OK, I’m a bit nervous about this one. I guess it comes down to whether Plant of the Week is about raising plant awareness or whether it represents plant promotion.
For I want to write about Cotoneaster horizontalis. In the list of what must be thousands of shrubs available for garden use, and which mostly fall into a category of frustrating amorphousness, Cotoneaster horizontalis finds itself on the extremely short list of garden shrubs with an inbuilt interesting and engaging form. But the trouble is that it, along with all cotoneasters as far as I can tell, has been condemned as being potentially invasive. And while I’ve never seen C. horizontalis as a garden escape, I understand the reason for concern and think that the threat needs to be taken seriously.
But first let’s talk about its unique qualities. The most obvious of these is its remarkable herringbone branching structure, which is combined with a curious ability to shimmy its way up a wall or fence in a kind of bracketing form, as if it’s elbowing its way up in a vertical commando crawl. At best this sprays the wall or fence with a pattern of deep green, horizontally-held triangular brackets, each of such geometric precision that old Fibonacci could no doubt come up with a mathematical equation to explain it.
Then there’s the flowering, during summer, with thousands of tiny, white nectar-rich blooms for which bees and other pollinators go entirely berserk. These are followed by a similar number of red berries, which are in perfect complementary harmony with the rich green leaves, and eventually sit in captivating analogous harmony as the leaves turn orange to deep red in late autumn/early winter.
But it’s the berries, of course, that cause the problem. Birds eat them and then distribute them indiscriminately. Consequently, it’s hard to find in nurseries, and maybe that’s as it should be.
At worst, C. horizontalis is a big, boring deep green shrub that takes up way too much space, being easily capable of getting to a metre high and two or three times that wide. When left unpruned, or worse, pruned into a neat shape, its greatest contribution – that triangular bracketing growth form – gets lost. It’s remarkable growth form is most evident when its pushing out into new territory, following hard-pruning as a free-standing shrub, or when it’s searching out new territory on a wall or fence. There’s nothing else quite like it.
Now does that constitute an awareness campaign, or pure promotion?
I first met it growing DOWN a substantial embankment, excellently pruned so that it seemed to grow in a single neat groundhugging layer. Spectacular.
I love it on a bank, on which you get that horizontal bracketing. Here in Australia, if we want a ground-hugging, naturally prostrate form, we tend to use C. dammeri.