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Fall a good time to acidify soil for rhodies

This Pacific rhododendron (R. macrophylla) leaf has chew marks along the leaf margins, characteristic of adult root weevils. Yellowed leaves may indicate the soil needs more acidity (lower pH). [Photo by Rhonda Nowak]

“There have been so many times since I have become a gardener that I have brought my family to the brink of bankruptcy just to have growing in my garden some treasure (to me) or another, something I felt I could not live happily in the garden without ...”

– Jamaica Kincaid, “My Garden [Book],” 1999

I can certainly relate to Jamaica Kincaid’s admission of financial irresponsibility when it comes to purchasing plants and garden accessories. For several years, I was guilty of overspending on plants that “I felt I could not live happily in the garden without” for my Shakespeare Garden in Central Point (after all, the Bard mentioned more than 200 different plants in his plays and poetry).

Long ago, Jerry and I agreed that I would pay for my garden treasures (to me), and I’m sure this wise decision is no small part of why my family has not been brought to the brink of bankruptcy, and why Jerry and I are still married after 34 years. (Kincaid and her husband divorced in 2002.)

Kincaid confessed that she has a particular weakness for rhododendrons with felted leaves, and she named some of her favorites: R. ‘Jane Grant’, R. ‘Anna H. Hall’ and R. smirnowii. R. ‘Jane Grant’ and R. ‘Anna H. Hall’ are descendants of a rhododendron called R. yakushimanum, which is uniquely native to the island of Yakushima in Japan, whereas R. smirnowii originated in Turkey.

R. ‘Jane Grant’ was bred at the famous White Flower Farm plant nursery in Connecticut and named after one of its co-founders. Kincaid noted this rhododendron hybrid with its beautiful, pink, ruffled flowers is particularly expensive and can only be purchased from White Flower Farm. I don’t know how much R. ‘Jane Grant’ set Kincaid back in the 1990s, but I recently checked out the White Flower Farm website and learned that a one-gallon potted R. ‘Jane Grant’ now costs $40 (not including shipping).

This price didn’t seem awfully exorbitant to me, and I might have been tempted to buy one, but all the R. ‘Jane Grants’ were sold out for the season.

The reason I might have been tempted to have an R. ‘Jane Grant’ in my garden is that before I read about them in “My Garden [Book]” I did not know that some rhododendrons, which are usually evergreens, have leaves covered in felt, called indumentum (from the Latin word meaning a garment or covering). Growing up in the South, I was familiar with Southern magnolias (M. grandiflora), which have cinnamon-colored indumentum on the underside of their leaves, but I thought this was a characteristic unique to some magnolias.

However, I’ve learned that not only do some rhododendrons also have indumentum but thousands of other plant species boast felted leaves (and other parts), as well. There are several different types of indumenta, some of which are: pubescent, hirsute, pilose, lanate, villous, tomentose, stellate, and scabrous. Forms of these terms often show up as species names in binomial nomenclatures, such as in Gossypium hirsutum (upland cotton), Kalanchoe tomentosa (panda plant), Phlox pilosa (downy phlox) and Digitalis lanata (woolly foxglove).

Plants have evolved leaf, stem and flower bud indumenta to provide protection from heat and predators, absorb moisture, control transpiration, anchor climbing plants and trap prey. Many insects, by the way, also have developed indumentum; examples are honeybees with hairy legs and bumblebees with hairy bodies to capture and transport pollen.

I was interested to learn that felted rhodies are especially prized by gardeners who know better than others how exceptional felted rhodies are. Less savvy gardeners will wonder what disease their rhododendron has contracted. Kincaid’s love of these highly valued plants led her to embark on a seed-hunting expedition across China, where she found out that rhododendrons with indumentum on the underside of their leaves are not rare; there are forests filled with them. (I wonder if this made her value the R ‘Jane Grant’ in her Vermont garden more or less?)

I found a thorough discussion of different types of indumentum on rhododendrons and azaleas in an online article called “Rhododendron Foliage: The Other 50 Weeks of the Year” by Donald Hyatt of the Potomac Valley chapter of the American Rhododendron Society (arspvc.org).

I was drawn to Kincaid’s thoughts about rhododendrons because I have two 20-feet-tall Pacific rhododendrons (R. macrophyllum) in Bandon. This broadleaf evergreen species is native to Oregon and common in mixed evergreen forests, although I’ve noticed that many are strangled out by more aggressive understory shrubs. On our land, the two rhodies have grown lanky and tall as they’ve tried to compete with huckleberry and gorse.

R. macrophyllum (literally “large-leaved” rhododendrons) do not have felted leaves, but I value them, nonetheless, because they are native plants that support native wildlife, particularly bumblebees, and because their pink flowers in spring stand out vividly from all the conifers. They are uniquely adapted to our wet winters and dry summers; mature plants do not need supplemental irrigation.

Once we cleared out the huckleberry and gorse crowding and overshadowing them, my Pacific rhododendrons started sprouting more leaves and producing more flowers; however, they are not quite “out of the woods” yet.

Some yellow leaves on the shrubs indicate the soil may be too alkaline, as rhododendrons like “sour” soils with a pH between 4.5 to 5.5. In fact, it’s a good idea to assess the overall health of your trees and shrubs at the end of summer in order to identify problems and amend the soil in the fall.

Oregon State University Extension Service recommends acidifying soils during colder, wetter times of the year by digging four 1-foot-deep holes just inside the shrub’s drip line and dividing one-half cup of elemental sulfur or sulfuric acid equally among the holes. Another way to acidify the soil is by mulching around the shrub with wood chips or pine needles.

Perhaps the biggest threat to my R. macrophyllum is root weevils, the adults of which are currently chewing large holes on the leaf edges and the larvae of which feed on the roots. Nematodes can be applied to the soil next summer to prey on the larvae, but until then I will need to hunt down the small, snouted beetles at night with a flashlight and find pleasure in squishing them between my fingers.

That’s one reason I don’t mind that my R. macrophyllum does not have indumentum on its leaves to protect itself: Otherwise, I wouldn’t have the opportunity to take on the role of eager weevil assassin.

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. See literarygardener.com or email her at Rnowak39@gmail.com.