area was seeded in a northern mix (PRG, FF, KBG) in the fall of 2013. It is primarily shaded until about 9am in the morning, but gets good mid-day and afternoon sun. The area nearest the road gets the most sun.
Each spring the area nearest to the road develops significant bare spots from what I think is a combination of winterkill and road salt, primarily affecting the PRG as far as I can tell. These bare spots get filled in by KBG over the course of the spring and summer, so that by fall, there is an ever-increasing amount of KBG in that area, with the exception of patches of fine fescue, which go dormant in the summer. (The fine fescue in my lawn may look fragile and goes dormant in the heat of the summer, but it is one tough grass that doesn't seem to be killed by anything, and comes roaring back each fall once the temperatures cool off enough.) Accordingly, after five years, the area nearest to the road is primarily KBG, but still with a fair bit of fine fescue intermingled.
The issue I have is that during July and August, the fine fescue largely goes dormant, resulting in a brown "understory" of fine fescue below the thriving KBG grass that loves the New Hampshire summer. The effect this has upon the visual appearance of the lawn is that when I mow the lawn, the brown "understory" gives the lawn a brownish color. However, as the KBG grows taller in the days following mowing, the lawn looks pretty good right before it is cut. Below is a photo of the area right before mowing on July 27th of this year. Looking across the grass from this angle, all that is seen of most of the grass is the tips of the KBG. The brown "understory" of fine fescue is hidden, with the exception of some that can be seen right along the roadside cobblestones. (The fine fescue really can't stand the heat of that area, but doesn't get killed -- it just goes dormant waiting for fall.)
My plan, based upon having read various articles from about a decade ago concerning the tolerance (or lack thereof) of fine fescue to higher rate Tenacity applications, was, while applying the Tenacity to control listed weeds on the label, to also "thin out" the fine fescue in this section of my lawn by making two applications of Tenacity at 8oz/acre at a 14-day interval. I was seeking to make the first Tenacity application just as the heat of summer was ending (basically at "seed-down" time for a renovation) so that the fine fescue would get hit just as it was coming out of dormancy. The hope is that in addition to killing off traditional weeds, this would kill off much of the fine fescue, without adversely affecting the Kentucky bluegrass. This hope is largely based upon the study described in a study conducted at Washington State University a decade ago: http://turf.wsu.edu/wp-content/uploads/ ... eminar.pdf
The relevant slide is #24, which is shown below.
There's also a similar relevant article, titled "Safety of Tenacity to Creeping Red Fescue Grown in the Northeastern United States" from the University of CT on page 46 of the following publication, which also shows significant injury to fine fescue in repeated heavier doses of Tenacity (two treatments of 5oz/acre at a 21 day interval): http://www.turf.uconn.edu/pdf/research/reports/2007.pdf