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Yak rhodies hold their own

Native to the mountains of Yakushima Island in Japan,
Native to the mountains of Yakushima Island in Japan, "yak" rhododendrons will tolerate more sun and cold than other rhodies. (Mike Prager / The Spokesman-Review)

The Yak rhododendron craze that swept Britain 60 years ago never made it to the Inland Northwest.

That’s too bad.

Gardeners here have been missing out on one of the most reliable, attractive and well-behaved types of rhododendrons on earth.

Yaks take their name from the island of Yakushima on the southern end of Japan where they grow on rain- and wind-swept mountain slopes, and from where the original species plants were taken for introduction into the British nursery trade in the 1930s.

I learned about them years ago growing up around rhododendron collectors. When I expanded my landscape here, I chose Yaks as an evergreen cover for a sunny front-yard bed. Planting Yaks was partly an experiment to see if they would thrive in the sun, but also because their compact nature fit the layout perfectly.

Over the past six years, they’ve proven themselves to be tough survivors, although a cold blast last winter burned some leaves.

Many of the Yaks, especially those that are closest to the original wild species plants, can handle a lot of sun, but as with any rhododendron or azalea rhododendron, their roots can never be allowed to dry out.

Yaks are special for several reasons. Although they come in a variety of colors, one of the most common forms starts with bright pink buds that open to a porcelain color splotched with yellow centers. The blooms are stacked in what are called full trusses.

They were good looking enough to a win England’s Chelsea Flower Show in 1947, sparking the Yak craze, according to Susan Clark, in an article posted by the Massachusetts chapter of the American Rhododendron Society,

Once the blooms fade, the new foliage emerges with a fuzzy surface that stands out at the tops and sides of the plants. The foliage is attractive in its own right.

The new leaves are covered with a silver felt on top, known as tomentum, and tan deerskin-like fuzz underneath, called the indumentum. The top fuzz wears off in a few months, but the soft underside maintains its curious texture year-round and can be a surprise to people who are unfamiliar with the plants.

The evergreen leaves are big long ovals in shape, and the leaf color runs from olive to deep green.

As a result, Yaks make handsome shrubs year-round.

Rhododendrons as a class include some 850 species and 25,000 cultivars or hybrids, according to the Royal Horticulture Society.

Yaks that are derived from the species plants are generally hardy to minus-25 degrees, with bud hardiness rated at minus-20. Hybrid Yaks should be hardy to minus-15 depending on the cultivar. But always, when buying rhodies or any other plant, check hardiness ratings.

In her article, Clark says that yakushimanum rhododendrons are now considered a subspecies of R. degronianum, a more widely dispersed Japanese rhododendron when compared with those from the island of Yakushima.

To grow rhododendrons and azaleas in the Inland Northwest requires a couple of soil additions. Their beds should be beefed up with compost, peat moss or similar organic matter to hold moisture and reduce soil pH since rhodies prefer acidic soil. (Inland Northwest soil is a bit on the alkaline side.)

Granular sulfur, iron sulfate or aluminum sulfate are among the acidifiers that can be mixed into the soil at planting. Continued applications are generally needed as the years wear on. If you notice yellowing of the leaves, add more acidifier to the soil. The shrubs need it to take up nitrogen for growth and good leaf color. Also, never let the roots touch concrete. Cut the roots with a shovel – they are only a few inches deep – along foundations, sidewalks or footings and remove any concrete chunks that might have been left from construction.

Make the planting area wide enough to accommodate growth.

(Light applications of soil acidifiers can benefit many types of plants, including tomatoes. That is partly because the region’s main water supply has a rather high pH. Aluminum sulfate is not for edible crops.)

Some rhododendron growers trim back old seed heads after they bloom to make the plants look tidier and grow better, but it is not necessary.

In choosing locations for rhododendrons, seek spots with morning sun or filtered high shade that hold their soil moisture. Avoid hot southern or western exposures. Shady north walls will work, too, but rhodies do best with more light. And this is where Yaks shine: They can handle more sun than other varieties.