Rhinanthus minor subsp.
stenophyllus O. Schwarz, a yellow-rattle

Account Summary

Growth form and recognition

A conspicuous, obligate hemi-parasitic, almost glabrous annual up to 1-30(-50) cm tall, with a shallow fibrous root and erect, simple or branched, usually black-spotted, hard, angular stems and opposite, narrowly oblong or linear toothed leaves and yellow, two-lipped flowers arranged in loose, leafy, spike-like racemes (Perring & Walters 1989). Yellow-rattle is unmistakable thanks to its very prominent, pale-yellow inflated but flattened calyx. Thanks to this distinctive, persistent calyx, even the still standing dead 'ghost' of the plant can be readily identified long after the seed has been shed. The only plant that might conceivable be confused with it, is the sticky and, in Fermanagh, extremely rare, Parentucellia viscosa (Yellow Bartsia).

Obligate partial-parasite ecology

Species of Rhinanthus are mainly found in low-sward, open grassland communities, and the chicken-and-egg question is asked, are they present due to their habitat preference, or is the vegetation a reflection of their presence? R. minor is shade intolerant and an open sward is likely to be important to allow its initial seedling establishment in spring. Being a hemi- or partial-parasite, the roots of seedling Yellow-rattle attach themselves by haustoria to the already established roots of neighbouring plants, mostly grasses, but also legumes, eg Trifolium repens (White Clover), creating xylem to xylem connectivity (Hodgson 1973). When this root connection is made, it enables the hemi-parasite to steal a proportion of its nutrient and solute requirements directly from the host plant. Having said this, there is no evidence of the parasite benefitting from acquisition of phloem-borne photosynthates from its host. The benefits are restricted to xylem-borne water, dissolved minerals and some organic compounds such as reduced nitrogen in the form of amino-acids, plus some organic carbon (Jiang et al. 2004). In order to withdraw sap from the host xylem, the parasite must compete with the host's transpiration stream by maintaining an osmotic potential gradient below that of the host plant. High concentrations of sugar alcohols, such as mannitol, help R. minor to maintain this osmotic balance (Hodgson 1973).

Subsequent survival of R. minor, especially in a productive grass-clover sward, is based on the parasite's ability to make use of its host and, at the same time, it modifies the competitive relationship between the parasite and its host in the former's favour. Consequently, the presence of R. minor in open grassland communities is strongly related to management and the associated soil fertility/productivity level of any particular site (Westbury 2004). Yellow-rattle can infect a wide range of host plants, enabling it to colonise and establish itself in different grassland types, although it cannot invade all grassland communities equally well. Nevertheless, hemi-parasites like R. minor can invade closed grassland habitats where other annual species are either rare or completely absent (Westbury 2004). R. minor plants have been observed to infect several hosts simultaneously. In sand dune vegetation, of a population of 65 R. minor plants studied, five were attached to just one host plant, and two to at least seven different hosts (Gibson & Watkinson 1989). The list of potential hosts for R. minor determined from four studies in Britain (Gibson & Wilkinson 1989) and one in Europe (Weber 1976), included 50 species from 18 plant families. Of these the Fabaceae (Legumes) accounted for eleven species and the Poaceae (Grasses) 16. Nine of the species were annuals or short-lived perennials, 36 were herbaceous perennials and five were woody species (Gibson & Watkinson 1989). Legume species are considered good hosts for hemi-parasitic species, the additional nitrogen supply enabling an increased biomass and fecundity (Westbury 2004).

Some degree of autotrophic growth without host support can occur in R. minor, but such individuals are frequently stunted, growing only 5-7.5 cm tall; they do not form branches and may fail to flower. In any event, they do not produce seed at levels comparable with heterotrophic hemi-parasitic individuals (Westbury 2004).

Apart from its predominant association with meadows on soils of moderate to low fertility, R. minor also features, to a more limited extent, in pastures and from ungrazed grassland, mire and roadside verges, as well as displaying atypical occurrences on rock outcrops (Grime et al. 1988, 2007).

Soil and habitat preferences

R. minor grows in permanent, less disturbed, infertile, damp grassland in a wide variety of soil and habitat situations. It can occur on substrates ranging in character from clay to sand, chalk, limestone and occasionally peat, but it is absent from sites with a soil pH below 5.0. It can become locally abundant in neutral or base-rich grassland. One of the soil types it rather commonly frequents is alluvial or glacial till laid down in moraines or in drumlins. This type of soil can have either poor to impeded drainage or be free-draining. In the latter case, the traditional farming practice of applying lime and farmyard manure will buffer the loss of minerals caused by leaching, thus maintaining the soil pH and favouring the survival of R. minor (Westbury 2004).

