Every garden has them, the extra hardy, and quick to volunteer plants that seem to take over every available spot, space, and pot. Colorful cosmos are welcome.
But I have Thalictrum, meadow rue, [which is lovely in its own right,] growing in about 10 different spots. Some of it so well established in the ginko tree's container that it is bigger than the tree. But I have a hard time getting rid of it when it is blooming, so I wait until it gets mildewy and then cut it down. There are grasses that have gone to seed in almost every pot on my patio. Don't plant millium effusium, a wonderful chartreuse green/yellow shade grass, unless you want it everywhere. And I mean everywhere! It is really stubborn and difficult to pull out from between the branches of hydrangeas, or any other plants. But oh so beautiful. Of course none of the really great perennials want to volunteer. Snobby golden hops, won't raise it's hand, nor will any of the beautiful zebra grasses reproduce. No they are too good for that. Its the workhorses of the garden that love to volunteer.
Plant euphorbia wolfennii in this climate and have a baby boom. Almost as many babies as Hollywood seems to be experiencing. But it is a lovely plant with those limey yellow blossoms. A real show stopper when planted. The trick is to wait until it blooms, and then drop the seeds in the neighbours yard....... wow a baby boom!