It's a Rafflesia flower. They appear in your town when you have under 3 PWPs and when you don't have that many trees.
You need to get rid of it. It means your town is not hood, and it's the opposite of the Jacobs Ladder. To get rid of it, plant more trees and build more PWPs. After you do this it will wilt. You can also remove it by putting a PWP on it. Good luck and I hope it goes away.
Ohh, there's one of those in the entomology section of the museum! I recognized the flower but I didn't realize they could appear elsewhere in the game. Neat!
Yes you can get them from not having enough tree and PWP. You can also get them from having too many tree in your town. My spare town has about 300 perfect fruit trees and has one. If a lot of trees die in a harvest it disappears for awhile and shows back up when the trees grow back. In the spare town I have no PWP.
It appears you have very few trees in your town. When your town lacks greenery, these weed-like rafflesias appear. Your town is the opposite of a perfect town, to summarise.
Its a rafflesia. If you have a rafflesia in your town then that means your town must be really dirty or full of weeds. But from your picture you provided it doesn't looks like it has any weeds at all. Maybe like others said : you font have that many tress and flowers and public work projects.