Archive for the ‘Tree Team and Propagation’ Category

Tree of the month: August 2015

Thursday, July 30th, 2015

Hybrid wingnut
What is tree of the month?
Pterocarya x rehderiana (hybrid wingnut)

Why is it tree of the month?
This vigorous grower named after the great Taxonomist Alfred Rehder (1863-1949) is a hybrid between the Caucasian (P. fraxinifolia) and Chinese wingnuts (P. stenoptera). The fruit, hanging in racemes, is particularly showy at this time of year.

Where can I find it?
Our specimen at Pool Gate is quite probably one of the original hybrids, with offspring from this growing strong on Sir Georges Walk.

Dan Crowley, Dendrologist

Old plants, new tricks

Monday, July 20th, 2015

Westonbirt Arboretum’s Tree Team is working with The Duchy College to grow new plants from some of the collection’s oldest and rarest rhododendrons.

Last month Westonbirt welcomed a special delivery of 19 young rhododendrons, from four different taxa, ‘micro-propagated’ and grown for the last four years at The Duchy College’s specialist facilities in Cornwall.

Micro-propagation is a method of vegetative plant production undertaken in laboratory-like, sterilised conditions using petri-dishes and Agar gel. Tiny cells taken from the rhododendrons’ flower buds one to three months before expected flowering were used to grow roots and shoots. One great advantage is that a vast number of plants can be raised from a single fragment of plant material.

The technique means that the team can grow new plants from rare hybrids introduced over a century ago by Westonbirt’s founder Robert Holford and his son, Sir George Holford. The Holfords used selective breeding and seeds collected by famous Victorian plant hunters to create the hybrid varieties, some of which are exclusive to Westonbirt’s collection.

The team at Westonbirt is used to creating and caring for young trees and shrubs at its propagation facilities. 1,511 specimens are housed in the glasshouses, polytunnels, shade house and standing down area at any one time and can remain there from two to five years before being planted out into the collection.

Usually, specimens are either grown from seed collected in the wild (such as on the recent collecting trips to the USA and South Korea), or from techniques such as air-layering. Air layering is when small areas of the branch are wrapped with moss and rooting hormones and sealed in black plastic, convincing the plant that it is underground. The roots are then left to grow on the plant until they are strong enough to be potted. Micro-propagation is a technique generally reserved for very old, more difficult, or less vigorously growing plants.

Westonbirt had its collection of rhododendrons professionally surveyed in 2007. Many were identified then as important Victorian hybrids, and so the programme of getting these rare plants propagated began.

“The rhododendrons we are reproducing are very exciting from an historical point of view; they represent one of the most significant periods of horticultural development at Westonbirt,” said Penny Jones, Propagator.

The young rhododendrons will stay in Westonbirt’s propagation unit for around four years until they are ready to be planted out in the collection.

Tree of the month: July 2015

Friday, July 3rd, 2015

Tilia oliveri, image credit Edward Parker

What is the tree of the month?
Tilia oliveri (Oliver’s lime)

Why is it tree of the month?
The beautiful foliage with a pale underside, along with the large flower bract make this species quite distinctive. Soon to flower, this is most definitely a seasonal highlight.

Where can I find it?
The champion grows on Specimen Avenue, whilst there are other mature trees in Acer Glade and on Holford Ride.

Dan Crowley, Dendrologist

Tree of the month: June 2015

Tuesday, June 2nd, 2015

Styrax japonicus snowbell tree credit Gina Mills
What is tree of the month?
Styrax japonicus, also known as the snowbell tree.

Why is it tree of the month?
An ornamental beauty, the white, bell shaped flowers which hang beneath the branches are not to be missed.

Where can I see it?
Specimens of varying ages grow in both the Old Arboretum and Silk Wood. One close to Savill Glade always catches the eye from Main Drive, where it grows alongside the also lovely S. obassia.

Dan Crowley, Dendrologist

Tree of the month: May 2015

Thursday, April 23rd, 2015

Acer amamiense

What is tree of the month?
Acer amamiense

Why is it tree of the month?
The new foliage flushes beautiful shades of purple. It really is a treat. The new growth is also rather hairy!

Where can I see it?
We have six plants. Three in 2050 glade, two around Savill Glade and one at the south end of Palmer Ride.

Dan Crowley, Dendrologist

> Find the locations of these plants using the Interactive Map

Tree of the month: April 2015

Monday, March 30th, 2015

Acer pycnanthum in autumn

What is tree of the month?
Acer pycnanthum – the Japanese red maple

Why is it tree of the month?
A rarity in the wild and less familiar in cultivation than it’s American counterpart Acer rubrum, it is going to flower for the first time here at Westonbirt!!

