I've touched on the politics of roadside trees elsewhere, and I've talked about urban trees for the future here. Any of those trees make good roadside choices, depending on location. Obviously, I'm willing to defend my own choices for what trees should be planted in urban spaces (go look at that blog post), but I also wanted to discuss tree selection based on principles that ought to be applied. Yes, this is in part about City Deal in Cambridge - but I hope these thoughts might be of wider interest too.
If we're looking at roadsides, there are for me 7 key principles we need to bear in mind.
1. Pollution resistance - if it'll die in traffic fumes its no good here.
2. Salt, flooding, and drying out - in restricted soil space on a road thats gritted this is all important
3. Root damage - if the tree will rip up the road lets not plant it
4. Appearance - lets plant trees that are beautiful!
5. Growth and shade - the right tree for the right place
6. Ecology and habitat - we should at all times foster a better environment.
7. Durability - lets not plant a tree that'll die of a disease months after it goes in
Now of course every tree is good or bad for some of those points - there is no perfect urban tree. But I think we can pick out a few species that are all-rounders and we should discuss those. And yes, I'm leaving out some lovely trees that are currently not doing so well due to disease in the UK (ash and horse chestnut in particular). And some that are great urban trees but rather less suitable to roadsides.
I should stress again that these are my 'likes' - there are many other options, but I'm aiming to get a discussion going on suitable trees for planting. And I'm not ashamed to put my own views forward to do so - these choices (and the ones in the earlier post) are personal, but I think defensible. Please, join in with your own views - but have a think about whether your preferences make good choices for the location you're discussing. I'll follow this post up with something about the specific locations we're looking at in Cambridge (although that might be on the Cambridge Cubed blog).
We all know this tree. While its typical of London streets its also planted across much of the UK, especially cities in Southern England. It puts on a lot of growth in the first few years after planting and can become a massive, spreading beast of a tree. Its famous for being disease and pollution resistant, and its entirely happy being pollarded hard (so you can cut it back and extend its lifespan massively in doing so). Thats all great.
But its really rather spreading - if you've got lots of space its good, so you want wide bits of earth with room for roots to go down and out. And it provides loads of shade - if you don't want that, don't plant it.
Its not amazing for wildlife either - nothing particularly to eat and its not one to foster the growth of a wide range of fungi. Its not awful for insects, but not superb either. I like this tree but for my money its over-used, too big for many places, and therefore perhaps best used sparingly now.
I quite like hornbeams, and they thrive on roadsides in London. There's a fastigiate (sticky up rather than spready out) form thats very popular on roadsides - that makes it a great choice if you're looking for a mid to large growing tree that doesn't shade too many things out. It has quite spreading roots but I haven't seen damage caused by this. Its great for insects and encourages fungi in the soil, and it has great autumn colour. The only word of caution would be that it rather seems to struggle in full light - its great for tightly shaded urban canyons, never seems as happy establishing itself in brighter light.
Or, lime trees. Tilia. Again, great urban trees capable of surviving in hostile environments, and which can be cut back again and again and still thrive. The commonly planted hybrid forms are near ubiquitous in England, and we all know them. They're great for insects and fungi - the only issue is perhaps they're a little TOO good for them. People complain like mad when they've parked their car under one of these in summer to come back to find sap all over the windscreen. There was a time when at Camridge Station the railway responded to that by putting the bike parking under the lime trees. Thanks guys!
They're big though - or at least have the capacity to get big. Shady, cooling, and lovely - but not for a narrow gap.
Loads of cultivars of this genus do well by roads, if they can get established. Some do well on clay (rowan), some seem happy in wetter conditions (some of the whitebeams), others are resonant of older periods in our history (wild service) - they're all middling to small trees, with blossom followed by heads of red (or orange, even green) berries. Fantastic for bids, of interest to foragers, not particularly messy, they're great for tighter spots - but they do still form a canopy. I'd like to see more varieties of Sorbus planted, they're all great fun.
Pear trees. Yes, I know. Go on, click on the link there and tell me you don't want to see that narrow, tallish, flowered and fruited tree by your roadsides. It can drop a bit of fruit - not like a wet mess of horrid like those ornamental crab apple trees, but some. And can struggle to get established in the driest of conditions - but its a grand tree and worth considering.
A native tree that does support wildlife, its not as big as its cousin the sycamore, nor is it quite as invasive. The helicopter seeds are almost as much fun as sycamore too. And it seems to survive nearly anything hurled against it. Great choice for roadsides, but, again, does need some spreading space.
You already know cherry trees. Flowers perhaps followed by fruit (and always plant a fruiting cultivar - why ever wouldn't you want beautiful berries for birds?) But have you spotted those lovely dark red leaved trees that have a fairly similar form, with red flowers and then dark red fruits? Thats a cherry plum. They're usually green leaved, with either green, yellow, red or even purple fruit following a white flower. Gorgeous trees that the birds go mad for. Then there are assorted varieties of almond and apricot worth looking at too - choosing a variety of types to add colour and interest to a planting scheme seems obviously a good idea to me.
You'll always find one of these trees that will fit in a reasonable space, and none of them are that massive. I'd urge some caution though - they're fast growing, but few really old specimens are around. They're not the very longest lived of trees - if you're wanting a treescape for your children then cherry and its cousins is a component of that but not the dominant type.
A tree that does well in a warmer climate - and as our climate warms this might be a winner. Lovely tree, and there are specimens thriving in London. Gorgeous, big flowering tree used by roadsides in some parts of the US and the far East - well worth investigating. But, again, not one for a very small space.