Plant of the Week – 30th August 2021 – Prickly Lettuce (Lactuca serriola L. )

Lactuca_serriola_Sturm45.jpg (706×1024)
Prickly Lettuce, Lactuca serriola by Johann Georg Sturm (1742–1793), a German natural history illustrator. Note that two different leaf types are shown, corresponding to two forms of the species. The image is public domain.

The Prickly Lettuce is rarely seen in Scotland but this may be about to change. According to Smith et al (2002) its close relative the Great Lettuce L. virosa occurs in several Edinburgh locations but the prickly one was last seen in the Capital back in 1910. I looked for reference to it in the more up-to-date survey, the Urban Flora’s database 2015 to 2021 but it wasn’t there (although, there are quite many records of Great Lettuce). Finally, I turned to the BSBI database for Midlothian and found that it was recorded 2015 – 2016 in the Leith docklands area; nowhere else in or around Edinburgh, but quite a few records in Glasgow.

It was a nice surprise for me to find a new population of Prickly Lettuce in the Bingham suburb of Edinburgh. I had to return two weeks later to be sure of the identification, because the Great and the Prickly are quite similar when young. Flowering stems and fruits are needed to separate them.

They are both said to be ‘probably native’. Seeds germinate in the autumn and young plants over-winter. They develop rapidly in spring and summer, flowering in August/September. Then they die. This is similar to the cultivated lettuce, said to have been derived from Lactuca serriola. I had a look at some lettuce plants in my garden that had gone to seed. They too had tiny prickles on the margins of the stem-leaves, though more delicate than those on Prickly Lettuce.  

A strange feature of Prickly is that it exists in two forms, one with simple spade-like stem-leaves and the other with indented-lobed stem-leaves. The artist who painted the picture above obviously knew this fact; he helpfully drew both types of leaf. The Great Lettuce never has the indented stem-leaves, so the Bingham plants, with their sharply indented leaves were almost certainly L. serriola.

Both the Prickly and the Great can be prickly, so this isn’t a good character for identification. The prickles are quite sharp and presumably deter slugs and snails, those enemies of cultivated lettuces.

But the most reliable character is the colour of the achenes (Prince & Carter 1977). The Great one, L. virosa, has maroon achenes turning darker with age of storage, but in L. serriola achenes are mottled olive-grey (see my photo below). There are other, less consistent, differences. The stems and leaves of L. virosa are usually tinged or flecked maroon but the Prickly has whitish stems and veins. The Great one is usually much taller, living up to its name.

In the small population from Bingham the achenes were mottled brown-grey, the leaves indented and the plants were not tinged maroon. So I was sure I had found another Edinburgh population of Prickly, well away from the Leith docklands. I don’t know if it will survive on the corner of the two streets where it grows now; the spot seems to have been overlooked by both the Edinburgh Council’s weed control people and the residents during this year but next year it may be different.

Achenes from the Bingham population, pappus attached. Colour and shape are important identification characters.

A few days after finding the Bingham population I found more wild lettuces whilst recording in the Muirhouse district of Edinburgh. The stem-leaves this time were spade-like and I wrote L. virosa in my note-book. But, back home, I remembered that L. serriola can have leaves like this too. I looked at the leaves on the photo I had made with my mobile phone. There is no tinge of maroon or the stems, and veins are whitish. Then something clicked – I remembered that the 1910 record mentioned in Smith et al (2002) was also from Muirhouse. I’ll need to return in the next few days to gather fruits, to complete the identification. In gardening books, cultivated lettuce seeds are said to survive for six years. Perhaps the seeds lie dormant until an especially favourable year, and thus a site has the potential to form populations over decades. This is, of course, merely an hypothesis. I noticed something else when I compared my photos. The walls against which the plants are growing are the same design, the same brickwork. Both sites are Council-built housing schemes, and perhaps soil was transferred between them during construction.

Still-to-be-confirmed L. serriola at Muirhouse, July 2021. Photo: John Grace

Prince and Carter’s 1977 paper in Watsonia is probably the best guide for identification. They also show the distribution as recorded in the 1970s. The plant was then distinctly southern, being absent north of a line from Exeter to Birmingham. Now it is rather common in the midlands of England and stretches into the north (see the map below).

This is what Prince and Carter say about habitat:

L. serriola behaves as a ruderal in Britain. It is most commonly, though not always, found in places where large amounts of earth have been moved; new housing estates and the verges of new roads are typical sites. Very occasionally it is reported from natural habitats such as shingle banks, but in the main it is confined to places affected by man’s activities. It is never found on waterlogged ground although it may occur on the well-drained sides of dykes in low-lying districts. It is not found on acid peat although it grows well on fen peat; it is rarely found on skeletal calcareous soils”.

The evidence for it being ‘warmth-loving’ is compelling. Prince et al (1985) examined its distribution along the M5 motorway from Exeter to Birmingham. Distribution was closely related to altitude; it seldom occurred above 85 metres above sea level, but below 60 metres it was common. The temperature difference between sites that are only 25 m different might have been about 0.2 degrees Celsius but this small difference was evidently enough! The species is most frequent in central Europe extending to north Africa but scarce in Scandinavia.

Distribution of Lactuca serriola in Britain and Ireland. Green squares are records from 2020. From BSBI.

There is one other Lactuca in Britain: L. saligna, the Least Lettuce. It’s rare and not in Scotland. You can find more about Lactuca by clicking on LACTUCA.doc (bsbi.org). You can also see my blog on the Great Lettuce (with achene images by Chris Jeffree showing the characteristic maroon colour) at https://wordpress.com/post/botsocscot.wordpress.com/2129 published in November 2020.

There is also the Wall Lettuce, Mycelis muralis (see the blog of August 2nd 2021 at https://wordpress.com/post/botsocscot.wordpress.com/4560. In the past it was called Lactuca muralis, and rumour has it that Wall Lettuce will be deemed to belong to Lactuca again in the future.

Lactuca serriola is also known as the compass plant because of a tendency of the leaves to twist round to align to the north-south plane. This isn’t a good character for identification – in urban settings the leaves are somewhat random.

If you find a wild lettuce you may taste the leaves. They are bitter, quite nasty.

John Grace

References

Prince SD & Carter RN (1977). Prickly Lettuce (Lactuca serriola L.) in Britain. Watsonia 11, 331-338.

Prince SD, Carter RN  & Dancy KJ (1985). The geographical distribution of prickly lettuce Lactuca serriola II. Characteristics of populations near its distribution limit in Britain. Journal of Ecology 73, 39-48.

Smith et al (2002) Plant Life of Edinburgh and the Lothians. Edinburgh University Press.

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