Trifolium medium L., Zigzag Clover

Account Summary

Growth form and preferred habitats

In B & I, this is generally regarded as a species of unmanaged grassland on heavy clay, neutral soils of intermediate or richer fertility. It is a robust, long-lived, rhizomatous perennial, tolerant of moderate shade and therefore capable of emergence through the tall canopies of derelict, neglected or lightly-grazed rough marginal grasslands and grassy banks at the base of hedges, along woodland margins and developing scrub, including on heaths. Typical habitat for T. medium is among medium-sized perennial herbs at the transition between unmanaged, rough grass and invading scrub (Sinker et al. 1985).

T. medium can also tolerate both non-calcareous and limestone soils of intermediate or near neutral pH (ie pH=7.0), provided they are neither regularly droughted nor waterlogged (Grime et al. 1988, 2007). It is typically absent from chalk (Crawley 2005). T. medium can also occupy more ruderal habitats such as along railway lines and occasionally in old quarries. In Ireland, it is sometimes found on heathland scrub and it can also occur in B & I on upland stream banks and in tall-herb communities on rock ledges (D.A. Pearman, in: Preston et al. 2002). Although after flowering it usually dies down in winter, in B & I, T. medium is described as being tolerant of frost and is said to be winter hardy (Duke 1981).

While it demands light and warmth for good growth and does not survive in gloomy woodland conditions, in fully open situations and on bare ground, T. medium finds it difficult to survive competition from better adapted colonising and meadow species. In Finland, it thrives in more open areas within hillside forests and does particularly well both in storm damaged woodland and in other areas open due to either logging or fires. Under Finnish conditions, T. medium appears to have exploited situations involving human activity, such as forest clearance, and the moderate rate of change involved in hillside forest conversion into grazing land. It has spread there from its original habitats to become a companion of people, including in neglected grass and ruderal situations (NatureGate website at https://luontoportti.com/suomi/en/kukkakasvit/zigzag-clover).

The established strategy of Zigzag Clover is described as intermediate between stress-tolerant competitor and the more general ecological role balance of C-S-R by Grime et al. (1988, 2007). It is important to bear in mind that Grime (1979, 2001) placed low nutrient availability at the core of his thinking on plant adaptation to survival in stressful environments (Craine 2009).

Identification

T. medium is rather similar in appearance to T. pratense (Red Clover), but it has narrower, oblong leaflets without white markings and stipules that taper evenly to a point, but do not possess the hair- or bristle-like leaf-tip characteristic of Red Clover. The English common name 'Zigzag Clover' refers to the appearance of the stems. The inflorescences are also stalked (not sessile) and the flowers are of a somewhat brighter red colour than those of T. pratense (Parnell & Curtis 2012). Although it is sometimes possible to distinguish T. medium from T. pratense by the leaflets of the former being longer and narrower, the only really reliable distinguishing character between the two is the appearance of the tip of the basal stipules of their stem leaves. In T. medium, the free part of these stipules is linear-lanceolate (ie like the blade of a medium-length sword), tapering evenly and coloured green almost to the tip. The free portion of T. pratense stem leaf stipules are triangular-ovate, narrowing abruptly to a brown, bristle-like tip (Webb et al. 1996; Stace 1997; Crawley 2005).

Although Zigzag clover looks very like what some refer to as the 'King' of useful plants, Red Clover, T. medium has never itself been a significant fodder crop.

Flowering reproduction

T. medium flowers from June to September. The numerous, globose, bright, purple-red inflorescences, 25-35 mm in diameter are borne on short stalks subtended by a pair of leaves. As with most clover species, T. medium must be cross-pollinated for any seeds to develop. The flowers are long-tubed, the corolla 12-20 mm long and the stamen filaments are fused from the base for most of their length, so that only quite large bumble-bees, honey- bees and certain butterflies and moths with a proboscis long enough can reach the nectar secreted at the base of the tube. Sometimes a small hole can be found at or near the bottom of the calyx-tube that indicates a visit from an insect with a short proboscis that has bitten a hole, broken into and robbed the nectar store. Obviously this robber activity does nothing to pollinate and fertilise the flower.

Fruit and seed dispersal

After fertilisation the legume pod develops, surrounded by the persistent fading corolla and the now spiny calyx. As in T. repens and some other clovers, the dead corolla plays a part in the seed dispersal by acting as a wing. In both T. medium and T. repens, the corolla does not enlarge, nor is it modified in any way as a flying organ, but it persists in a withered state, dry and scarious and enclosing the pod which contains one (or more) small, globose seeds. The withered flower becomes detached from the head of flowers and is blown away across the ground by the wind (Ridley 1930, p. 117). Eventually the 2.0 mm long membranous pod splits to release the solitary seed.

Apart from this and the rather remote possibility of the fruiting Zigzag Clover plant being grazed and the seed carried in an animal gut, there is no other form of seed dispersal mechanism. While it might be imagined that the seed probably has the usual legume hard coat dormancy and associated prolonged buried survival in the soil, the NW European survey of soil seed banks found that all five references consulted had concluded T. medium seed is transient, surviving for less than one year (Thompson et al. 1997).

