Centranthus ruber, red valerian. Whether you consider it a weed depends on your viewpoint :-) It self sows with wild abandon!!

And it *is* a plant, you know how you tell? It's alive, it's green (due to chlorophyll) and it doesn't move on it's own nor eat other things. That usually qualifies it as a plant.

Now, if you meant to say, desirable ornamental, that's different....