This week we are looking at one of the many daisy-like flowers in the family Asteraceae. The Sea-Mayweed, Tripleurospermum maritimum, is a favourite of mine. I once grew it in my garden where it formed a huge sprawling mass of feathery leaves and many bright flowers. It got out of hand, and my family persuaded me to remove it to the compost heap. It’s a short-lived perennial plant, but I have not been able to determine how long it lives.
Tripleurospermum maritimum on the seashore. Ballantrae, Ayrshire. Photo: John Grace
Sea Mayweed is found in coastal habitats including towns all around the Scottish coast, where it flowers freely from June to September. It usually has several sprawling stems, mostly trailing on the sand or shingle where it grows, with finely divided leaves which are slightly fleshy. The fleshiness, a sure indicator of salt tolerance, is one feature that distinguishes it from a closely related inland species, the Scentless Mayweed Tripleurospermum inodorum.
Tripleurospermum maritimum on the rocks, Maidens, Ayrshire. Photo: John Grace
The stems of both species are somewhat woody and branched; those of T. maritimum frequently turn purple in late summer and autumn. The flowers are much the same in the two species: 3-5 cm across, with yellow centres (made of a few hundred tiny tubular ‘disk florets’), and 15-30 white ‘petals’ (each one is a ‘ray floret’). Both species are visited mainly by flies and each is cross pollinated. Both are more or less ‘scentless’. In the British Isles both species have 18 chromosomes but in Central Europe T. inodorum is often polyploid and many individuals have 36 chromosomes (Certner M et al. 2017).
Tripleurospermum inodorum as a garden weed, Edinburgh. Photo: John Grace
T. maritimum is a tough plant. It can tolerate high levels of salinity and high concentrations of heavy metals (Ievinsh et al. 2021). It responds well to nitrogen fertilisation and that may be one reason why it flourishes so well where sea-birds are nesting. Sobey and Kenworthy (1979) studied breeding sites of gulls on the East coast of Scotland. Sea Mayweed was one of the six species they commonly found, whereas the sites with no gulls were covered by a perennial grass sward in which Red Fescue was the dominant species. Four principal gull activities affected the vegetation of breeding sites: treading, digging of scrapes and collecting of nest-materials, clashes associated with territorial disputes and defaecation.
T. inodorum, leaf tip, showing the tiny bristle at the tip. T. maritimum lacks the bristle. Photo: John Grace
It’s a highly variable plant; usually prostrate in its maritime habitat (drift-line and spray-zone). Being near the ground is probably a useful trait on the sea-shore, helping the plant to avoid scouring by wind-borne sand, and also on cliffs where it might otherwise be torn from the ground. But in sheltered spots it is taller, more like the annual Scentless Mayweed Tripleurospermum inodorum. It was once thought that the prostrate/erect variation was simply an environmental influence but when the contrasting forms are raised in a common garden the differences remain (so they must have a genetic basis). To make matters complicated, T. inodorum and T. maritimum can interbreed to form fertile hybrids (Kay 1972).
Disk florets of the two species, 3 mm long. Each floret has an outer tubular calyx and an inner yellow flower (tubular with five petals). I find no difference between the two. Google Lens thinks they are components from some electronic device! Photo: John Grace
Kay (1972) examined the differences between mayweeds at sites all over Britain and recently Crossley and Skilbeck (2021) looked for hybrids in Orkney. The conclusion we can draw is that hybrids may be rather common. This has a bearing on what name we should give to those plants that look like T. maritimum when we find them far from the sea, and also raises the question of whether the two Mayweeds are in fact merely forms of the same species (and clearly it depends on the answer to the age-old question of ‘what is a species?’).
There is one morphological trait which seems to be a reliable indicator of which species we have. You must look at the achenes with a hand-lens to see it. You need flowering heads that have ‘finished’ so that the achenes are ripe and loose. If you are new to this, you need to know that the achene is the seed and its dry seed-case, and in the daisy-family the achenes come away easily of you attack the inflorescence with your fingers. In ripe seed-heads, using a x6 hand lens you will see two darker patches on each achene – they are called ‘oil glands’. In T. inodorum they are more-or-less circular and in T. maritima they are somewhat elongated.
