In addition to the big shrubs we know as rhododendrons with their sturdy presence and voluptuous flowers, there are other less well-known rhododendrons in the genus.
These can extend the performance of this group of plants beyond spring to all four seasons of the year.
More than a thousand species and even more hybrids fall within the genus Rhododendron, bringing with them a multitude of characteristics for use in garden design all year.
The good news is that if you have one of those big rhododendron shrubs in your garden, the same conditions that support it will likely support many of the other lesser-known ones.
Fortunately for us in Skagit County, most of the growing requirements for rhododendrons occur here naturally.
According to a world expert, “the perfect site for Rhododendrons is characterized by deep, well-draining sandy loam soil; sheltered or open woodland; rainfall (of) 50 inches during the summer; and a minimum temperature of 14° F” (Cox 1993).
A soil acidity of 5.5 to 6.5 pH is needed. Our minimum temperature and our natural soil pH tend to fall within the requirements. Also, rhododendrons tend to do well in our moderate summer temperatures. Where Skagit County obviously falls short is low rainfall in the summer.
Like any home garden, the Discovery Garden, located at the WSU Mount Vernon Northwestern Research & Extension Center, turned out to be less than perfect.
Then soil was added to create beds of well-draining soil, and drip irrigation was installed to keep the soil moist.
After a soil test, fertilizer was added before and after flowering, and the pH was lowered by adding iron sulfate. Once these basic requirements were met, the rhododendrons graciously reciprocated with healthy growth and ample flowers — a complementary blend of climate and plant, with only a bit of human intervention.
Pests and diseases
Rhododendrons are generally free of pests and disease if grown in the conditions described above. The two pests most likely to cause problems are the root weevil and lace bug.
The small, dark root weevil feeds on leaves at night, leaving notches at the edge. The best way to destroy the weevils is to sneak out at night and place a blanket or pizza box under the shrub and shake until they fall and can be destroyed.
The lace bug is a tiny whitish-tan insect with lacy-looking wings. Its leaf sucking causes yellow stippling on the upper surface of the leaf and tar-like spots on the lower surface.
The best defense is to direct a strong stream of water at the underside of the leaves to dislodge the insects. Most lace bug damage takes place in early to mid-July.
Consider fall — not the usual season for rhododendrons to stand out in the garden. More likely, a nice Japanese maple with lacy red leaves gets the attention.
But there are rhododendrons that can compete: the deciduous azaleas (yes, azaleas are a type of rhododendron).
They bloom in the spring, but many follow up with leaves as eye-catching in color as that Japanese maple.
Colors range from red, bronze and maroon to orange and yellow. Some deciduous azaleas with outstanding fall leaf color are “Cecile,” “Rosy Lights,” “White Lights” and the species R. luteum.
Note that fall is best time to plant rhododendrons of any type.
Winter is the season when the commonly grown big evergreen rhododendrons can save the garden from the Northwest’s interminable grayness.
Here are plants you can count on to loyally maintain some substance and color through our dark days.
One example would be “Taurus,” a hefty specimen whose large size and dark green leaves keep a garden looking alive throughout the winter.
The ultimate in beautiful winter foliage, however, is the species R. bureavii (common name: Bureau Rhododendron) that, unfortunately, is not suited for an open garden, because its leaves will “burn” in the heat of summer and should be planted where there is some shade.
Sited correctly, this rhododendron grows in a rounded form covered with leaves that are shiny dark-green on the upper surface and rust-red on the lower surface. In a breeze, the lower surfaces show themselves like little red flags.
Spring brings a deluge of flower color beginning in March and running through May. Because the number of varieties of rhododendrons is so large, a rhododendron can usually be found to satisfy the gardener’s needs and tastes. A good place to find that rhododendron is to search the American Rhododendron Society website: rhododendron.org. Here you can search for flower color, plant size, bloom time and more.
By summer, rhododendrons have mostly finished blooming and now present their new leaves, a show in itself.
There is one rhododendron — a species from China — that blooms as late as July, R. auriculatum. It can grow to about 6 feet tall, with white, fragrant flowers.
A hybrid, “Polar Bear,” which has this species in its parentage and is similar to it, also blooms late. The hybrid may be easier to find in nurseries than the species from China.
All of these types of rhododendrons can be found at the WSU Discovery Garden in Mount Vernon.