It's true that you can't make an ID from just the flower, however there aren't that many yellow composites blooming this early in the year around here. I'll bet it's one of the Groundsels (Packera spp, they just changed the name recently so it's listed as Senecio in older books.) Scroll down for some pics very like yours. There are a bunch of different species, so yours might not be this exact one. WW Here is a link that might be useful: Ragwort, Packera (Senecio)

Not Linaria, but closer---this plant is taller than Linaria, and the flowers are smaller and spaced farther apart. Not Verbascim thapsus either---this plant is totally UN-like V. thapsus.

It's a St. Johnswort for sure, however, that's the best I can do. Most of these are native but you will have to check with local experts on the pictured species. This plant is not the eurasian species that is used as an anti-depressant herb.

It's a member of the Pea Family(Fabiacea) and appears to be the Showy Rattlebox, Crotalaria spectabilis (an introduced species). Without a photo of the leaves and lower stem, which are usually purplish in color. can't be sure. Check the photos on the link and see if that is it. Rb Here is a link that might be useful: Crotalaria spectabilis