What's this that's seeded itself?

I'm clearing out some unused pots of dead compost and came across this little chap, who appears to have seeded himself. Before I consign him to the compost bin, I'd like to know what he is so I can decide whether to keep him or not. I'm thinking some sort of tree 'cos he's got a fair old root ball on him (roots completely filled the pot and I'd removed half the roots before taking the photos).

Not much to go on I know, but any ideas would be welcome

Thanks in advance

 

Looks like a goat willow (salix caprea).

Very fast growing and a bit of a pest, wouldn't bother keeping it.

 

I think willow too, would you say the stems are reddish and pliable?