The ‘oil glands’ of T. maritimum are the yellow patches on the white surface of the achene. This one is a fairly early stage: they get darker. It isn’t easy to see them without a dissecting microscope but this image comes from my mobile phone. A hand lens can be used but sprinkle the achenes on a table top to avoid wobbling in and out of focus. In this image, the glands are elongated (more than 1.5 times longer than broader), confirming that the plant is Sea Mayweed not Scentless Mayweed. Image: John Grace.
There are other differences in the structure of the achenes – if you are interested you should access Crossley & Skilbeck’s 2021 paper by clicking on the link below. They have images. My guess is that the differences in the achenes relate to flotation. I think that seeds of the maritime species will be better floaters and therefore be efficiently dispersed by wave action.
This difference in the achenes has been realised for some time and is well described in Clapham et al. (1987). The best of the popular illustrated handbooks try to illustrate the feature (Streeter et al. 2009). But for the beginner it is hard to see details of the achenes!
There is another character that I’ve been investigating. The leaf tips of T. inodorum end with a tiny bristle (see my attempt to photograph it above). Not all leaves have it: possibly it gets brushed off.
Of course it can be frustrating to determine species that are so closely related. I notice that the BSBI website has separate maps for Tripleurospermum maritimum s.s. and Tripleurospermum maritimum s.l. (s.s. means sensu stricto in the strict sense and s.l. means sensu lato in the broad sense). Those in the broad sense are inland as well as maritime. Some have attempted to allocate observations to various subspecies, following Stace (2019).
The two species have different histories. T. maritimum is a native species, first recorded in Gerard’s Herbal in 1597; but T. inodorum is an archaeophyte (a species thought to have been introduced by humans before the year 1500 CE). Stace and Crawley (2015) make the point that many of our archaeophytes are ancient weeds of gardens and cornfields: T. inodorum is one of these along with Corn Chamomile Anthemis arvensis, Stinking Chamomile Anthemis cotula, Cornflower Centaurea cyanus and Corn Marigold Glebionis segetum. We still find T. inodorum in neglected corners of fields but mostly it’s on waste ground, soil heaps and derelict sites.
Tripleurospermum maritimum on the foreland at Ballantrae, in Ayrshire, growing under the bows of an abandoned fishing boat, where it escapes competition from the more aggressive Lolium/Festuca grass sward. Photo: John Grace
References
Certner M et al. (2017) Evolutionary dynamics of mixed-ploidy populations in an annual herb: dispersal, local persistence and recurrent origins of polyploids. Annals of Botany, 120, 303–315.
Clapham AR, Tutin TG and Moore DM (1987) Flora of the British Isles, 3rd edition. Cambridge.
Crossley, J. & Skilbeck, C.A. 2021. Sea Mayweed Tripleurospermum maritimum and
Scentless Mayweed T. inodorum (Asteraceae) intermediates in Orkney. British
& Irish Botany 3(3), 297-323. https://doi.org/10.33928/bib.2021.03.297
Ievinshi G et al. (2021). Tripleurospermum maritimum from a coastal shingle beach: nitrophilic status, tolerance to salinity and heavy metals. Environmental and Experimental Biology 19, 265–273. http://doi.org/10.22364/eeb.19.25.
Skilbeck, C.A., Lynch, I., Ellenby, M. & Spencer, M.A. 2019. Achene Morphology of
British and Irish Mayweeds and Chamomiles: implications for taxonomy and
identification. British & Irish Botany. 1(2), 128-166.
https://doi.org/10.33928/bib.2019.01.128
Sobey DG and Kenworthy JB (1979) The relationship between herring gulls and the vegetation of their breeding colonies. Journal of Ecology 67, 469-496. https://www.jstor.org/stable/2259108
Stace CA (2019) New Flora of the British Isles, 4th edition.
Stace CA and Crawley MJ (2015) Alien Plants. Collins, London.
Streeter D et al. (2009) Collins Flower Guide. Collins, London.
©John Grace
Botany in Scotland 