I love when stuff like that happens -- and marvel at mother nature.
One summer, I had a row of planters on my deck railing filled with a variety of flowers -- petunias, moss roses, etc. By August one pink petunia had sprout in the patio below from the seeds that fell from above.
The patio was on the south east of the house, very protected from cold and wind. The petunia lasted through Thanksgiving that year and had seeded so well that the next year I had tons of petunias on the patio.
The moss rose had also seeded itself down there -- only I didn't know it. I was pulling out the seedlings because I thought they were weeds. After going out of town for a couple weeks, I came back to blooming moss roses and finally realized what they were
Sigh, sigh -- now I'm in a townhouse with a concrete slab for a patio.
I miss my garden!