Pseudofumaria lutea (synonym Corydalis lutea). Yellow Corydalis.
You can see this plant growing on walls in towns and cities. It appears as a clump of bright yellow flowers, each flower shaped like a tiny trumpet. It belongs to the poppy family, Papaveraceae, but it looks nothing like a poppy. Formerly it sat alongside Fumaria and Dicentra in the Fumariaceae, but that family is now relegated to a subfamily of the Papaveraceae, called the Fumarioideae.
It is a ‘garden escape’, first grown in Britain in 1596 and recorded in the wild in 1796. Its true home is the foothills of the Alps in Switzerland and Italy. However, it now occupies much of Europe and also sites further afield. In the British Isles it is most common in the south-east of England, especially in London where it appears to have thrived in recent decades (Kent 1992). It is much less common in Scotland, where it tends to be on the east, whilst in Ireland it is hard to find.
The European distribution suggests it avoids extreme heat and cold, yet it is known to be winter-hardy, tolerating temperatures as low as -50 oC. Judging from its British distribution, it likes an equitable summer temperature, with an annual July temperature above 15 oC. Its expansion in London in recent decades may have been the consequence of heat-waves.
It’s a short-lived perennial that flowers for much of the summer, and sometimes its leaves remain green throughout winter. Its roots seek out cracks and crevices in mortar, and it clings on firmly. I first noticed it near my house in 2014, on the main A702 road out of Edinburgh. What a beauty. Quite other-worldish, with its finely divided pale green leaves (looking like Maiden-Hair Fern) and its dazzling yellow flowers. Each year I admired it but last year the householder prized it off the wall and chucked it in the bin. Some people! When will they ever learn?
How do the seeds get into the wall? It seems that ants pick them up and take them home to share with their comrades. The phenomenon is called myrmecochory. In Fumaria and quite many other plants the seeds have a lipid-rich external structure (the elaiosome, see ‘m’ in the old print above) which ants find irresistible (Ohkawara et al. 1997, and see https://kindheartsandcorydalis.co.uk/myrmecochory/). They carry the seeds to their nest inside the wall. Evolution works in wonderful ways: in this case the considerable investment in the manufacture of lipids ensures the species finds a safe germination site.
It is loved by gardeners for their suburban rock-gardens. An interesting question is whether the plants we see on walls escaped recently from gardens or are descendants of an ancient escapee. Possibly the populations have diverged from the ancestral form. I should try to get seed from the Alps and run a comparison. Perhaps we’ve got subspecies Britannicus or even Scotica. However I could find no work on molecular genetics of this species or any other Pseudofumaria. In fact, the plant is distinctly under-researched. However, it pops up in a journal that we botanists seldom consult: Construction and Building Materials. So let me tell you what I saw.
I read that researchers in Delft have become interested in what they have called the ‘bioreceptivity’ of building materials (Lubelli et al. 2021). Traditionally, many householders in the Western World consider any wild plants growing on their walls as ‘weeds’ and clean them off by scraping or poisoning. But attitudes are changing and some (usually younger) householders would like a ‘green wall’ rich in wildlife. So the Dutch researchers looked at the chemical and physical properties of mortar and bricks as far as it influences the establishment and growth of Pseudofumaria lutea and Ivy-leaved Toadflax Cymbalaria muralis. Their report is quite technical and I didn’t have a friendly civil engineer by my side when I read it. They mixed the seeds with the mortar. Rather surprisingly (to me) seeds germinated on quite fresh mortar, and mortar with plenty of added lime worked best. The addition of vermiculite was helpful (but as one might expect, the vermiculite weakened the bonding of the bricks). Of course, it’s no use making walls bio-receptive if they are going to fall down; it looks as if further research is needed before these ideas are transferred to Bob the Builder. Let’s wait and see.
The big question of whether plants weaken the walls they grow in will be tackled in another blog. Plant roots secrete weak acids which will speed up chemical weathering of the mortar, and some of them have tap roots which can force stones apart in some cases.
Plants on walls have been discussed on this site before: see Brian Ballinger’s blog https://botsocscot.wordpress.com/2020/06/07/50-walls-in-dundee/ and another one by Brian in his capacity as a BSBI recorder for Easter Ross: https://bsbi.org/wp-content/uploads/dlm_uploads/Plants_on_Walls_in_E_Ross.pdf. He finds Pseudofumaria lutea but not as often as Ivy-leaved Toadflax Cymbalaria muralis. Like Pseudofumaria lutea, the Toadflax also has its headquarters in Southern Europe, but it has spread further as an alien to many parts of the world, including Australia, South America, Japan and South Africa. However, whilst the species Pseudofumaria lutea seems a shy traveller, the genus itself, Pseudofumaria (aka Corydalis), has speciated greatly and now occupies sites well beyond Europe: The species are most diverse in China and the Himalayas, with at least 357 species in China. There is one in particular that we want to cultivate: Corydalis flexuosa. Its flowers are a most unusual shade of blue, nearly turquoise. It comes from central China.
Kent D.H. (1992) Pseudofumaria lutea. BSBI News 60: 20-21.
Lubelli B. et al. (2021) Influence of brick and mortar properties on bioreceptivity of masonry Results from experimental research. Construction and Building Materials 266: paper 121036.
Ohkawara K. et al. (1997). The evolution of ant-dispersal in a spring-ephermeral Corydalis ambigua (Papaveraceae): timing of seed-fall and effects of ants and ground beetles. – Ecography 20: 217-223.