‘Wall Plants’ Our urban limestone pavements
One of my favourite landscapes in the Dales are the iconic limestone pavements. I love kneeling on the blocks of limestone, known as clints, and peering into the crevice-like grykes. There you see a huge array of plants. The humid atmosphere, away from direct sunlight, forms a miniature botanical garden.
You don’t need to travel up to the Dales to get a glimpse into plant life in the grykes. We have a similar manmade habitat in our towns and villages. Old stone walls are cemented together with lime mortar, made from lime and sand, mixed with water. This produces a very similar habitat to the limestone pavements, which is why we tend to see the same plants, especially ferns – Hart’s Tongues, Wall Rue and Maidenhair Spleenwort.
The tongue-shaped fronds have given Hart’stongue its name, a Hart being an adult male Red Deer.
Wall Rue has leaves similar to the Mediterranean herb Rue, which was used by Roman physicians as an antidote to venomous snake bites. Rue comes from the Latin ‘Ruta’ meaning ‘to set free’, in this case from poisons.
Maidenhair Spleenwort also has a medical origin. Spleenwort was derived from an old belief, based on the medieval doctrine of signatures, that the fern was useful for ailments of the spleen, due to the spleen-shaped, spore-producing sori on the back of the fronds. Although after forty plus years, studying and practicing medicine, I am not convinced they look particularly splenic.
It is not just ferns that have made their home on walls, but flowering plants as well. It is not an easy environment, with a paucity of water and exposure to the glare of the sun. Here are just three, each with their own interesting storey.
Who hasn’t admired walls covered with the beautiful Ivy-leaved Toadflax? This small plant with its two lipped purple flower probably arrived in the UK as seeds on Roman statues imported from Italy to decorate the country estates of the aristocracy in the late 1600’s, from where it has spread. It has an amazing method for propagation. Like nearly all plants it is positively phototropic, i.e. growing towards light. But once the flowers have been pollinated and fertilized, the flower stalk becomes negatively phototropic, and grows away from the light, back towards the wall. Ivy-leaved Toadflax therefore ends up ‘planting’ its own seeds into the dark crevices of rock walls, where they are more likely to germinate. How clever is that?
Another good example is Wall Lettuce, ‘it does what it says on the tin’ and grows on walls. A member of the Daisy family, it has just five yellow florets, compared to the Dandelion’s 150-200. The stem and leaves are frequently suffused with a maroon colour. It can be baking hot growing in full sun, and the maroon pigment acts as the plant’s own suntan lotion. Wall Lettuce is also related to our Salad Lettuce. Romans named the plant Lactuca (as in lactation), which is where we get the name lettuce. Cut the stem and a milky latex exudes, just as in a Dandelion flower stalk.
Our last wall plant is a stunner – Yellow Corydalis, again another non-native, this time from the foothills of the Alps, and brought to the UK as a garden plant in the 16th century. The unusual name Corydalis, comes from the Greek korydales, meaning Crested Lark (similar to our native Skylark), which the flowers are said to look like. Following recent genetic analysis, it has also surprisingly been moved from its own family to join the Poppy family.
These are just six of the plants to be commonly found growing on walls. There are many more. When you are out and about around town, do take time to stop and look, the older the wall, the better. They are fascinating places, and I haven’t even mentioned the insect life.