Looks like some type of Ash tree. Fraxinus is the genus name, there are several species of it. We have Green Ash in my area.

Token has it right, common names are evening primrose and nine o clocks. You can watch these flowers open, right before your eyes, as they open very quickly & sometimes they will even shake a bit-we always watched them as children & I still enjoy doing so. They get lanky & weedy looking but I always let some reseed to watch open & show others. Look for the fattest bud in the early evening, then check about dusk & you will soon have their schedule. Mine opened about 8:45 pm last night. The blooms close during the day but will linger if it is overcast or rainy. Susan

I missed that you had come back with a reply, Coach! It sounds like you have some species of Asclepias! One of the common names is butterfly weed, and people pay money for the stuff you're trying to get rid of! Like dandelions, the seeds "float" in the air, and my guess is that some neighbor pretty close has one/some they're cultivating to attract butterflies! The most common species is A. incarnata--but I really have no way to know if that's the species that's showing up in your yard or not. Some info at the link below if you're interested in more! In terms of getting rid of it, I recommend Weed-B-Gon. You can use that in the grass--it kills only broad leaf plants, and you can use it where there is no grass too. From my experience it works better than Roundup, and it's what I use anywhere I need an herbicide these days--works quite well at getting rid of cottonwood and aspen suckers in the grass! As far as I know all, or at least most, milkweeds have taproots, so the sooner you get rid of them the easier it will be. If you decide to try to pull them, you'll need to get all of the root out or they'll come back from anything you leave behind! If you have a place where you want to grow some to attract butterflies (it's also a host plant for the caterpillars), you can just deadhead the flowers before any seeds form to help keep them from "filling up your yard!" Can't help with "the neighbor" if I'm right that that's where the seeds are coming from! Skybird P.S. The "plant" you were getting into as a kid was most likely dandelions! Here is a link that might be useful: Asclepias incarnata

The leaves and wood of elder have a somewhat unpleasant smell, which is easy to check for. And, as the link info says - the stems are hollow. Fairly easy to remove as it is not deep-rooting, though the roots do spread widely. Better to remove it entirely as it will resprout from the stump. If there are other elders nearby you are guaranteed more because the birds will bring seeds to your garden. Makes an 'interesting' jam from the black berries teamed with apples. Sort of a strong toffee flavour. (I don't think it was my jam-making...;-) )