The weed looks like henbit, which is a winter annual (sprouting in fall, growing in n spring, dying in summer). I have a similar dilemma to yours. I overseeded and have a lot of new grass. I used Tenacity at seeding (4 oz /acre rate) and did a second application around 5 weeks at the 2 oz/acre rate. My initial plan was to do two follow-up applications and then use prodiamine as a more effective pre-emergent in November (trying to ward off poa annua). I haven't done the second follow-up and don't think I will and I think I'm going to wait until March for the pre-emergent. I will save the Tenacity for spring when I can identify poa annua (if I have it now I don't recognize it) and use spot sprays (mixed at 2 oz/acre rate) then. Reason for change of plans: all herbicides are a stress and I want the new grass to grow as well as it can now before the season shuts down. I can deal with weeds later. Some of my new grass has been slow to come in. I wonder if the Tenacity hindered germination and growth of some of the fescue. I put more seed down at the same time as the follow-up application of Tenacity because some areas were very thin. They are still somewhat thin. I never had this problem with other overseeding and this is the first time I've used Tenacity.

Scroll down to the question from John: http://www.domyownpestcontrol.com/tenacity-herbicide-p-1877.html I'll copy it here for readers browsing: John from Great Neck, Ny QWhat are the application instructions for Tenacity on fine fescue turf? My lawn is fine fescue, and I overseed every spring and sometimes in fall. The label states not to use Tenacity at the same time as seeding fine fescue. Thus, what would be the best strategy to use Tenacity on my lawn for pre and post-emergent purpose? Should I look for an alternative product? A Per the Tenacity label, you will not exceed 5 fl. oz. of product per acre for fine fescue. This is just the max amount that should be used per year and per acre. The rate that should be used is you will need to mix 0.5 tsp of Tenacity and 1.5 tsp of surfactant to one gallon of water covering 1,000 sq ft. Also, Tenacity should NOT be applied at the time you are putting down new seeds or overseeding. This will likely inhibit their growth. You will want to wait until the lawn has been established before applying this product for pre-emergent control. This is generally after three mowings. There are no restrictions for Tenacity with regards to post-emergent control of weeds. - See more at: http://www.domyownpestcontrol.com/tenacity-herbicide-p-1877.html#sthash.VFzwKzyo.dpuf So it sounds like it's fine for one application per year on established fine fescue. The 0.55 teaspoon to one gallon of water over 1000 suare feet is the 4 oz per acre rate. Do you want to use this as a pre-emergent or post emergent? What weeds are you targeting?

Tenacity likely will take more than one spray to kill the crabgrass. But it will “light it up” so maybe the next time(s) you can just spot spray. Also you don’t have to wait between Tenacity application and overseeding. Scott’s Starter Fertilizer with Weed Preventer actually contains the active ingredient in Tenacity. So you don’t have to wait until 9/21.

It'll come back. I've hit it with a ton of Tenacity. It seems dead but new sprouts come out when the Tenacity applications end. Even on a blank canvas of a new seeding, triv has no problem sprouting--maybe less but who really knows. I had wanted to try Velocity but seems it is no longer made and the ag equivalents are super expensive. I still try to hit it with Tenacity very early spring and avoid irrigation in the summer to let the lawn go dormant. Has kept it in check.