Trees for small gardens: Olea europaea, the olive tree, grows slowly in our climate and this makes them perfect for containers. Pinch out the tips when the trees reach around to keep them bushy. Photo by Paul Debois.
Multi-stemmed birches in square planters - disadvantage: they're deciduous so only their tracery to look at in Winter. If we planted Betula pendula 'Jacquemontii' it does however have amazing white trunks