As with so many fine clematis, my first glimpse of C. 'Chalcedony' was in Barry Fretwell's beautiful first book, Clematis (Capability's Books, 1989). He reported that the clematis was named after the semi-precious gemstone, pronounced either kal-said'-æ-knee, or to rhyme with "pony", kal-said'-ony. The gemstone, a silica oxide quartz, is not as changeable or fiery as an opal, but it does appear in a pale array of opaque tints from pinky-lavender through ice blue, varying with the light it's seen in as much as with mineral gradations within each stone. It is also a fairly hard gem, scoring a 7 on the Mohs scale. The clematis is much the same: luminous and sturdy. Every good thing Fretwell claims for this plant is true. It was bred by Strachan in 1984, producing an always-double hybrid of C. 'Vyvyan Pennell' × C. 'Marie Boisselot'. Given the ability of C. 'Vyvyan Pennell' to change color over the life of the spring double flowers—mauve and ending much more blue—the mutable nature of each C. 'Chalcedony' blossom comes as no surprise. The serene milkiness of C. 'Marie Boisselot' is echoed in the softly opaque base color of the gemstone and the flower.
One sees where the flower gets its name.
Spring blossom at the Rogerson Clematis Garden Bed 16, the Founder's Garden.
The gemstone Chalcedony as displayed at the
Smithsonian Museum of Natural History.
C. 'Chalcedony' has the additional advantage of producing flowers of extremely durable substance. The sepals are thick and slightly wavy, with the C. 'Duchess of Edinburgh'-like tendency to show green flares in those outermost. The sepals are all within a half-centimeter of being the same length, especially in the spring crop, making a generous pompon. The result is perhaps the most reliably rose-like profile of any of the always-double large-flowered clematis. In warm weather it takes well over a week for the flowers to expand, and once fully open they resist rain or hail with admirable fortitude not seen in, for instance, C. 'Belle of Woking', which is closest in color. C. 'Chalcedony' also has stronger-neck (both pedicels and peduncles) than knowing C. 'Vyvyan Pennell' was a parent would lead one to expect, with none of that cultivar's blowsiness. C. 'Chalcedony' is not as wildly vigorous as one would hope for in a vine with this quality of blossom, but it can be long-lived. And the fact that it doesn't get huge (expect 2-2.5 meters in height, more or less) makes it a good candidate for growing in containers with old-fashioned shallow-rooted annuals at its feet, perhaps lobelia and sweet alyssum (now Lobularia maritima). It also partners well with some of the shorter sweet peas in the softer color harmonies. Because it will always flower double, don't be afraid to give C. 'Chalcedony' a hard-prune to 0.3m every other year or so to get good rejuvenation from the crown. In the Pacific Northwest we see the first wave of flowers from May into June, followed by deadheading and a restorative handful "rose and flower" fertilizer to produce more flowers in September. C. 'Chalcedony' will often try to produce flowers into November, but the density of the sepal count means these will rarely open well, if at all.
The cooler the spring nights, the longer it takes
the buds to form, and the more tepals result.
Tanglevine Cottage, Sellwood Oregon, 2006.
The color is changeable, and summer blossoms
are sometimes not quite as full.
Szczepan and Barbara Marczyñski Garden, Poland.
As one would expect with such a fine, sturdy double, C. 'Chalcedony' has become a plant of influence. Fretwell used it as a parent for his magnificent C. 'Kiri te Kanawa'. It would not surprise me to learn it is also the parent of some of the newest doubles from Japan, England, and Holland whose parentage has been withheld from publication. Were I into that form of sport, I would try crossing C. 'Chalcedony' with C. 'Duchess of Sutherland', to finally create a double red large-flowered of outstanding reliability.
An early November effort results in paler flowers and fewer tepals.