I have 2 plants that i put in about Feb, they both looked very healthy until about 2 months ago when the lower branches just fell off , although they were very green and healthy. The top of the plants are still looking good but the lower branches are now bare... antone know why please.
I have a group of Mahonia and this is the way that they grow, naturally losing their lower leaves as they grow taller, so I wouldn't worry too much. If you want them to branch out more then I would suggest doing some pruning after flowering to promote this.
Are they in full sun? If so then they need watering. They grow lanky in full sun with the leaves going red with more shade and moisture they stay bushier and green. mine is in semi-shade in a damp spot and it is dark green.