Friday, 14 June 2013

An East Coast Favourite at Westonbirt - The Tulip Tree

For some visitors to Westonbirt, the tulip tree (Liriodendron tulipifera) is among the first trees they come across, with a maturing specimen growing on the borders of the current main car park. Others may notice a young specimen growing close to where a large Norway maple (Acer platanoides) once stood near Waste Gate at the entrance to Silk Wood, whilst those who make it as far as Jackson Avenue in the Old Arboretum will see the examples that make up part of the unique species combination of this avenue (the others are lime Tilia x europaea and cedar Cedrus atlantica, flanked by beech Fagus sylvatica and yew Taxus baccata). Numerous specimens are dotted around the collection, including a number of recent plantings from wild sourced seed.  

Liriodendron tulipifera is a member of the primitive Magnoliaceae and has just one other member of the genus for company, L. chinense (from China, as the name suggests!). The latter is more of a rarity at Westonbirt, with rabbits (who seem to be particularly fond of species in the Magnoliaceae) playing no small part in the demise of our mature specimens in recent years. We currently have just one specimen, located not far from Loop Walk in the Old Arboretum, but rest assured, new plants are on the way!

Introduced at some point in the mid to late 17th century, there are a number of venerable specimens of L. tuipifera around the country, but none have quite achieved the stature of those in the surviving old growth forests in its native range on the east coast of the United States, which I was privileged to see back in 2010 (courtesy of the Friends Of Westonbirt Arboretum). There, they attain heights of 120 feet and equally impressive girth (see photo below!) and are every bit as impressive as they sound.

A monumental wild specimen of L. tulipifera

Both species of Liriodendron are notable for their unusual leaf shape, which are four lobed with a broadly notched apex, appearing almost to have been cut. The leaves of L. chinense are ‘narrower waisted’ than those of L. tulipifera, offering a useful characteristic when distinguishing between the two. Both are also known for their tulip-like flowers, hence the specific epithet tulipifera of the American species. Our L. tulipifera aren't in flower just yet this year, but should be soon, so time keep a close eye on them! 

Slightly dull in colour, the flowers of the two species differ in size, with those of L. chinense being smaller, whilst also lacking the orange pigmentation present in L. tulipifera (see photo below). Our sole L. chinense is still a bit on the young side to be flowering quite yet. As well as flower and leaf characteristics, the Chinese tree also tends to attain a smaller size than its American counterpart, at least in cultivation. 

L.tulipifera flower

There is also a hybrid of the two found in cultivation, of which we have a fine example on Broad Drive in Silk Wood. Planted in 2002, it is intermediate in leaf characteristics of both parents and though again yet to reach the age where we would expect to see flowers, it is certainly showing plenty of hybrid vigour, so we shall eagerly watch this space!!

Liriodendron tulipifera x chinense leaf

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