| sounds like it's a combination of things- Kathy's made a good point about the wind drying that side out... the sun is certainly part of it- everything you listed is a 'part sun' lover- they don't want all sun, all the time- but they want to be close enough to SEE the sun.
if one side gets even five more minutes of sun, the plants will tend to head that way- they are photovores at heart :) I'd simply move some of the younger 'volunteers' to the shady spot, making up for growth with plant density- or I'd want to put a non-growing THING there- a planter, or a bird feeder, or a statue. the shed will certainly make a good windblock- though if you're getting consistent winds from one quarter, I'd do a little math to make sure that the tree remains in the 'lee' of the shed, otherwise, you may end up with it sitting in the very spot where the two 'waves' come back together, which will make it even windier in that spot than it was before. | 
