Royal Botanic Gardens Kew

Shaun Haddock invited the IOS membership on July 19, 2010 to an Oak Open Day at the Royal Botanic Gardens, Kew, near London. The aim was to focus on the Mexican oak species in their vast oak collection.

Seventeen oak enthusiasts from the UK, France, Belgium. Luxembourg, Germany, the Netherlands, and Finland, met at the Main Gate. Summer heat and nearly 12 weeks without real rain had changed the green lawns and other plantings at Kew into a prairie-like landscape. It was hard to imagine that we were in rainy England - it looked far more like California or the steppes of Inner Anatolia to us.

The group was welcomed by Shaun and Tony Kirkham, head of the Living Collections Department. Tony is a well-known author and an authority on woody plants in general.

We gathered at the staff offices, were we met Tony Hall, an expert in alpine plants and an honorary research fellow, and Ray Townsend, the Arboretum Manager, who accompanied us throughout the remainder of the day.

Over tea and coffee we had the opportunity to listen to Tony's very interesting presentation about a somewhat new pest in England - the Oak Processionary Moth (Thaumetopoea processionea). Beginning about five years ago this moth has caused major problems in oak wood management, especially in the surroundings of Kew and nearby parks and gardens.  It was clear soon after the discussion started that this moth has been quite well known for years in Continental Europe. Their original South European range is expanding northward, possibly or partly as a result of climate change.

Here in England the moth is fairly new, known since only 2006, and horticulturalists, environmentalists and foresters are desperately searching for an adequate control. As they mature, the caterpillars form a nest-like bowl, where they are well protected from insecticides delivered as spray. The major problem with Thaumetopoea is not the defoliation of the oaks, but rather the more or less extremely allergic reactions which humans suffer when they approach or touch this caterpillar. The backs of older caterpillars are covered with up to 63,000 pointed defensive bristles containing a toxin similar to that found in the bristles of nettles.  The hairs break off readily, become airborne and can cause epidemic caterpillar dermatitis. 

Tony informed us that in the Kew collections, oaks of the Sections Cerris and Robur are vulnerable; it was found particularly frequently on Quercus cerris, and with somewhat lesser rates of infection on Q. castaneifolia, Q. robur, Q. petraea, Q. x turneri and Q. ilex.  He described several more or less effective--but costly--methods used in oak processionary moth control.  The destruction of 1000 nests cost up to 65,000 British Pounds (equivalent to 105,000 US Dollars) in 2009 at Kew alone.

After the presentation and discussion, the group went directly to the collections. Here Tony and his colleagues described for us the maintenance of the living collection. An effective weed- control with a minimum of herbicides has been developed by Ray, who is called "Round-Up Ray" at Kew. He has developed a schedule for spraying at the end of winter, which at Kew is the end of February.  A concentrate of only 10 ml per liter of a Glyphosate-containing herbicide is sprayed on weeds, and this has proved to be a very effective control while reducing the need for herbicides.

We were all eager, of course, to see the oaks, to which we now turned.  One of the first oaks we encountered was a surprise - a giant Quercus crassifolia (fig. 1) from Mexico, planted in 1934, and the British Champion for that species. 

We all gathered easily under the canopy of this majestic and rare oak. Some of us had seen Q. crassifolia in Mexico last year, but this specimen in far off England is possibly the largest in cultivation outside of Mexico. Another Mexican species which attracted our attention was Quercus insignis, the oak species with the largest acorns in the whole genus.  Q. insignis has been quite hardy at Kew, showing no significant frost damage from temperatures down to - 7 °C.   In recent colder years it has defoliated but nevertheless survived even though unprotected (see remarks about this species, and photos, in Winter 2010 number of Oak News and Notes ).

The oak collection at Kew is amazing: it includes more than 1400 oaks from more than 300 taxa.   Many are unique in Britain or Europe, and many of them are original introductions and thus of enormous scientific and horticultural value. 

A huge Q. variabilis was propagated in 1909 from seed sent from the Arnold Arboretum after being collected by Ernest Wilson in China (fig. 3). 

Another surprise was the rare and only endemic African oak species, Q. afares, which was planted here as early as 1869.

Still another giant turned out to be a familiar species: Quercus castaneifolia, planted in 1939; it is quite comparable to specimens of the same species growing in the forests of northern Iran. It is hard to believe that this specimen is just 70 years old (fig. 4). Another interesting encounter was Quercus rubra harboring the Yellow Mistletoe, Loranthus europaeus.  This is a rare semi-parasitic plant similar to mistletoe but with yellow fruit; it grows  mainly on oaks and chestnuts.  It is believed to be the one and only Loranthus specimen growing in the British Isles.

In addition to oaks, we saw a great many interesting trees, shrubs, perennials and…weeds., including Cathaya argyrophylla, a rare conifer from Szechwan discovered in 1955 and extremely rare in collections, and the shrubby West-Chinese Sambucus adnata, with it's typically winged rachis. Tony also introduced us to the art of “bark sniffing!”  One of the differences between Pinus ponderosa and Pinus jeffreyi is that the bark of Pinus jeffreyi smells like caramel candies.

Although we had spent the whole day in the midst of this tremendous collection of trees, we nevertheless finished our tour with a touch of regret…we had really only scratched the surface!

Our sincere thanks to Shaun, who facilitated the gathering, but most especially to Tony Kirkham for his warm welcome and the conduct of the tour, together with Tony Hall and Ray Townsend for their ideas and immense knowledge, not only about oaks.