Q: This was referred to as “Missouri primrose” in northwest Arkansas.
I have it in a south-facing garden that is very dry but it spreads profusely. Can you tell me what it is?
A: Most of us call it “evening primrose” (Oenothera speciosa), without referring to a state of origin.
Since it is native throughout the Southeast and is distributed far into the Midwest, states of origin don’t matter much to this plant. As you note, it can thrive in dry sites. I commonly see it growing happily between sidewalk cracks.
In rich soil it can take over, choking out all garden plants in sight.
My advice is to keep an eye on it; evening primrose can become a pest before you blink twice!