Existing CG will die off after the first frost. Tenacity and Siduron reportedly will kill off CG, but it will take a couple of weeks to work. WBG Max with CG control will also kill CG but it will also take a couple of weeks and seeding should not be done within a month of application. So your best option is to overseed this Fall and use the pre-emergent next Spring.

Siduron (also known as Tupersan) does not kill crabgrass. It is a pre-emergent that prevents crabgrass from successfully germinating and establishing, but allows for seeding of other grasses. It does nothing for existing crabgrass. The active ingredient in WBG + CG Killer that kills crabgrass is Quinclorac.

And that's why I shouldn't pass along "information" from what someone else has reported. I was hesitant to post anything about Tenacity or Siduron as I have not used either and other than the one report, it has been my understanding that as Andy pointed out, Siduron/Tupersan is a pre-em.

This pre-emergent chemical for spring application is easy to find, frequently included in step 2 of four-step lawn care programs. I have found that just using steps 2 and 4 (which kills broad leaf weeds including dandelions) gives me satisfactory results.

Wow - the mistakes continue. Step 2 products DO NOT contain pre-emergents for crabgrass. They contain broadleaf weed killers, which are neither pre-emergent nor effective against crabgrass. Many Step 1 products contain pre-emergents for crabgrass. Step 4 products typically are fertilizer-only.

To the OP: the pre-emergents for crabgrass are generally applied in Ohio at the time that the Forsythia are blooming in Spring. With all pre-emergents except for Siduron/Tupersan, you cannot plant grass seed after you have applied it (the period is 90 - 180 days, depending on the specific pre-emergent).

Andy. I have read reports that Tenacity will work as a pre-em. True?

True. Definitely true. But not the same as other "drop it and forget it" pre-m's.

Tenacity is applied by spraying it onto the grass with a pump-up sprayer or equivalent. Amounts are tiny - a teaspoon does 1000 sq ft. And while it is sold as a post-emergent, it provides some pre-emergent coverage for the weeds it treats. Some are grassy and some are broadleaf.

Of course, there are consequences to using synthetic preemergents like Siduron. It's not going to leave your cool season grass totally unscathed.

From a different K-State document than last time (Kansans are goo-goo for tall fescue; it's the most popular turfgrass for lawns in that state):

"Siduron (Tupersan) is the only preemergence crabgrass preventer that can be used at the time of seeding [tall fescue]. Use only the lowest label rate. Even then, a 10-percent reduction in seed establishment can be expected. Do not use any other crabgrass preventer until after the grass has been mowed several times. Broadleaf weed killers should not be used until the grass has been mowed three times. The best way to avoid weeds during establishment is to plant in mid-September." [end quote]

You'll note that this document was written in 2000, about 11 years before Tenacity came on the market, so it says "Siduron (Tupersan) is the only preemergence crabgrass preventer that can be used at the time of seeding [tall fescue]."

Since I planted some tall fescue just after Labor Day, I've been reading up on that grass lately. Also planted some TTTF two or three months ago for my neighbor in his needy side yard. Good grass, but I'm not goo-goo for it like I am for zoysia :-)

....actually just checked my calendar. It was 4.75 months ago on May 6 that I seeded TTTF in my neighbor's shady side yard that sometimes gets submerged a bit during heavy spring rains. I guess it was getting kind of too close to summer to seed a cool season grass, but I figured his partial shade would help out, and I suppose it must have because the grass is doing very well.