Epilobiums are very efficient in this. Their seeds have no dormancy period and start immediately. I figured this out during winter, playing with small epilobiums in my lighthouse. The whole cycle from seeds to seeds take only 2-3 weeks. Nice model plant for geneticists! And this is probably the reason why fireweeds appear before any other plants at burnt sites.
So I haven't been imagining spontaneous generation of willowherb in a pot or bed I cleared only yesterday!