Something in the pea family. You might compare to Chamaecrista fasciculata, partridge pea.

Thank you so much for that input! I'm thinking it might be Chamaecrista flexuosa. I'll try to get a closer look at the plant tomorrow.

Many thanks!

a rather noxious weed ... the MI variation anyway ...

and that may be proved by your comment that it just popped up.. and the fact that it looks 4 feet tall in your pic ... that might be part of the definition of aggressive.. to many of us ...

ken

Chamecrist flowers aren't pea shaped. Sesbania sp?

Thinking that is likely Sesbania herbacea. You could also see S. drummondii in your area, but that should only have 10-20 leaflet pairs. Your plant seems to have at least 26 on some branches.

From the looks, it could also be a senna species...

Thanks again for everyone's input. I believe there are around 36 leaf pairs. I was hoping it wasn't a weed, but it's looking more and more like it might be. Here's a closeup of the flower.

Here is another shot of the plant.

Now that I can see the flowers more clearly I agree it is not Chamaecrista, however Chamaecrista is in the pea family as is this plant.

I think it is Sesbania herbier. It has been found both East and West of Travis county according to USDA, so my guess is that it is here also in areas that are not as Alkaline. S. drummundii is different in both flower and leaf. Both are not appropriate for an in town garden. I would definitely deadhead them if you are thinking of letting it grow. Me , I would pull it before it seeds and definitely not let it show you if it is a perennial and sends out plants from its roots. USDA identifies it as both annual and perennial. I do know that the relative . S. drumundii AKA Rattlebox forms LARGE thickets out on the gravel bars of the Pedernales River.If you start to google it it comes up with Sesbania herbacea CONTROL. Always a bad sign. LOL. Rattlebox is an unattractive plant but it serves the purpose of being the first succession of plants to establish plant matter and root mass into an unstable stream bank. I like to hover under it on the gravel bank for shade while I am sitting by the river on a 100F day.

Thank you all for your amazing feedback. This plant has to go. Since it appeared quickly and has grown so rapidly, it's way too suspicious. We visited the Wildflower center in Austin yesterday so I have some better ideas on planting some native plants in that area.

Thanks again!