I got talking to a guy at the City Council about their tree strategy recently, and I've got to say I was impressed - Cambridge is a fabulously green city, and the city council does want to keep it that way in what may be a rapidly changing world. They're aware that a diverse range of urban trees is far more robust against various diseases - such as the one that wiped out most of our elms, and another which is taking such a heavy toll of ash trees now. When you look at the urban trees across a lot of cities in the UK, there are certain trends that are worrying, such as the prevalence of relatively few species of often closely related trees, which might mean that the our urban tree stocks could be vulnerable to diseases or climate change in the future.
So I thought that maybe I could contribute a few thoughts on this subject. I started out with a list of suggestions for urban trees I think could do well, and after half an hour of writing and dozens of listed cultivars I decided to cut things back a bit and just list a few I'd like planted in and around our cities based on four goals.
(1) Being uncommonly planted in towns and cities in the UK now - it isn't that linden or plane are bad, but broadening out what we're planting is a good idea
(2) Provide good cover/shade/aesthetics and being maintainable in this context (including resilience to polution or a warmer climate)
(3) Be in some way of use, producing fruit, nuts, leaves or something else of practial value
(4) Support wildlife - whether by being a rarely used native tree or in some other way
Yes, I know, they aren't independent goals, indeed every tree will do all of these things to some extent or another. But its a way of looking at things that might be useful. None of the suggestions I'm making do all of these things perfectly, but they all do some of them.
One last thing - at heart I'm a forager. I love the idea of planting as diverse a range of food producing trees as we can. I tend to think if we can plant a beautiful tree, or plant a beautiful tree that at some stage might be worth someone scrumping fruit from, the latter is obviously better. I accept my choices may be skewed thus, but I'd defend the selection of fruit producing trees as better for people and for wildlife.
So here we go with my list...
'Exotic' Prunus trees
I know, 'exotic' depends on context. But what I mean here is trees of Prunus genus that aren't commonly used in urban planting schemes in the UK. That means trees other than plum, cherry plum, cherry, blackthorn and damson.
There's a variety of peach that I believe would make a great park/city tree, a cultivar called Avalon Pride. I've had one in my garden for years, and unlike every other variety of peach tree that withers under the attack of peach leaf curl when we get one of our typical humid UK summers, Avalon Pride is largely resistant to this. So as our summers get ever warmer and our season for fruit becomes shorter as cherries, plums and damsons get earlier, broadening that season with other stone fruit seems sensible. Likewise, apricot trees (Prunus armeniaca) can do well, especially in the South of England, and I see no obvious reason not to plant them where there's space for a good sized tree. Not maybe for next to the busiest of roads (they can shed a lot of fruit), but a beautiful tree in flower, leaf and fruit.
Loquat and Medlar
I do like trees that look good through different seasons, so I'm of the opinion that trees that start off with great flowers and follow up with beautiful looking fruit are among the best options - something for bees, something for birds and other animals. And generally speaking our councils plant a lot of these - depending where you are there are certainly plenty of whitebeam and rowan trees in most cities.
One that I've seen far too rarely is the loquat, Eriobotrya japonica. Not a big tree ususally, rarely more than 4m tall, but the spikes of white flowers it has in Spring are followed by distinctive yellow fruit later in the year. Great looking thing.
Another thats planted just occasionally is the medlar, Mespilus germanica. This one has an old, old heritage in the UK and across Northern europe because the fruit stay on the tree into the dead of winter, only becoming good to eat when they go soft (or 'blet' - not as weird as it sounds, several tree fruit benefit from this). As a fruit crop it has long since been superceded - but its another tree that provides a point of interest in any planting scheme, with flowers followed by fruit that don't look like anything else you'll ever find.
We're used to the good old varieties of oak trees we find in woodlands in Britain, but many cities seem loathe to plant them. This is a real shame, they can grow to a wonderful size and have a truly extraordinary lifespan, offering a host of sub-habitats for wild animals, fungi and even other plants. If we're going to continue to diversify what is planted in our cities, I'd go for things like Turkey oak (Quercus cerris) and Hungarian oak (Quercus frainetto), being species suited to maybe slightly warmer climates and able to withstand pollarding. Beautiful, shady, majestic trees.
Black (Morus nigra) and white (Morus alba) trees are glorious things - 15-20m high when grown, but slow growing. They're big, shady beasts. Trees of real stature, shedding delicious fruit that birds go mad for, and they're excellent habitat trees for insects and birds. Yet, strangely, they're not planted all that often in our cities. As our climate gets warmer I think this one could be a winner.
I don't really know why these really delightful relatives of elm aren't grown more commonly in the UK. They have lovely dense foliage, they get to a good size, and produce some small, vaguely edible berries that birds seem to love. The species native to Southern Europe is Celtis austraulis and I see no reason it shouldn't do really well in the UK, but Celtis occidentalis is a North American species thats maybe prettier. There are a number of species of this tree though, and for any local authority looking to broaden out the range of species it has I think this is a great bet. Quite a good size, 20m or so, but fast growing too so ought to be good in urban settings where cutting back is part of the management strategy.
Blue Bean Tree
This is a real oddity (Decaisnea fargesii) but where you see it growing in the UK it seems to do well. Only a small tree but one with delicate, alien looking flowers followed by almost metallic blue bean-pod shaped fruit. Again, of only tangential use (the fruit pulp is kind of edible but unexciting), but its a great little tree that has the huge advantage of not being closely related to anything else you're likely to see planted in British cities - if we want to ruggedise our urban tree stocks against future risks, thats got to be a good thing.
Ok, I'm going out in a limb with this, its another really weird one. But thats the point of this article. I present the most darling little tree you'll ever meet, Poncirus trifoliata.
This is presently as close as you can get to a citrus tree happily growing outdoors in the UK. Its a spiky, slow growing, massively aromatic plant. Especially when in bloom, and it can produce the most unlikely looking small, bitter oranges (that as a push could be cooked into marmalade, but its really not worth it). The real reason for wanting to see more of this one planted is for the flowers, which are a brilliant bee-magnet. They love this tree. And its a good option for roadsides and verges where something smaller, slower growing and alltogether shrubbier will grow - I doubt it ever gets to more than a 2.5m in the UK, and it takes years to get that far. I've rarely seen one in the UK that was bigger than a good sized shrub, and its not renowned for being a big tree - surely a plant like this has to be worth a punt in hedges if we're looking to diversify?