Rhododendrons come in a wide range of variants, so many that there are several suited to any given climate. Rhododendrons are beautiful but finicky plants, as they can easily be killed by improper planting conditions, or soil or watering mistakes. Be sure all the right conditions are in place for you to plant rhododendrons that will thrive.
Plant rhododendrons at the right time of the year. In cold and temperate areas, spring planting is best, while hot or subtropical climates require fall planting.
Get your soil tested. Most native U.S. soils have too much clay for rhododendrons to thrive, and will need amending. Use a soil-testing kit to determine the composition and density of your soil. Rhododendrons are acid-loving plants, so you may need to add peat moss or a sulfur-based amendment if your soil is very alkaline.
Amend the rhododendron planting site. Choosing a spot that is mounded or on high ground is ideal. Your goal is to create a shallow, well-drained spot rich in organic material. This will be ideal for rhododendrons. Turn over the soil a foot deep throughout the area, making a wide, shallow draining field for the plant. Mix half rich compost with half dry peat moss and sand, and mix it into the soil well.
Choose the right kind of rhododendron. Not all are equally cold hardy. Check cold hardiness requirements for varieties at your local nursery or greenhouse, and pick one that is rated to withstand average cold temperatures in your area in winter. There are rhododendrons available that are hardy down to --25 degrees F.
Plant carefully. Much rhododendron advice tells you to bury the plant's root ball in a hole twice as deep as the ball itself, but the plants have shallow roots and can be killed by burying too deep. Dig a hole just the depth of the root ball, letting the top of the root ball protrude slightly aboveground, and add the soil mix around it.
Water twice a week when the plants are young, directing the water into the ground, not onto the plant, to avoid fungus problems. Once they are a few years old, you can water less frequently. If you choose to fertilize, do so in small amounts, sidedressing rather than applying it directly to the base of the trunk.