Traditional agricultural management of meadow/pasture grassland by first cutting hay in late-July, followed by autumn grazing cattle on the same ground, leads to an opening up of the sward for many species, including R. minor, by reducing standing biomass and creating bare ground through trampling. The effect of this management is greatest in soils of low to moderate fertility (Westbury 2004).

In terms of soil moisture regimes, Yellow-rattle cannot tolerate drought, but it can cope with meadows that become temporarily waterlogged in winter and the species extends occasionally into the margins of soligenous mire (ie lake-shore fen conditions) (Grime et al. 1988, 2007; Westbury 2004).

However, Yellow-rattle is most typically a plant of hay meadows and is especially associated with those of high floristic diversity (Grime et al. 1988, 2007). Being an annual, its survival in any location depends on its ability to release seed before the hay crop is cut. The very widespread agricultural move in B & I from hay to silage fodder production, the latter of which is cut much earlier, together with the reseeding of meadows and the spread of artificial inorganic fertilizers and slurry, have all resulted in a decline in the field prevalence of R. minor populations nationwide. Yellow-rattle persists, however, in sites that are more marginal and in areas where intensive agriculture is either uneconomic or impossible, for instance due to being inaccessible to large, modern machinery.

Phenology and flowering reproduction

Seed germinates in spring and plants flower from May to August. Each terminal raceme inflorescence contains less than 30 bisexual flowers, each one with a flattened and inflated calyx with four teeth and the upper lip of the bright or pale yellow corolla flattened from the side, enclosing the four stamens. Nectar is secreted at the base of the ovary and the flowers attract insects, mainly bees and bumblebees, that carry out pollination. If this fails to occur, the flowers can self-pollinate. As commonly happens in numerous other tubular flowers (eg Gentiana spp.), some Bombus bee species bite a hole at the base of the calyx and corolla and steal nectar, thus bypassing the legitimate flower entrance and avoiding enacting the pollination mechanism of the species.

Seed is set from June through to September. The capsule, 12-17 mm, is brownish, ovate and compressed (Sell & Murrell 2007). Seed production obviously varies enormously with vegetation, environment and season, but as an indication of typical reproductive success, 171 R. minor plants in pasture had a mean capsule number of 17.30 ± 0.96, each containing an average of 9.7 ± 0.1 seeds, resulting in a calculated seed output of 168 ± 11 per plant. "It does not therefore appear that the production of a large seed output is in any sense a necessary concomitant of the semi-parasitic habit."(Salisbury 1942).

Whenever the capsule is mature, the large pale yellow ovate seeds, 3 × 2 mm, each of which is laterally compressed and possesses a broad semi-circular wing (Butcher 1961), become loose inside it, and thus older fruiting racemes make a distinctive rattling noise when shaken, a feature reflected in the large range of English common names given to the species (see below). The ripe capsules split open at their apex and the stiff stems of the plant are shaken by wind or passing animals, thus scattering the winged seeds on the breeze.

The distance travelled by seed in this manner is slight or poor, unless aided by man. Seedlings tend to be located at no more than 1.45 m from the edge of an existing population. Mowing machinery has been shown to assist seed dispersal both in terms of distance and seed number dispersed and hay cutting and transportation have also been shown to be an effective means of seed dispersal (at least 4 m), better even than sheep grazing (a maximum of 0.9 m) (Westbury 2004).

Dormant seed is reported to be ± transient, ie not persistent for more than a year in the soil seed bank, germination taking place in the spring following production (Roberts 1986).

Variation in population density

Populations of R. minor are often observed to fluctuate greatly in density from year to year and from site to site. On a sand-dune grassland in Norfolk, natural population densities varied from 100-200 flowering plants per m² in dense patches, to 0-50 m² in less dense areas (Gibson 1986), while in a SSI meadow in W Yorkshire, populations ranged from 596 to 1004 individuals per m². These figures are considered rather low in comparison to populations in a twelve year old meadow in Canada, where mean high density was measured at 4,382 ± 1,236 per m², and low density at 405 ± 183 individuals per m². However, these Canadian figures were for densities at the beginning of the growing season, and losses of plants occur throughout the year, and obviously more noticeable at the higher density at the start of the season (Westbury 2004).

