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Where did it come from?
I hadn't planted it, certainly, and there are other singles that have popped up elsewhere.
It didn't seem like squirrels would have moved a bulb (since daffodils are distasteful), but establishment from seed didn't seem obvious (in the middle of a lawn), either. Nor would having come in with leaf mulch make sense in this spot.
A bit of research revealed that some daffodils, especially the simpler-flowered ones, do set some viable seed. So, it's undoubtedly a reflection of an good year, weather-wise, for germination and establishment of several new daffodil plants in unlikely spots!
How the seeds were dispersed there in the first place is another question entirely.
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Notice that there's another daffodil in front of the flowering one that's established, but not flowering...