Rhododendrons are, in general, easy to grow if they are given the conditions they enjoy, loose, acid, evenly moist soil and a thin mulch. Sometimes they fail to grow, lose leaves and die back, branch by branch. If you catch them before they die completely, however, you can often give them a fresh start by replacing the old growing conditions with new and better ones.
Choose a pot large enough to hold the root ball with about half an inch of extra room between the roots and the side of the pot. If you rhododendron is too large to fit into a pot, scrape away enough soil from around the edge of the root ball to leave a four to five inch deep ditch.
Fill the pot (or ditch) with loose, fibrous, acid potting mix, being careful not too put soil on top of the roots themselves. Rhododendron roots are thin and can be smothered by extra soil or a too-deep mulch.
Make sure, however, that the root ball is not sitting higher than the surrounding soil, leaving the roots exposed to the air. The root ball would dry out faster than the ground around it, causing yellow leaves and eventual death.
Water your rhododendron whenever the top of the soil feels dry. Too much water, constantly wet soil, will rot the roots. Too little will stress the plant.
Fertilize with rhododendron fertilizer monthly until the middle of summer. Use only as much as is recommended by the manufacturer, as too much will cause brown edges to the leaves.