Tuesday, 4 June 2013

A Rare Birch for Westonbirt

Returning from my recent visit to Stone Lane Gardens I was accompanied by a specimen of a rather rare birch – Betula ashburneri. The plant was a gift from Paul Bartlett, the Garden Manager at Stone Lane, for the collection at Westonbirt.

As described in the new Betula monograph, the species was ‘discovered’ in 1997 when on an expedition in south east Tibet, morphological differences were observed between the B. utilis at lower altitudes and those growing around the tree-line, by plant hunters Hugh McAllister and Keith Rushforth. Whereas lower altitude B. utilis tend to be single stemmed trees, those observed higher up were multi-stemmed bushes. Along with this difference in form, these high altitude specimens had smaller leaves with fewer veins than B. utilis as well as small, erect fruiting catkins. Whilst these plants could have been just a peculiar form of B.utilis, subsequent chromosome counts established them as genetically distinct. Examples of B. ashburneri were also found in cultivation from an earlier collection of B. utilis, along with a possible hybrid of the two. The species is named after the late Kenneth Ashburner, creator of Stone Lane Gardens and co-author of the aforementioned (and excellent!) monograph.

A number of young examples of B. ashburneri growing  in a grazed area at Stone Lane,
complete with  alder catkins from a neighbouring tree, if you can spot them!

The specimen I acquired has been potted on since its arrival at the propagation unit here at Westonbirt and currently resides in an air-pot, as do virtually all other plants on our nursery. The somewhat knobbly design of these pots (see image below!), with air holes and a perforated base, discourages root spiralling and encourages fibrous growth as roots grow outwards, finding their way to these air holes, where they then dieback (or are ‘air-pruned’), stimulating further growth behind the tip. A mass of roots results, with no need for any ‘teasing out’ during  planting, whilst we also find our trees establish quicker. Can’t wait to see this one out in the collection!

B. ashburneri, in an air-pot on the nursery at Westonbirt


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