Managers of historic landscapes and cityscapes should ditch the list of trees they learnt by rote at college and resist the temptation to pick the latest fad species when replanting areas of historical interest, says chartered forester and arboricultural consultant Clive Mayhew. Mayhew hopes he will be listened to for the sake of safeguarding the country's historic treescapes.
A tree officer for over 20 years, Mayhew believes the principles of design and historical development have been left out of arboricultural training for a generation, often resulting in ill-considered and mismanaged landscapes planted with little thought of context and setting.
"The bit I hate the most in any plant book is the grid at the back, where you end up picking the plant that ticks the most boxes," he says. "So if you see a plant that offers autumn colour, berries, nice spring flowers and a bit of flaking bark in winter, it gets chosen because it has something for everyone. I remember this element to plant selection at college very well. You would always arrive at the same choice - Amalanchier."
Fast-changing planting tastes fuelled by nursery marketing don't help either, he says. "Increasingly, plant selection is made on what is in fashion at that moment. A couple of years ago it was Corylus colurna Award of Garden Merit (AGM). A few years earlier, it was Alnus rubra. You can see those fashions emerging on the street scene. A wet-behind-the-ears tree officer chooses the latest fad species and fills our streets, parks and gardens with it one year, only for another fad to take its place the following season."
Examples of this can be seen in most towns and cities, but Mayhew highlights a street in Barnet, north London. "You have some Ginkgo trees, then down the road are dark-leaved Acer, and then the next officer has applied his attributional planting and we have Sorbus hupehensis with its berries and autumn colour - a nice restricted-growth street tree, but taken together the selection of trees just doesn't mix well."
Mayhew admires many of the results of planting formulas advocated and adopted by the likes of landscape architect Sylvia Crowe, whose designs based on space division and aspect ratio were influential in landscape planning of post-war new towns.
Mayhew has successfully applied such formulas, but believes this approach is best suited to new design projects, as it often fails to take into account the surrounding landscape, architecture and historical context of existing spaces.
"Trees are the most precious natural asset we have and there must be a better way to use them in our urban environment. The issue is trying to develop a winning method of making the right planting choices for the space you are working with."
This rarely presents a blank canvas. "The spaces we are working with are surrounded by other things," he says. "Our cities and towns are complex, multi-layered, historical places. Sometimes the trick is working out where those layers meet and end, and identifying the historical setting.
"Whether it's London, New York, Paris or a small village in the West Country, there is a historic aspect to most of our urban spaces. What I suggest is that we can use that to inform our species choices to complement the areas we are working in."
The principles advocated by Mayhew stem from the work of plant historian and author Mary Campell-Culver, who proposes a periodic timeline of UK plant introductions based on invasion, settlement, trade and plant-hunting expeditions (see box). The 11 partly overlapping periods make it possible to plot the progress of plants arriving from around the world into the UK.
This, he says, allows more informed decisions to be made when it comes to the appropriate planting or re-planting of trees within a historic landscape, while allowing a broad interpretation of the sequential nature of tree introductions.
"If you have a tree that you know comes from the eastern seaboard of the US, like Magnolia grandiflora or Acer negundo, you know it has been in the UK 200 years longer than trees like Sequoia, Douglas fir and Thuja from the west coast. Likewise, if you have a tree that comes from Japan like the ornamental cherries, you know it is going to be 100 years later than the sequoias."
This approach can inform managers when faced with the multitude of trees on offer at nurseries, he says. "There are some species that will throw you off track, but generally the closer they originate from Britain, the earlier they will be in the timeline."
Putting this into practice, he suggests: Aesculus flava AGM, introduced in 1764, would make a good species choice for a Georgian square re-planting; Zelkova serrata AGM, introduced in 1861, would be good for a Victorian park; and Prunus 'Pink Perfection' AGM, introduced in 1935, would suit a late Arts and Crafts garden.
Mayhew believes a better understanding of trees in their historical context will provide arborists and specifiers with a better appreciation of how the sites they assess have developed over time. This way of categorising trees allows familiar landscapes to be seen in a new light, he says.