Vegetative reproduction and population decline

The underground rhizomes of Zigzag Clover branch and spread to form clonal patches of the plant, but since the flowers are largely self-incompatible, many clones produce little or no seed. Fragmentation of the rhizome creates separate daughter colonies of identical genome, but a possible over-reliance on vegetative reproduction for population survival helps to explain why this clover is associated with older, neglected, undisturbed grasslands, such as occur on Lough Erne lake islands and on more inaccessible rocky shore headlands around the larger lakes in Fermanagh. It also suggests a reason for the current decline in this legume's occurrence throughout the whole of B & I, since T. medium was always fairly thinly scattered and was never a widespread, common species with a crop-based reservoir of repeatedly sown seed like other clover species. This is particularly the situation in Ireland, where the decline of the species from a low peak population is all the more clearly marked.

Fermanagh occurrence

Half of the 23 records of T. medium in the Fermanagh Flora Database are of pre-1950 date and RHN and the current author (RSF) consider it a very local, decidedly rare and casual, more-or-less ruderal species in the VC. The status of the plant is unknown, but as it previously would very likely have been included in agricultural clover seed widely and frequently sown for fodder, even as a contaminant, it might well be an undetected introduction. The Fermanagh records are scattered across a total of 20 tetrads, 3.8% of those in the county.

Irish occurrence

The New Atlas map shows T. medium widespread in Co Antrim (H39) and scarce in the other five VCs of N Ireland. Time will tell whether this indicates under- or over-recording or both! In the Republic of Ireland, nowadays, T. medium is a rare or very rare plant confined to damp roadside verges and banks (Doogue et al. 1998; Green 2008), or recently disturbed grassland in damp limestone pasture on the turlough-like bottom of shallow valleys (Reynolds 2013).

Previously, in the Burren, Co Clare and Connemara, Webb & Scannell (1983) regarded it as a rather rare species of roadsides, waste places and rocky ground and, "in all cases apparently a relic of cultivation." These authors also considered it, "obviously transient in many of its stations" [they listed a total of just nine sites], "in a few, however, it seems well established."

British occurrence

Zigzag Clover is much more generally distributed in lowland areas in Britain than in Ireland, although it remains a frequent to uncommon plant of heavy basic or clay soils. There are definite distribution gaps where it becomes local or rare, for instance in C Wales and around the English Wash. In Scotland, it becomes rare or absent in the N & NW, most likely due to the prevalence of unsuitable soils. T. medium reaches its maximum altitude in Britain at 610 m on Helvellyn in the English Lake District. The BSBI Monitoring Scheme Survey of 1987-88 concluded that none of the widely reported losses of T. medium populations across B & I that varied from -4% to -35 % were statistically significant (Rich & Woodruff 1990, map and figures in 2, p. 87). The New Atlas hectad map suggests possible under-recording by modern botanists, or perhaps equally probably, mistaken over-recording of it in past years. Analysis of the New Atlas survey data indicated a marked decline in S & E England since 1950 (D.A. Pearman, in: Preston et al. 2002). Evidence gleaned by the current author (RSF) from the biology and ecology of T. medium indicates there is little doubt that populations are declining in abundance and in some areas also losing ground across both B & I, most probably due to the near disappearance of lowland permanent grassland habitats in the last 60 years, together with progress in producing cleaner clover seed for agricultural grassland ley sowings (Grime et al. 1988, 2007; D.A. Pearman, in: Preston et al. 2002). However, the 'Local Change' monitoring sample survey of Britain, carried out from 1987-2004, did not detect any further decline (Braithwaite et al. 2006).

European and world occurrence

T. medium occurs throughout most of Europe except the extreme N & S. There is more variation within the species on continental Europe than exists in B & I, Flora Europaea 2 listing four subspecies, with intermediates occurring. Two of the subspecies are very local: subsp. sarosiense (Hazsl.) Simonkai is restricted to the foothills of the Carpathians; and subsp. balcanicum Velen., to the Balkan peninsula. The other two are subsp. medium, which is found throughout the range of the species, possibly excepting Greece; and banaticum (Heuffel) Hendrych, which is recorded from Czechoslovakia, Hungary and Romania (Tutin et al. 1968).

This polymorphic species was originally confined to Europe and adjacent parts of Asia but has been spread by man and his activities, although not to anything like the same extent as T. pratense, T. repens and a few other grassland ley clover species. T. medium has been introduced, however, to N America, north central China, Japan, Tasmania and both islands of New Zealand (Hultén & Fries 1986, Map 1246). In New Zealand it is regarded as, "a rare casual [introduction] of waste places and cultivated land, formerly more widespread" (Webb et al. 1988).

Threats

Older undisturbed grasslands are vital to its mere survival, and the NI Government Environmentally Sensitive Areas (ESA) policy should have assisted its maintenance.

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