Thanks for bringing this up, htmb. Nice photo of hog plums. I think we'd just call them wild plums hereabouts but wild plum groves are a not common in this area; the only one I remember seeing was at WIlson's Creek Battlefield.
I'm in the process at work of making chart of MIssouri fruits and berries and when they ripen. Mostly the wild kind, but also garden varieties, to help our poison center staff quickly ID plants on the phone, by color, habit, to get an idea what people have when their 2 year olds eat a mouthful. I have gotten over being surprised at how ignorant people are about the plants that surround them. I'm going to mine this thread for information.
I don't even know when holly is supposed to have berries, but this seemed very early to me.
Nope, holly berries ripen in the late summer so the birds have food for the fall/winter. The seeds of which they then poop all over my place and then I have to pull and tear up baby holly trees everywhere. Unless you tear out every piece of root of a holly it comes back up. We have a holly tree we have been trying to kill for three years by cutting, poisoning and burning, and it's still sprouting up. I hate holly.