Where can I see it?
Close to Loop Walk on the north end of Holford Ride.

Dan Crowley, Dendrologist

Stuck between rot and a hard place…

Monday, March 16th, 2015

Deciding when to remove old trees is one of the hardest decisions we have to take here at Westonbirt. All of the arboretum’s trees are regularly inspected for safety reasons and we try to strike the balance between allowing old and valuable trees to remain and visitors’ safety.

As trees reach the end of their lives they tend to go into decline (known as senescence) and as managers we try to care for specimens through this process, for example by gradual crown reduction. However, we can only delay, rather than prevent, the inevitable. Sooner or later we have to say goodbye.

The time has sadly come to fell a magnificent giant redwood (Sequoiadendron giganteum) that has graced Main Drive since the Holford family started their arboretum in the mid-nineteenth century, as its health has been in decline for many years. We have taken the difficult decision to remove it following lots of close monitoring and some recent investigations. Although the decline of this specimen is fairly obvious, it can be tricky to get a clear picture as to what is actually happening inside a tree’s trunk.

The external signs of decay

The external signs of decay

To help us in this instance we recently arranged a specialist test on this tree with the Treetronic system, which uses electric current/voltage to investigate the internal properties of the tree. The result of the measurement is a two-dimensional map of the electrical impedance (EI) of the tree. Importantly, each tree species has a typical impedance (water/moisture) distribution, and to properly analyse the EI results, the operator should have a good working knowledge of how the subject tree species grows and how the water/moisture distribution may vary in different seasons.

As suspected, our results indicated a central column of dysfunction/decay, which corroborates the external characteristics. The resulting diagram for this tree revealed a high degree of decay (red areas), which matches the outer visual evidence. Based on past experience, we can assume that the decay also extends into the root plate, which would adversely affect the tree’s stability.

Graphic showing internal decay in red

Graphic showing internal decay in red

On a brighter note, we do have over 50 other giant redwood specimens throughout the arboretum, and we also hope to collect more wild seed later this year from western USA.

Finally, if a reminder were ever needed that our trees do not live forever and that we do need to manage older specimens in particular, then just a short distance away on Loop Walk you can still see the uprooted remains of a large Nootka cypress (Xanthocyparis nootkatensis).

The uprooted remains of a large Nootka cypress

The uprooted remains of a large Nootka cypress

This tree was being monitored closely due to decay in the base, but unfortunately fell during high winds over the Christmas period before we had chance to remove it.

Mark Ballard, Curator

Tree of the month: March 2015

Friday, February 27th, 2015

Illicium simonsii

What is tree of the month?
Illicium simonsii

Why is it tree of the month?
It is a lovely pyramidal shrub with small, pale yellow, fragrant flowers that are worth seeking out for both their look and scent. It usually flowers towards the end of the month, so keep your eyes peeled!

Where can I see it?
It can be found in both the Old Arboretum (close to Main Drive and Savill Glade (tree number 25.0799)) and in Silk Wood (Sand Earth (31.1424)).

Dan Crowley, Dendrologist, Westonbirt, The National Arboretum

>> Visit the Westonbirt Map to search for trees by name or tree number

Tree of the month: February 2015

Sunday, February 1st, 2015

Picea farreri
What is this month’s tree of the month?
Picea farreri
Farrer’s spruce

Why is it tree of the month?
Graceful form, glaucous (blue-ish!) foliage.

Where can I see it?
Sand Earth in Silk Wood, set slightly back from Willesley Drive.

Dan Crowley, Dendrologist, Westonbirt, The National Arboretum

The Wolfson Tree Management Centre: An Update

Friday, January 30th, 2015

Work is currently underway on what will become the new Wolfson Tree Management Centre. The new facility will provide all that Westonbirt’s expert tree team needs to manage the tree collection.

The foundations and floor slab are now complete for the new machinery store. The contractors have now started work on the drainage and completing the new yard.


This is a photo taken from the edge of the new yard marked out with a timber edge. Drainage channels form a boundary around the new building to protect it from heavy rainfall and to ensure any rainwater runs along the channels and pipes to a soakaway.


This is a photo of the first section of the new yard which has been finished. This area will become the tree team’s new vehicle wash down and fuel fill up point, their own a miniature fuelling station! The waste water and any potential spills of oil or diesel will drain along the new channel, where it is then filtered by a very large oil interceptor tank, see photo below. This tank holds any leaked oil and fuel which we can then remove safely.


The new yard and building floor slab have been created with a very high level of care and attention to detail although the brush finish across the site has been created by using just a brush and some rope!


Sophie Nash, Project Manager