I was fascinated by the plants growing out of stone walls in Cornwall, Wales, the Channel Islands, and elsewhere in Britain.
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| Plant growing on a vertical wall, Wales |
We have stone walls in northern Colorado but by comparison, they are few and very new; the first European settlement in this area was about 1840, most settlement after 1880 and as far as I know the Ute and Arapaho didn't build stone walls. Furthermore, the rainfall and humidity are much higher in Britain, fascilitating life on a wall. Here are some of what I have seen.
Older neighborhoods were rich in stonework, including lots of property-line walls
These rock walls create opportunities. Below, a fern in the middle of a wall
Getting to a suitable spot halfway up a wall seems very difficult. Fern spores are tiny and float through the air.
But this plant has seeds, not spores:
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| Mexican fleabane on a wall |
This is probably Erigeron karvinskianus, Mexican fleabane or Santa Barbara daisy, a small composite from Mexico and Central America that is widely grown as a garden plant, It has escaped to become a weed, all over the world. When you look carefully at the descriptions of it, it has seeds that, like dandelions, are distributed by the wind. So it flew there.
When plant books say "escaped from cultivation," mostly I think of growing on roadsides. Ok, this is a roadside wall--I was walking the sidewalk--but nevertheless, I don't usually think of plants as slipping out of the garden and to grow half-way up a stone wall. But Mexican fleabane did.
Mexican fleabane, above, is flowering despite growing with its roots in a wall and about 2' off the ground. It must get its nutrients out of the wall's materials. Britain's frequent rain and high humidity make flourishing possible.
Wall plants must be the bane of the owners of the walls. This wall, below, nicely whitewashed, has nevertheless been colonized by a plant (green spot half way down)
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See the plant, halfway down the wall, more than
a foot off the ground. |
Here is the plant close up. My best guess, based on the leaf shape, is butterbur (Pedasites hybridus, dandelion family, Asteraceae) though the middle of a wall is not a typical location for butterbur.
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| butterbur, Pedasites hybridus ? |
I can't identify this one at all, but look at the size of it. Apparently walls are comfortable locations for the plants that can get there.
Britain has lots of English ivy (
Hedera helix, ginseng family, Araliaceae) which you often find growing all over walls. Mostly English ivy cheats, coming up from the ground somewhere. The roots that let it climb do not bring in sufficient water and nutrients to sustain it. That said, if it finds a hole in the wall that holds water and dirt, it can certainly use that as its base. Its seeds are dispersed by birds. It would not be easy for a bird to drop a seed into a hole in a rock wall, but certainly not impossible.
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| English ivy, Hedera helix, on wall |
The older the stone, the more opportunities for plants
This is Conwy Castle in Wales. Finished in 1287, it was neglected most of the next 800 years, so of course the plants colonized it. Current maintenance focuses on keeping it from falling down and safe for tourists, I doubt scrubbing plants off the walls is high on their list of projects. If you look carefully at the towers, you can see even where they aren't obviously green, there are plants all over them.
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| Plants on Conwy Castle |
And this is Conwy Castle's well: plants all over the walls, despite relatively poor light.
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Well at Conwy Castle, lots of plants
on the vertical walls |
This (below) was a very old wall. Leaves accumulate on its flat top, and it has a vigourous grass among the mosses of the vertical wall.
I was in Cornwall and the Channel Islands right at the end a prolonged drought, so I call the grass above vigorous despite the brown leaves because nearby, on a Cornish hedgerow (they build them with stone) many grasses have died. Not just the big ones near the top of the wall
but lots of littler ones in the lower half of the wall. Most of what I see are grasses, some are other small plants.
Water is a significant problem to plants growing on walls. In Britain, rain usually falls often enough to maintain them, but if it doesn't, they die.
The Cornish hedgerow above is more than 1000 years old. Perhaps rock walls without mortar were a training ground for plants to evolve the necessary air-borne seeds, tolerance of the minerals in the rocks, and the resistance to drought necessary to be good modern wall plants.
Here, three maybe four fern spores have germinated on this wall.
Plants are so very opportunistic and adaptable!
Comments and corrections welcome.
References
In this blog: Plants of Conwy Castle
link
Streeter, D. 2009.
British Common Wildflower Guide. William Collins, Publisher. London.
Google, for suggesting names.
Kathy Keeler
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