Here are a couple photos of it taken last fall and detailed descriptions.
Because it reproduced abundantly, I’ve been pulling every seedling I see so it doesn’t take over the neighborhood. Still, I’m curious to know what it is.
I’d say it’s definitely a Composite of some kind. Of course, we’re talking about the largest angiosperm family on Earth…
Possibly a Senecio. Still looking…
Wow! ** jayjay,** that was quick! You are way in the ballpark and I’m sure what I have is a very close relative. The main leaves of mine have five leaflets while the rayless tickseed you linked to has three. Plus, the flowers of mine are a bit larger and have seven or more involucre (I thought they were sepals) while yours has five.
What really confirms that both plants relatives is something I’d forgotten about. It says at the end of the article, “The genus name ‘Bidens’ means ‘two teeth’ and is so given because of the two awns of the fruit.” This reminded me that the seeds were flat ovals with two prongs at one end. The cat from across the street used to visit me a lot and was always bringing them inside on her fur. Sigh. I haven’t seen that cat since Feburary and don’t know what happened to her.
Anyway, thanks so much for solving my mystery. I owe you a great big hug!
Damn, I’m late. 
My guess would have been a Bidens also.
Oh, actually jayjay’s first picture has seven involucre so it could be that one.
Maastricht, thanks anyway. You get a big hug too.
Since people found the OP’s plant, want to give another a shot?
I haven’t actually knowingly seen this plant, but it smells. strongly. For the past 2-3 weeks some plant that’s all over this part of the state (central NH) has smelled very strongly of yeast of the sort you smell when you take a tour of a beer manufacturing plant. I never noticed the smell before, but you can smell it in the late afternoon all over the place right now. Any ideas?
Triffids?
Lindenblossom?
Hops?
That’s interesting elfkin. I don’t know what it is. Maybe there’s a lot of home brewers there?