How do I make this bed usable?

I've rented a house and I want to put some flowers in. I've got some pots at the front but have got this at the back. Part of the drive was fenced off to create this nook - which is handy - and a tree was cut down. Only problem is, the bay you can see, the only one I have, has the tree roots in it - which seems to have some growth still. Is there any way of killing off the remaining growth? I did think of just chucking top soil over it and planting things in that but that seems like a very short term solution.