Street tree remix - alternatives to pest prone trees

Horse chestnuts have all but disappeared from buyers' shopping lists. Jack Shamash asks what alternatives suppliers are recommending.

Bleeding canker has attacked horse chestnuts - image: GAP Photos/Richard Loader
Bleeding canker has attacked horse chestnuts - image: GAP Photos/Richard Loader

Among tree workers throughout Britain there is genuine concern. A variety of pests and diseases, some of which appear to be linked to climate change, have started attacking common trees. For those responsible for maintaining the street scene, this is a serious problem.

A number of common trees have been affected. The most obvious is the horse chestnut (Aesculus hippocastanum). As everyone in the industry is painfully aware, these trees have been hit by the twin scourges of leaf-miner and bleeding canker (the bleeding canker is caused by a variety of pathogens - notably Phytophthora and Pseudomonas syringae pv. aesculi). The result is that horse chestnuts throughout England and Wales look awful, lose their foliage swiftly and can even die from the condition.

Oaks have been affected by oak processionary moth. This pest does not harm the tree, but the caterpillars are responsible for generally mild allergic reactions among park workers and visitors. Species such as Japanese larch have meanwhile been hit by Phytophthora ramorum, sparking huge swathes of the trees in forestry situations to be felled to prevent further spread.

Plane tree worries

There have been signs that a condition called massaria has spread to Britain from France and Holland. Massaria (Splanchnomena platani) is a fungal condition that attacks - and can kill - plane trees. In France, it has caused serious problems on the lines of planes along the famous Canal du Midi, forcing many to be cut down.

Last year, it was identified in London parks and in some cases diseased branches have had to be removed for safety reasons. This is particularly dangerous for the tree population in Britain because so many towns and cities have streets that are lined with plane trees. If the disease took hold, it could spread rapidly from street to street.

The most notable effect these diseases are having on tree sourcing is that almost nobody is buying horse chestnut trees. Wyevale Nurseries amenity sales manager Andy Congera explains: "It has not been part of normal production. Nobody wants it throughout Europe. We are one of the few nurseries that has kept some stock of horse chestnut. We sell around 2,000 a year, but primarily to people replacing old trees. The market for horse chestnuts is virtually dead."

However, tree buyers have not reacted in the same way to other pests and diseases. Keith Sacre of Barcham Trees, which is one of the largest suppliers of trees to local authorities, says: "We have to be careful not to be panicked. Sales are pretty much as they have always been. Aside from horse chestnut, we're not seeing any real variation. Most local authorities haven't really changed their buying habits."

Wait and see

Certainly, the London Tree Officers Association is adopting a "wait and see" attitude. Chair David Lofthouse says: "I've rung around a few colleagues and the consensus is that, apart from horse chestnuts, there are few signs of new diseases having an effect on species choice. Massaria is not affecting the use of London plane. And we're not sure how the situation will develop."

But there are signs that people are becoming more aware of pests when specifying trees. Barrell Tree Consultancy managing director Jeremy Barrell says he is starting to see some reluctance to take the affected trees. "I can't say it's very common, but recently we had an incident where someone wouldn't take oak trees because of oak processionary moth."

Similarly, at Deepdale Trees, sales manager Mark Godden has noticed that his customers are more wary than usual. "They want to know where the trees are being sourced from and they are asking a lot of questions," he says. But most growers have not noticed any drop in sales. Congera points out: "Oaks are still in serious demand as boulevard trees. We haven't seen any effect."

Hillier Nurseries director Hossein Arshadi concurs. "Most of these diseases have not had a huge impact. Phytophthora is mainly waterborne, so unless you are near rivers it doesn't usually do too much damage. It affects alders and larch, which aren't that commonly specified anyway. And massaria hasn't really developed as a problem in Britain - although this may change.'

Arshadi believes that changes in the trees specified, can help to reduce problems considerably. "We won't, for example, put in ordinary elms. But we do a lot of Ulmus New Horizon, which is sold by Hilliers and is resistant to Dutch Elm Disease," he adds. Similarly, he will not recommend Sorbus aria 'Lutescens', which is prone to mildew, but will happily supply the similar-looking Sorbus aria 'Majestica', which is more resistant.

Horse chestnut alternatives

Arshadi has a wide variety of trees that he currently recommends in places where he might have used horse chestnuts previously. These include walnut trees and sweet chestnut trees. He will also deploy lime trees, Norway maple and beech, although he accepts that these lack the beautiful displays of flowers that are found on the horse chestnut.

Another stunning specimen that is starting to gain popularity as a street tree is the Indian bean tree. "For streets, we want things with a predictable habit, so apple trees, for example, are not very useful," Arshadi explains.

Lofthouse would like to see more Indian horse chestnut (Aesculus Indica) - a variety that does not appear to be affected by leaf miner. "It's a very good urban tree and it would be useful if it was more commonly available," he says.

Majestic Trees, based in St Albans, is also recommending using new varieties. Sales manager Sara Phillips says the problem has been climatic rather than pests. "A lot of Robinia 'Frisia' died in the hash winter weather. So we are now suggesting that people use Gleditsia triacanthos 'Sunburst', which has very similar characteristics but is more hardy."

Many nurseries point out that, despite the pests, it is largely "business as usual". Deric Newman, sales manager at Civic Trees, based just outside London in Watford, says old staples such as Pyrus 'Chanticleer' and Acer - freemanii are still in big demand for street projects. "There's a fairly limited range being used, although some schemes are now specifying the occasional more exciting varieties, such as tulip trees," he says.

However, consultants are now coming to the view that fighting pests and diseases is not just a matter of choosing new trees to replace affected species. They believe that new strategies are needed, for instance planting a wide variety of trees to minimise any risks.

Jonathan Cocking of Jonathan Cocking Associates believes that we have to avoid a monoculture, through which diseases can spread with alarming speed. "If you only have two or three species, a few diseases can wipe out the lot. We need greater diversity and texture in planting," he maintains.

He points out that long avenues of single species such as limes, horse chestnuts and oaks may look lovely but are very prone to infection. Instead, he recommends using a variety of trees with a similar appearance. "We're now recommending that people use a mix of things that look roughly the same - such as beech, southern beech and hornbeam - if they want an avenue of trees," he adds.

Barrell also believes that a mix of trees will be essential if streets are to avoid the sort of carnage that affected the elm population in the 1970s as a result of Dutch elm disease. "We have to protect our future stocks from climate change as well as pests and diseases. Until now, most local authorities have specified that we should use native trees. I think this is short-sighted."

Linking green spaces

Barrell has his own formula. He believes that only between 50 and 60 per cent of the trees planted in towns should be native. "This will allow us to link the rural and urban green spaces, and allow species of animals and insects to move around," he says.

However, he would like to see around 40 per cent of non-native trees being used. "Beech and birch could be struggling in 100 years' time. We need more variety," he insists. He points out that in the USA, an insect called the emerald ash borer has destroyed vast numbers of ash. "This sort of disease could easily come here. We don't know what's going to arrive. We should be constantly looking for new species that will do well in the urban environment," he points out.

He suggests that such trees as liquid amber, tulip trees and acacias should be included in the mix. "We should have a wide palate from which we can choose," says Barrell. "If we have a wide variety of trees, then our street scene will be able to withstand new strains of disease. If we stick to a monoculture, then we could be in trouble."

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