shrub or small tree in wet ground

I have a small brick built planter which measures 4 ft x 3 ft and the soil is made up of lots of manure which i dug into the soil this year . As a result the ground is very moisture retentive! I would like to grow a small tree of large shrub to be the middle feature which i will surround by planting bluebell bulbs. Has anyone got any advice on what would be best in this location. 

I hope that the manure was well-rotted?

Kilmarnock willow would do well in damp ground.

A hydrangea would like moisture retentive soil, you have the  choice of several types, I think one of the serrata cultivars would look lovely http://apps.rhs.org.uk/plantselector/plant?plantid=998, or you could go for a more traditional mophead or the gloriious paniculata - lots of choice 

A Cornus- Dogwood- would like it too.

They are primarily grown for their stem colour in the autumn/winter, but some have nice foliage colour as the leaves die back. You prune them back quite hard each Spring, so it wont get too big & it's the new stems produced that year that colour up the best. Some varieyies are more prone to suckering than others, so check first if you dont want a grove of them.

I've got a rich purple Iris Reticulata 'Georg' under my C.Alba Elagentissima. J.