Populations of R. minor are strongly regulated by sward composition and structure, the number of individuals being relatively low in more productive swards, indicating the limited competitive ability of Yellow-rattle when faced with vigorous growth in its potential host plants (Westbury 2004). The established strategy of R. minor was categorised as R/SR, meaning intermediate between a straight Ruderal and a Stress-tolerant Ruderal (Grime et al. 1988, 2007).

Toxicity and grazing

Being totally dependent on seed production each year for its local survival, Yellow-rattle in pasture land is also extremely sensitive and vulnerable to heavy grazing pressure. However, as the plant is somewhat poisonous, containing glycosides that presumably render it unpalatable, for this reason animals tend to avoid grazing it. Stock will only browse on R. minor if they are really very hungry or are actually starving (Cooper & Johnson 1998).

On account of its semi-parasitic nature, Grigson (1955, 1987) referred to Yellow-rattle as, "another of the pretty infirmities of the land". He also quotes Gerard's Herbal, "What temperature or virtue this herbe is of, men have not as yet been carefull to knowe, seeing it is accounted unprofitable." (Gerard 1597).

Fermanagh occurrence

Yellow-rattle is a common and widespread species in Fermanagh, having been recorded in 150 tetrads, 28.4% of those in the VC. It is found chiefly in and around damp or even flushed patches of ground in agricultural grasslands, but also on drier parts of fen-girdled lakeshores, by rivers and on cliff ledges and in quarries. It is more prevalent in the SW half of the county.

Variation

The genus Rhinanthus is very variable and contains about 50 species in the north temperate regions of the world (Sell & Murrell 2007). R. minor is a very variable species and a number of different schemes of subdivision have been proposed. While Praeger was referring plants of wetter habitats to R. stenophyllus in 1936 (see the account below), now sometimes considered one of six subspecies in the flora of B & I (New Flora of the BI 1991, 1997), others feel we do not yet have sufficient information to put names on Irish variant forms (An Irish Flora 1996; Parnell & Curtis 2012).

Six intergrading and inter-fertile subspecies, based mostly on geographic and ecological evidence (F.J. Rumsey, in: Preston et al. 2002; Stace 2019), are now recognised in the flora of the BI, however the variation is complex and continental plants are even more complex. Some forms in Britain simply do not fit comfortably into any of the proposed subspecies and Stace (2019) has suggested that it might indeed be better to abandon the subspecies altogether.

Variation in R. minor is described by Sell & Murrell (2007) as, "a combination of ecotypes, geographical races and seasonal variants".

British and Irish occurrence

As the New Atlas hectad map indicates, R. minor is very widespread throughout the whole of B & I with the exception of intensively cultivated arable land around Cambridgeshire (VC29), wet ground in the English Wash, plus in parts of N Essex (VC19). In Ireland, it is least common or absent in parts of Co Wicklow (H20) and further west in N Kerry (H2) and Co Limerick (H8). Populations declined in both B & I during the 20th century, but have recovered somewhat due to the use of R. minor as a component of colourful 'Wild-flower seed mixtures' widely and commonly sown in gardens and council amenity areas (F.J. Rumsey, in: Preston et al. 2002).

European and world occurrence

R. minor is a mainly European species, being widespread throughout most of the continent from Iceland southwards, although becoming rare around the Mediterranean basin. Having said this, it does penetrate the whole length of Italy from N to S. In N Sweden, it is regarded as an alien introduction (Seel & Press 1993). It is widely naturalised beyond its native range and consequently is probably not native in Asia and N America, having been transported into the latter in imported hay (Fernald 1907; Hultén & Fries 1986, Map 1702).

Names

The genus name 'Rhinanthus' is a combination of the Greek 'rhinos', 'a nose' and 'anthos' 'a flower', ie 'nose flower', referring to the trunk-like upper corolla lip of the Mediterranean species R. elephas L., which is now placed in a completely different genus, Rhynchocorys (Gilbert-Carter 1964). The Latin specific epithet 'minor' means 'lesser' or 'smaller' (Gilbert-Carter 1964).

Grigson (1955, 1987) lists a total of 48 English common names for Rhinanthus minor of which at least twelve include 'rattle' as a word element, all clearly referring to the characteristic noise made when the ripe capsules are shaken. Numerous other names such as 'Money in the purse', or 'in the box' or 'the basket' also allude to the same property of the fruit. Vickery (2019) notches up a list of 50 English common names, indicating that 'Shackle- 'bags', or 'basket' or 'box' or 'caps' also refers to 'rattle' in various parts of the country.

Threats

Changes in agricultural practice associated with intensification have reduced the incidence of the species, but it persists in more marginal sites and in areas where intensive agriculture is uneconomic or impossible.

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