When I've purchased daffodils in the past so often the key word in selecting my choice was "naturalize". But it's not just daffodils that naturalize. Any perennial that puts out its share of seeds is going to naturalize as I've learned in the pond garden. The two clumps of yellow iris I purchased now completely surround the pond.
I have black-eyed susans growing where I didn't plant them.
And now the Joe Pye weed is naturalizing.
This is the first year that its done it in earnest.
Other years there have been a shoot here and there that I would purposely transplant with no luck. This year, new clumps are forming in other spots around the pond. It's decision time: do I let it naturalize? I had no choice with the cattails which I didn't plant.
They did that all on their own despite my attempts to stop them. A willow tree has appeared that I consistently cut down to noavail.
Then there is the ajuga. 
I planted it to be a ground cover along the edge of the pond long before the irises took off. Friday I discovered it had spread quite a bit under its layer of Japanese stilt grass. But then I had intended that it naturalize.
Here's one last plant growing in the glen garden. I've left it there undisturbed because I like the looks of it.
I didn't plant it. Who did? Nature naturalized.