Weed

Our garden has seen some strange new growth recently. Over the course of a couple of wet but sunny days, the commonplace generic weeds and daisies were joined by...
little trees.

There's two mature trees of this species (don't know what it is) in the garden and now there's about fifteen little saplings in the garden. Now I don't want to cut the grass, chopping down little trees seems more destructive than just grass and daisies.