The Genus Betula

Betula ermanii 'Grayswood Hill' in the Yorkshire Arboretum, October

Earlier this year, the  monograph The Genus Betula  by Kenneth Ashburner and Hugh McAllister appeared from Kew Publishing. With birches now looking good in the beauty of their bark, and recently in the gold of their autumn leaves, this seems a good time to post the full text of a review I wrote for Garden Design Journal, which appeared (slightly abridged) in its October 2013 issue.

‘Long-awaited’ is an apt expression for this book, which has not only been long-delayed, but long-needed as a contemporary review of the whole genus Betula from both botanical and horticultural perspectives.

Curtis’s Botanical Magazine Monographs, published by Kew, are a series of finely-produced, authoritative monographs that continue the tradition of the oldest scientific journal in the world of blending botanical accuracy with horticultural information and fine illustration. The Genus Betula is no exception, being a chunky book full of good photographs and enhanced by many excellent paintings by Josephine Hague and line drawings by Andrew Brown. The authors, Kenneth Ashburner and Hugh McAllister, worked together on Betula for many years before Kenneth’s death in 2010, after which Hugh completed the book, but it is very definitely a joint production bringing together many decades-worth of study, including travel by both authors to study birches in many remote places. One of the results of this is the inspiring arboretum at Stone Lane Gardens, Ashburner’s former home in Devon, where groves of birch saplings from the same collection were planted together. This gives a unique opportunity to study natural variation, but is also very beautiful and well worth visiting. 

Betula medwedewii: a rare species from the Caucasus and eastern Turkey (Yorkshire Arboretum, October).

The diversity of Betula is perhaps surprising. Occurring throughout the cooler – and often coldest – parts of the northern hemisphere, 45 species are recognised here, a mixture of the familiar elegant trees and a number of shrubby species. Although often attractive in the wild, with nice autumn colour, most of the small species are of ‘botanical interest only’, seldom performing well in gardens. Of the taller trees it is clear that there are many possibilities for new introductions bringing new characters to the garden, such as the Vietnamese B. insignis var. fansipanensis with purple-flushed new growth, and for selecting superior cultivars. This book should also serve to remind gardeners of the existence of many fine species that are seldom planted, such as the magnificent B. grossa for which there are no current suppliers in the UK. In these times when tree diseases are so alarmingly prevalent we need as much diversity as possible.

 B. utilis subsp. jacquemontii from the western Himalaya usually, but not always, has good white bark, and numerous selections are valued for this quality. This specimen is at Arboretum Kalmthout (Feb 2013)

The Chinese B. utilis subsp. albosinensis typically has richly coloured bark:, but this and other features form a continuum with the broad species B. utilis. This one from Sichuan is at the reddest end of the spectrum (Yorkshire Arboretum, 2012)

The Genus Betula is principally a botanical book, discussing birches from a taxonomic and biological view. The authors’ reasoning for their opinions is clearly stated and I find it convincing. There are two major nomenclatural changes that will raise a few eyebrows, but from the evidence presented seem sensible: the inclusion of B. albosinensis in B. utilis, as subsp. albosinensis, and the inclusion (also as subspecies) in B. pendula of the Chinese B. szechuanica and the Far Eastern Asian -Alaskan B. mandschurica. In these cases they are taken as representing the extremes in a continuum of variation. For the horticulturist there is a general chapter of the cultivation of birches, reminding us particularly of their shallow-rooted nature and the great desirability of planting them young, and each species has a note on its particular merits or demerits in cultivation. A chapter on birch cultivars by Paul Bartlett, who runs Stone Lane Gardens, is useful but I should have liked to see a fuller description of each, and they are not illustrated, which is unfortunate. All in all, however, this is a useful book and will be a standard reference for years to come.

A less extreme colour form of B. utilis subsp. albosinensis, Purdom 752 from western Gansu, long-known in cultivation as B. albosinensis var. septentrionalis, a name not upheld in The Genus Betula (Yorkshire Arboretum 2012).