If anyone knows better, please correct me, but I don't see any reason why RoundUp would not kill Japanese stiltgrass. There might be a problem getting the RU to stick to the leaves. They make a "sticker" for that. Basically it is liquid dish soap or shampoo. Spray some water on it and look to see if the water beads up and rolls off the blades of the weed. If so then you need the sticker. Before you spray the real RU product, experiment with plain water and soap to make sure you use enough soap to make it stick well. Once you know the amount of soap, mix the RU with the soap and spray. Wait a week to see the results. During that week water daily to sprout all the weed seeds there are. Then, a week later, spray the RU on all the new weeds. There are weeds sprouting this time of year. Generally they are not visible until spring, but they do sprout. So even if you don't see anything the second week, spray the RU again.

That will kill everything and you'll be starting over. You might want to do that in the spring. If you get lucky you'll sprout all the summer weed seeds and knock those down before you put real seed in. Spring seeding is a bad time of year because all those summer weeds are waiting to sprout. It is also a bad time because the grass sprouted in the spring does not have much time to grow hardy roots that can withstand the summer heat. But at this point you don't have many options.

I will offer you the (hard) truth about Japanese Stiltgrass. I know this truth because I have fought it, and won. BTW, there is tons of bad info about Japanese Stiltgrass out there.

I will also try to tell you that a I-Want-A-Solution-Now is about the worst thing that you can do when dealing with Stiltgrass. Both Morph and David can vouch for me that I'm not a seat-of-the-pants guy.

Stiltgrass is very aggressive. It can take over an entire lawn in 2-3 seasons. And the stuff you need is not cheap, available in Home Depot, and must be applied exactly at the right time. You can't say "I was busy that weekend".

Stiltgrass has a very definite life cycle. In the North (including NJ) it grows ONLY from seed - it does not overwinter. The seed for next year's crop is already on/in your soil. This means that if you rototill, you will INCREASE the amount that occurs next year. But that also means that a pre-emergent will work. Stiltgrass emerges in June. That means that a pre-emergent for crabgrass goes down at the time of Forsythia blooming, and an additional treatment will be needed around Memorial Day. DO NOT listen to people that try to tell you that you can use Tenacity/Mesotrione to defeat Stiltgrass and re-seed at the same time. It does not work for Stiltgrass.

After the emergence period is over, you will still have some Stiltgrass that breaks through. To deal with that, you will need an herbicide named "Acclaim Extra". It is not cheap, but it works. Apply it exactly as the instructions state - no changes.

You will be able to reseed in mid-August of next year, but the lawn should be thickening/recovering well by then.

Stiltgrass often creeps out of woodland areas where this invading grass can be very common. If that describes your area, you will need to repeat the treatments year after year. A new lawn will be invaded every bit as easily as your current lawn.

BTW, seeding and rototilling now is a recipe for disaster and wasted money. You may not like all that I said, but ignoring the lessons learned by me (the hard way!) will bring you back here next year with an even tougher problem.

Thanks guys for the advice.@Andy, acclaim extra is indeed expensive, $82 for a pint will be perhaps the most expensive thing I will ever buy for lawncare but the review of that thing looks pretty solid.

Thank you Andy. I wonder if that is the approach needed to get rid of King Ranch bluestem?? That is the grass that the Texas highway department has been using for several years in the median of all the highways. Now it's in every yard and field with a bare spot. I have it fairly prominently in my new yard and as a full fledged border in the green space off my property. Keeping it mowed seems to help, but I'll know more as time goes on.