Clematis of the Month for March 2017
described by Linda Beutler
Although native to just a scant four western counties in the state of Virginia, Clematis addisonii is a surprisingly adaptable garden plant. One might assume, since it grows on older, broken-down shale barrens (more dolomite than shale) and largely in partial shade, that without gardening near a handy wall of compressed undersea silt over a dolomaceous substrate, one could not hope to make it happy. But it transfers cheerfully to well-amended garden soil producing a charming and long-blooming addition to the sunny perennial border if its feet are shaded. The beautiful specimens seen in Dublin gardens during the I.Cl.S. Ireland trip in 2006 will attest to this.
During the Virginia extension to the 2014 I.Cl.S. Conference in the Delaware Valley of the United States, 15 intrepid attendees were led to Clematis addisonii in the wild by local naturalist and renowned garden author C. Colston Burrell (Cole). We found it obligingly in bloom and not uncommon in its particular habitat. The occasion underscored how much gardeners can learn from seeing desired species growing in situ. My return to Oregon saw all of the specimens in the Rogerson Clematis Garden moved to the shady end of the rockery, pieces of shale from road maintenance piles tucked in around their roots, and an annual side dressing of crushed oyster shell to raise the pH.
(Note: Cole would not allow us to take shale from the formations, but shale crumbles and flakes off into roadways and is pushed into great berms by devices similar to snowplows. He did allow us to take shale from these “unnatural” formations. I returned home with five pounds of the stuff and it was apportioned to those clematis who seemed most homesick, with excellent results.)
C. addisonii in horizontal shale 
C. addisonii lolling over rocks
Clematis addisonii is an herbaceous perennial growing not more than a meter in length, and rather lax in its habit. In gardens it is wise to grow it in an open surrounding support to keep it upright. In the wild it lolls over surrounding rocks. The foliage is broadly ovate and bright to blue-green; lower leaves sometimes appear almost perfoliate with little or no petiole, and a simple outline. As the leaves progress up the stem they may be lobed, and the 2-4 cm petioles form the leaflets into compound leaves with just 2-3 small leaflets. In the wild the succulent-looking foliage is often marred by bites and nibbles. In the garden, I slug bait C. addisonii with religious devotion.
The flowers are the star attraction, solitary gems located at stem ends exclusively, with little branching. The nodding urns are 1-2.5 cm long, and the color is strongest at the receptacle, ranging from pale pink to plummy cerise, but usually a medium rose shade, all color forms gradually lighten to cream at the exterior tips. The four tepals appear thick at the margins, which are cream to creamy green.
The seeds are large and the seed tails are glossy golden-brown. It comes easily from seed if the pericarp is removed after soaking the seed in plain water for five days, and flowers the second year. This may seem sacrilegious to avid seed collectors, but plain old gardeners will appreciate that if deadheading after the first flowering for 3-4 weeks in June, Clematis addisonii will rebloom. If one grows several plants, one can have it both ways.
This species was first described by Britton in 1890, and named for Mr. Addison Brown, President of the Torrey Botanical Club of New York, who was along on the first expedition to gather herbarium specimens and write-up the species. They found the type specimens slightly upland from the banks of the Roanoke River, in Roanoke County, Virginia. The native range includes the three adjoining counties through which the Elbrook Formation of limestone and shale extends. It is hardy to USDA zone 7, and at the Rogerson Clematis Garden winters down to -13C have proved no challenge for it.
It is somewhat mystifying that Clematis addisonii has not been used more in plant breeding. One imagines it would give interesting results to cross the showy Clematis texensis hybrids with C. addisonii in an attempt to create short, leafy non-climbing versions of favorites such as C. ‘Princess Diana’ or C. ‘Gravetye Beauty’. Perhaps someone interested in this form of gambling will read this suggestion and be inspired. In the meantime, Clematis addisonii will continue its dominion as one of my top ten clematis species for the garden.
Linda Beutler