Tripleurospermum inodorum (L.) Sch. Bip., Scentless Mayweed
Account Summary
Introduction, archaeophyte, occasional. Eurosiberian temperate, but widely naturalised in W & E Asia and N America.
1900; Praeger, R.Ll.; Co Fermanagh.
May to November.
Growth form and preferred habitats:
T. inodorum is an erect or spreading, winter- or summer-annual, ruderal weed growing up to around 60 cm tall. It is a plant of lowland, mainly cereal-growing arable farmland and is occasional on lightly, but regularly disturbed roadsides and in trampled wayside paths. While essentially lowland, Scentless Mayweed has been recorded at around 560 m above sea level in Britain. In soil terms, it occurs on both heavy and well-drained, mostly fertile substrates of mildly acid to calcareous nature. It is much rarer on heavier soils than on those of light and medium texture. Scentless Mayweed shows no definite calcicole or calcifuge tendencies whatsoever in B & I, although it may do so in Continental Europe (Kay 1994). While this Daisy-like Mayweed is capable of germinating at almost any time of year, and has rosettes that are perfectly hardy and wintergreen, T. inodorum can also, very occasionally, behave as a short-lived, polycarpic, semi-rosette perennial (Grime et al. 1988, 2007; Kay 1994).
Since arable agriculture in many areas of B & I, including Fermanagh, has almost ceased, it appears to be capable of surviving as small populations in a range of other types of disturbed, ecologically open sites in both rural and urban settings. T. inodorum also occurs as an impurity of grass seed (Fryer & Makepeace 1977) and, locally in Fermanagh, this helps to explain its continuing occurrence on lakeshore meadows and pastures in the area. T. inodorum is now essentially a common ruderal colonist of disturbed, artificial habitats and it is likely that it will increase its presence, thanks to its very large seed production capability and considerable seed longevity in the soil seed bank (Grime et al. 1988, 2007).
Flowering reproduction:
The characteristic large, scentless flowerheads range from 2.0-4.5 cm in diameter and are produced from July to September on long, smooth, terminal flower-stalks. The flowerhead is hemispherical and the involucral bracts are in several rows, nearly of equal length and with brown jagged margins. There are about 25 white ray-florets, 10-20 mm long and female, while the inner yellow tubular disc-florets are bisexual and number around 400 per flower-head. The flowers are mainly visited by flies that pollinate them, and they are largely, but not completely, incompatible (Kay 1994).
T. inodorum is like many other ruderal weeds, phenotypically plastic and extremely variable in size and habit. Thus the number of flowerheads per plant can vary enormously with the growing conditions, from one to around 1,200. However the usual range of flowerhead numbers is much more conservative and lies between 15 and 200 (Kay 1994).
The achene fruits, 2-3 mm in length, are very distinctive in appearance: they are yellowish-brown or greyish in colour and have three broad ribs on their inner face and two dark oil or resin glands attached near the top of the outer face (like a tiny pair of eyes) with a thin, wavy, frill-like reduced pappus rim above (Melderis & Bangerter 1955; Salisbury 1964; Perring & Walters 1989).
Fermanagh occurrence:
T. inodorum has been recorded in 46 Fermanagh tetrads, 8.7% of those in the VC. As the tetrad distribution map indicates, it appears confined to lowlands in the C & E of the county.
T. inodorum and T. maritimum s.s. (Sea Mayweed) are interfertile to some extent and they were taxonomically and nomenclaturally confused until 1969 (Kay 1972). Since not all botanical recorders distinguish them, the true distribution of the separate species is not yet known (New Atlas). Fermanagh records pre-dating the current VCs (RSF & RHN) work were assigned in the Revised Typescript Flora to T. maritimum subsp. inodorum (L.) Hyland. ex Vaarama.
British and Irish occurrence:
T. inodorum is an archaeophyte (ie ancient introduction), present since at least the Bronze Age; as the BSBI Atlas 2020 hectad map shows, it is well distributed throughout B & I, but is scarce in the Scottish Highlands and in parts of C & W Ireland (H.J. Killick, in: Stroh et al. 2023). Scentless Mayweed is the most widely distributed large-flowered, annual Mayweed species in B & I and in most parts of both islands it is, or was, by far the most abundant of the four annual Mayweeds that grow as weeds on arable land, the others being Matricaria recutita (Scented Mayweed), Anthemis arvensis (Corn Chamomile) and A. cotula (Stinking Chamomile), and only the latter of these definitely occurs in Fermanagh (Kay 1994; H.J. Killick, in: Stroh et al. 2023). T. inodorum has been in decline for some time in both B & I, but since this species was only distinguished and separated from T. maritimum 50 years ago (Kay 1972), not every recorder recognises the separation and the interpretation of mapped records must therefore be approached with some degree of caution.
In the longer term, since the majority of T. inodorum populations consist of individuals with flowers that are strongly self-incompatible (Kay 1994), viable populations must consist of several genetically distinct individuals in order that fresh seed may be set. Regular, or at least occasional, introduction of new genetic individuals is considered essential for survival, otherwise Scentless Mayweed will suffer the decline to extinction witnessed in other arable annual weeds.
European and world occurrence:
T. inodorum belongs to the Eurosiberian temperate phytogeographical element and its range stretches from N Spain, N Italy and the Balkan Peninsula northwards to include most of Scandinavia, reaching 66oN. The distribution extends eastwards to Russia, W Siberia, C Asia and southeast to the Caucasus. Scentless Mayweed has also been introduced to E Siberia, N America and New Zealand plus to other temperate regions as an agricultural weed (Hultén & Fries 1986, Map 1809; Kay 1994). The map published by Hultén & Fries (1986) shows T. inodorum as a subspecies of T. maritima with it being very rare or absent from much of the Mediterranean. The likely reason for the latter absence is that T. inodorum populations cannot survive high summer temperatures and the associated drought conditions (Kay 1994).
Names:
The current genus name 'Tripleurospermum' (previously it was in the genus Matricaria) is a combination of the Greek 'tri', 'three', 'pleuron', 'rib' and 'sperma', 'seed', an allusion to the three-ribbed achene fruit (Stearn 1992). The Latin specific epithet 'inodorum' means 'unscented' (Stearn 1992).
Threats:
Small isolated populations are unlikely to persist in the absence of arable cultivation.
References
Kay, Q.O.N. (1972); Kay, Q.O.N. (1994); Fryer, J.D. and Makepeace, R.J. (1977); Meikle,R.D.(Ed.), Carrothers,E.N., Moon, J.McK. and Davidson, R.C. (1975); Stearn 1992; Hultén & Fries (1986); Melderis & Bangerter 1955; Salisbury 1964; Perring & Walters 1989; Grime et al. 1988, 2007; Stroh et al. 2023; New Atlas