My philosophy about volunteer plants is that, until I have enough of them or give them out to all my friends and family, I will keep taking new growths and pot them up, if I don't have space for them. Then give those pots away to friends and family...sometimes even strangers!

Wallflowers, (erysmium cheirii) are traditionally grown as biennials (often as part of a 'bedding out' system) but mine have been robustly perennial for over a decade. Nicotiana sylvestris is often good for another go-round, but both of these probably point up the shadings of classification and expectations between true biennials (Campanula medium), short-lived perennials or tender perennials grown as annuals.
Have just been on a seed buying digitalis rampage after being hugely heartened by last years robust growth of fat rosettes, just beginning to extend skywards (D.purpurea, grandiflora, lutea) in our bedraggled and wild woods - expecting a froth of various umbellifers and many white foxglove spikes.

Interesting, I grew Erysmium 'Bowle's Mauve' sometime back which is supposed to be perennial but it acted like a biennial for me. I loved the plant, it bloomed in late winter & was evergreen with very attractive blue foliage which I would have liked even without the blooms. The plant got very big with a woody base so I trimmed it back after blooming the second year, it slowly died so I figured it was a biennial. Just looked it up, its perennial.