Humid it is, but not really shady. Nevertheless, not a doubt you are right, given the information those links present. I've not yet experienced it as a big problem, but maybe it's because I got other stuff that's even harder to control. ;-) Anyway, thanks!

Looks like Carduus.

I must admit, it was certainly worth the effort. Inbetween the stones of the paved area it's still growing. Small sprouts, but because of the stones I can only pull the leaves, and it keeps growing back, however slowly. If I would have done this in the ground too, I still wouldn't be able to control it, but the digging method proved successful. Just some small sprouts at first, less and less over time, and by now it's all but gone. One big effort, then one season of closely guarding against new growth, and that's it. No poison, no weekly (or more) removal of lots of new growth.. I'm happy.

Thank you very much. Turns out it is not a native species here, but very invasive. It breaks the germination of other plants. Apparently the Dutch Food and Consumer Product Safety Authority even launched a campaign a couple of years ago to eradicate this "Alsemambrosia". I'm glad I asked before it fully flowered, and I'll report it and remove it in time to prevent further spreading in the neighborhood.

@kevinsky Sorry I forgot. It's in The Netherlands, North-Western Europe. I've added it to the question.

Thanks. :) Just phone pictures, but I had good daylight, and I think clear pictures do help when identifying a plant. Didn't think of fingerprints, though. Now you got me worrying. ;-)

Maybe your Clematis was grafted on the root system of another variety. Sometimes, the roots can grow new stems of their own, and suddenly you got two different plants sharing the same roots.

Drooping as going soggy? If so you watered your cactus too much and you're probably too late.

I think pepper mint is more purplish, so it would be C, if any. B might be Moroccan mint. Anyway, I think your first line contains the real answer and the varieties are better distinguished by taste than by looks.