Smartweed

To ensure accuracy, I suggest leaving this at the genus level. As chesterbperry mentioned, more detail (overall plant, close up of leaves showing veining and shape, etc) is often needed for a species ID and even then, it's not always possible.

This is definitely not P. caespitosum, it lacks the characteristic bristles that would be sticking out between the flowers. It is important to take comprehensive photos of plants, not just the pretty parts, if finding an accurate ID is your goal.

Hi Lainey, this looks like a type of Smartweed. Was it in water or a very damp area?