Rhododendrons are the signature plant of many Northwest gardens. While they grow to the size of small trees on the West Side with little effort, they are a bit more of a challenge in the Inland Northwest.
There are hundreds of species and thousands of cultivars to choose from. For our area, be sure to choose rhododendrons hardy to at least USDA Zone 5-6. Resist the urge to choose a particular color before checking the plant’s tag for its cold tolerance and other care instructions. Ones to watch out for are the yellow rhododendrons, which tend to need at least Zone 6 or higher. I tend to pick plants that are hardy to at least minus 15 degrees. Keep in mind though, while the plants are hardy, the buds will often freeze out before the rest of the plant if we get a really cold winter and you will lose a year of blooms.
Two of my favorites are the PJM and the Northern Starburst; they are hardy to minus 25 and minus 30, respectively. The PJM is as tolerant of heat as it is cold and is widely planted in our area. It puts out a cloud of almost electric blue purple blooms in early April. Northern Starburst is an improved form of PJM with larger flowers and thicker leaves.
Rhododendrons need an acidic soil to grow well. Because our soils here tend to be alkaline, it is a good idea to plant them in a mix of fine conifer bark or peat moss and good quality compost. To help maintain an acid soil, apply a rhododendron fertilizer with iron early in the spring before the flower buds begin to swell and again after the plant finishes flowering. The iron helps release nutrients in the soil and prevents them from turning yellow.
Rhododendrons have fine, shallow roots so water them generously throughout the growing season, especially in July and August and again late in the fall before the ground freezes. Consider putting all your rhododendrons in one area of your landscape so you can water them efficiently but not overwater other plants. Small micro spray sprinkler heads set on a timer independent from other irrigation work well to get them enough water. Mulch plants with a thick layer of bark to further reduce evaporation.
Rhododendrons grow best on the east and north sides of buildings where they are shaded from the hot sun. Their leaves can be fairly large and can lose a lot of moisture in the heat of the day. If the soil dries out and the plant can’t keep up with the loss, the leaves will be quick to droop.
Plant rhododendrons in the lee of fences, buildings or other shrubbery to protect them from cold north winds in the winter. They can also be wrapped with burlap or shielded with wooden panels. As a defense mechanism of their own, they roll their leaves up when it gets extremely cold to reduce desiccation.