JARS v57n4 - Species Profile: Rhododendron fastigiatum Franch. 1886 and Rhododendron impeditum Balf.f. & W.W. Sm. 1916

Species Profile: Rhododendron fastigiatum Franch. 1886 and Rhododendron impeditum Balf.f. & W.W. Sm. 1916
Steve Hootman
Seattle, Washington

First published in the Rhododendron Species Foundation newsletter and since revised for publication in this issue of the Journal.

These two species represent the ultimate in dwarf "blue-flowered" rhododendrons, easily grown, hardy, attractive and floriferous. I will discuss them together as they are very closely related, visually and functionally interchangeable in the garden, and often confused in the nursery trade.
Rhododendron fastigiatum was first collected by western botanists on the famous Cang Shan (shan = mountains) in western Yunnan, China, where it was found by the Abbe Delavay in 1883. It was subsequently introduced into cultivation by George Forrest in 1906. Rhododendron impeditum was first collected and introduced to the west by Forrest who collected the plant in the high mountains of western Yunnan near Lijiang. We now know that the range of the first species extends throughout the high mountains of northern Yunnan where it occurs in open rocky meadows and on screes and cliffs from 10,500 to 16,000 feet (3150 m to 4800 m). The latter species is found from northern Yunnan into southwestern Sichuan Province in similar habitats from 9,000 to 16,000 feet (2,700 m to 4800 m). I have vivid memories from 1995 of R. fastigiatum growing en masse among scattered specimens of R. sphaeroblastum var. wumengense . This scene appeared as we climbed to 13,000 feet (3900 m), emerging from forests of Rhododendron bureavii and lacteum onto the windswept rocky meadows of the flat-topped Wumeng Shan in northeastern Yunnan. We had no idea of the flower quality in this particular population as it was late October, but were impressed with the extremely attractive foliage. Plants raised from this seed have proven to be equally fine in blossom, with stunning large, deep blue-violet flowers.

R. impeditum in its native habitat at 11,800 ft. on
the Cang Shan, W Yunnan, China.
R. impeditum in its native habitat at 11,800 ft
on the Cang Shan, W Yunnan, China.
Photo by Steve Hootman

Rhododendron fastigiatum and R. impeditum are members of Subsection Lapponica , a large and taxonomically confusing group of primarily dwarf, lepidote (scaly-leaved) species. As mentioned earlier in the text, they are very closely related to each other and can often be quite difficult to distinguish from one another. This is especially true in the wild as the two species often merge into each other when they occur together. The confusion between the two is aggravated by the fact that virtually all of the "impeditum" (and "litangense" see below) sold in nurseries is actually fastigiatum . This is easily discernable by the fact that these plants typically have very attractive glaucous blue-green leaves with very pale scales on the undersides - good indicators for fastigiatum . True impeditum (incl. litangense) will have shiny deep green leaves with brown scales on the undersides. In virtually all other characteristics the two species are quite similar.

R. fastigiatum SBEC# at Glendoick, Scotland
A planting of R. fastigiatum SBEC# at Glendoick, Scotland.
Photo by Steve Hootman

In most cultivated forms, these two species grow as dwarf and mounding or even prostrate, evergreen shrubs up to 2 feet in height (they are quite variable in the wild however). In full sun they will remain quite compact, covering themselves with tiny scaly leaves and masses of colorful flowers each spring. Rhododendron impeditum Litangense Group (formerly R. litangense ) is a prominent exception to this, forming an upright plant to 3 feet or more. The leaves of these two species are typical of the subsection with prominent scales covering both the upper and lower surfaces. The leaves are generally around 1/2 inch in length and elliptic to oblong or ovate in shape. The small, widely funnel-shaped flowers (mid-spring) are borne in clusters of up to five. They range in color from pale purple to violet or deep blue-purple, rose-purple and (rarely) white.
These are superb dwarf species for the rock garden or as choice specimens with other smallish plants. The striking colors and sheer abundance of their flowers every spring are more than enough to strike lust in the hearts of even non-rhodoholics. Both of these species are easily grown in virtually all of the temperate "rhododendron-growing" regions of the world, excluding those areas with extremely hot and humid summers. It is easy to extrapolate from the native habitats of these two species that they will perform best in the garden if provided with exceptional drainage and plenty of light. Supplemental irrigation during dry spells in the growing season is advised (as it is for almost all rhodies!). Little maintenance is required as both pruning and dead-heading are unnecessary although a shearing back of several inches every few years can be beneficial if the plants become leggy (this should not be necessary if planted in sufficient light). At the RSBG, R. fastigiatum and R. impeditum are cultivated and displayed in the Alpine Garden where the plants grow together into large groupings for a natural and aesthetically pleasing effect. In the Pacific Northwest they flourish in full sun (with irrigation!) but will perform better with light afternoon shade in those areas with warmer summer temperatures. Both species are probably hardy to at least -15°F (-26°C).
Many garden-worthy hybrids have been raised from crosses utilizing either of these two species including 'Purple Gem' and 'Ramapo' ( fastigiatum ) and 'Azurika', 'Saint Merryn' and 'Mayfair' ( impeditum ). A form of R. fastigiatum grown by G. Reuthe of Kent, England, was awarded the Award of Merit from the Royal Horticultural Society in 1914. A form of R. impeditum grown from Rock #11469 by Sunningdale Nurseries of Surrey, England, was awarded the same honor in 1944.

Accessions in the Collection of the RSF
R. fastigiatum

1973/101 J. Henny: Brydon. Deep purple flowers.
1981/140 F#5847: RBGE. Slow-growing dwarf & prostrate form w/ lavender flowers.
1984/110 †SBEC#0804: Glendoick. Selected form.
1994/398 'Blue Steel', Glendoick. Very glaucous foliage.
NOTE: We also have numerous plants grown from my collection of seed on the Wumeng Shan (SEH#214) accessioned and growing in the Alpine Garden for evaluation.

R. impeditum
1976/379 (Litangense Group) Windsor. Purple flowers.
1977/617 (Litangense Group) King & Paton. Rose-purple flowers.
1984/087 King & Paton. Purple flowers.
1984/088 (Litangense Group) King & Paton. Purple flowers.
1994/399 F#20454: Glendoick. Dwarf form.
NOTE: We also have numerous small plants grown from my collection of seed made on the Cang Shan in 1997 (CCHH#8253). These will be accessioned and planted out for evaluation in the fall of 2003.

References Consulted
Cox, P.A. & Cox, K.N.E. 1997. The Encyclopedia of Rhododendron Species . Glendoick Publishing. Perth, Scotland.
Davidian, H.H. 1982. The Rhododendron Species. Vol. I . Timber Press. Portland, OR.
Hootman, S.E. 1995-1997. Unpublished field notes.
Rhododendron Species Foundation: Records 1964 to present.

Steve Hootman, a member of the Seattle Chapter, is Co-Executive Director/Curator of the Rhododendron Species Botanical Garden in Federal Way, Washington.