"Looking south from the Orangery (at the Royal Botanic Gardens, Kew), one is presented with a fairly typical scene - a great number of trees of many species spread in an apparently random fashion. A cursory examination would suggest that William Chambers' building has little in common with the treescape in front of it.
"On closer inspection, however, certain trees become conspicuous, not just because of their great age, but also their consistency in terms of planting and introduction dates. Single specimens of Robinia pseudoacacia, Ginkgo biloba AGM, Sophora japonica and Magnolia acuminata all survive in close proximity to the Orangery. These species have a similar introduction date and evidence exists to suggest that the trees were planted in the winter and spring of 1761-62, to coincide with the completion of the building.
"Today, all these trees are widely recognised as individual veteran specimens, but this process of collective understanding also allows them to be recognised as part of Chambers' original landscape design, and endure as a key component of Kew's valuable landscape history."
Mayhew hopes that by uncovering the past, managers can move into the future with informed decisions on how best to nurture and develop the country's many historical treescapes, parks and gardens.
"There are something like 8,000 conservation areas in England alone, identified by their architectural merit, but I'm pretty sure most local government officers aren't too aware of that when developing the planting schemes for these places," he says.
Historic landscape designers will be familiar with this concept and are using it to great effect in restoration projects around the country, he says - but those looking after the street planting next to them might not be. "Using a timeline is about getting tree officers to look at a street or space, identify it as Victorian or Georgian, for example, and then turn to a palette of trees and plants introduced to the country in the same era to produce a planting scheme or treescape fit for its surroundings."
Clive Mayhew will run a seminar on trees in historic landscapes at Kew on 21 May. Places can be booked through the Arboricultural Association.
Campbell-Culver's periodic timeline of UK plant introductions
Period 1: 1000-1560
Plants arrive from Continental Europe due to the Norman invasion, the Crusades and monastic expansion. Trees include Morus nigra AGM, Prunus persica, Picea abies and Platanus orientalis AGM.
Period 2: c. 1560-1620
Plants appear from the Balkans, Asia Minor and the Near East. Trees include Corylus colurna AGM, Pinus pinaster AGM, Cercis siliquastrum AGM and Aesculus hippocastanum AGM.
Period 3: 1620-22
The "Tradescant period", when plant hunter John Tradescant the Elder travels to Russia and North Africa, while his son makes journeys to the east coast of North America. Trees include Larix decidua AGM, Robinia pseudoacacia, Taxodium distichum AGM and Acer rubrum.
Period 4: 1660-1720
Introductions from the eastern seaboard of North America. Trees include Liquidambar styraciflua, Acer negundo, Quercus coccinea and Catalpa bignonioides AGM.
Period 5: 1730-60
Species from China begin to appear. Trees inlcude Thuja orientalis, Ginkgo biloba AGM, Sophora japonica and Koelreuteria paniculata AGM.
Period 6: 1680-1774
Introductions from southern Africa and the Cape of Good Hope. No notable tree species.
Period 7: 1772-1820
The "Antipodean Period", following the return of Joseph Banks and Captain Cook from the southern hemisphere. Trees include Dick- sonia antarctica AGM, Acacia dealbata AGM and Cordyline australis AGM.
Period 8: 1820-60
Plants arrive from California and the west coast of North America. Trees include Pseudotsuga menziesii AGM, Pinus radiata AGM, Cupressus macrocarpa and Sequoia sempervirens AGM.
Period 9: 1840-70
South American plants appear, though few noteworthy trees apart from Araucaria araucana, the monkey-puzzle tree, first introduced by Archibald Menzies in 1795 and later (more widely) by William Lobb in 1844.
Period 10: 1849-90
Expeditions to India and the Himalayas bring further introductions. Trees include Aesculus indica, Cedrus deodara AGM and Betula utilis var jacquemontii.
Period 11: 1890-1930
Newly explored areas in China and Japan result in the introduction of new trees, particularly maples and flowering cherries. Trees include Malus tschonoskii AGM, Prunus 'Shirofugen' AGM, Acer grisium AGM and Davidia involucrata AGM.