Before vandal interference in my garden along with other difficulties in our lives made gardening as my late dh and I enjoyed it impossible, bronze-leafed fibrous begonias used to self-sow in a shady spot where years of working in compost and planting potted plants with commercial potting soil made the perfect place for these begonias to self-sow. The vandal interference has included such hassles as digging out our topsoil and removing the landscaping ties with which we edged the flower beds. We had stabilized the landscape ties by drilling holes into them into which we pounded rebar sections. The vandal does cute things like moving some of those 6-8" rebars around where they can be tripped over.
So, they don't come back now either, but once upon a special, vandal-free time in Oella, it was possible to have a garden dream and then make it come true - and self-sowers like fibrous begonias would self-sow in following summers - late - by which time the summer was pretty far along.
Incidentally, before we grew tall, shrubby roses with other naturalized plants and flowers there, cherry tomatoes on tall vines used to regularly self-sow.
There's a discussion in this forum on another thread that has crossed into this sort of phenomenom about flowers known to be killed by freezing after they were leafed out, but which can be germinated by exposure to freezing temps using and adapting the technique called "winter sowing":
The thread "Hardy annuals that bloom in spring and summer in Maryland zone 7b" in
Annuals forum .
I think this is a timely, relevant discussion since now is the best time in z7 to sow "hardy annuals." Our choices for which flowers to sow now that would flower this coming summer are expanded by realizing the parameters for germinating these sorts of flowers are greater than commonly thought.