Plant of the Week – November 22nd 2021 Yellow Rattle (Rhinanthus minor)

Rhinanthus_minor_(Icones_Florae_Germanicae_et_Helveticae_1862,_v20,_plate_1738,_BHL-29363)_clean,_no-description.png (1973×2497)
Reichenbach, L.; Reichenbach, H. G.;, Public domain, via Wikimedia Commons

The species was first recorded by the herbalist John Gerard in 1597. He called it Penny-grass because the husky brown seed capsules looked like pennies. On habitat, he noted that it “groweth in drie meadows and is to them a great annoiance”. Its modern name, Rhinanthus, was given by Linnaeus and is derived from the ancient greek words for ‘nose’ and ‘flower’. He thought the upper lip of the flower looked like a nose. Readers can examine the image below and see if they can see the nasal resemblance. The modern English name ‘Rattle’ is given because the rather large seeds rattle around noisily in their roomy capsules.

Rhinanthus minor, flowers. Photo: Chris Jeffree

My first memory of this flower is from Ireland, many years ago. I was a young student of Botany – we were taken on a field course to the Burren in the west of Ireland, a wonderful other-worldish site in County Clare where so many species grow on the limestone pavement. Just outside the little town where we stayed, Lisdoonvarna, was a hay meadow with masses of Yellow Rattle and other brilliant wild flowers. What a great sight in the summer sunshine!

Rhinanthus belongs to the family Orobanchaceae, the broomrapes. Until recently it was placed in the figwort family (Scrophulariaceae) and it does indeed look somewhat like a figwort, but after deliberation in 2016 by a panel of scholars known as the Angiosperm Phylogeny Group, and taking into account evidence from molecular genetics, it has been moved.  In fact, the figwort family was largely dismembered to the dismay of many.

Nearly all broomrapes are parasites, with root-like structures called haustoria that engulf roots of other plants and steal nutrients. In the case of Rhinanthus, the plant is merely hemiparasitic. Unlike its fully-parasitic relatives, it has chlorophyll, and all the required photosynthetic machinery. Moreover, it does have roots of its own and it can live by itself in a plant pot, albeit not quite as well as when it has a host. In the wild it thrives when attached to an unsuspecting host. Sometimes there is more than one host per individual plant.

Indeed Yellow Rattle seems rather promiscuous when it comes to the choice of host. Gibson and Watkinson (1989) painstakingly dissected turves to identify which plants were hosting R. minor. They found 20 hosts, most of them common plants. Adding their data to those of other researchers, there were 50 host-species from 18 families with 30% in the grass family and several legumes.

It is a native plant throughout Europe, Asia and northern North America and it can be seen almost anywhere in Britain. However, we have six subspecies with somewhat different distributions. They vary in morphology, hairiness and flowering time, some with branched stems and others unbranched. Stace (2019) has some helpful line drawings and a key, so if you find a Rhinanthus minor then you might go to Stace and try to identify the subspecies. The most common northern one is subspecies stenophyllum which is branched, and one of the tallest (up to 50 cm). However, there is much variability here and even more on the continent, leading Stace to suggest that the subspecies “may better be abandoned”. There is also another, completely different, species in Britain, R. angustifolius. This one has been recorded around London but there are only two records in Scotland, a recent one at Carnoustie.

Mature seeds of R. minor. Note that they are ‘winged’. Photo: Chris Jeffree.

Rhinanthus minor is a summer annual, germinating in the early spring and flowering in the same year. For germination it needs temperatures of only about 6 oC. It flowers and seeds from June to September and produces a small number of winged seeds per flower (2-10) which, as mentioned above, ‘rattle’ in their dry capsule. The seeds remain viable for only a few months unless stored under very special conditions (as in the Millennial Seed Bank at Kew). In the wild it requires several weeks of low temperature to break dormancy. Evidently it gets this necessary chilling in much of Europe (but will it always be so?).

As for habitat, Rhinanthus minor is typically found in meadows, often ‘hay meadows’ hence its alternative English name Hay Rattle. These days, hay meadows barely exist as they are the product of low intensity farming. However, the species is often seen in urban settings, being recorded 43 times in our Urban Flora of Scotland data-base of over 60,000 records of wild flowers. Sometimes local councils have added it to a seed mixture sown in parkland. not just because it is pretty and native (although it is definitely both of these), but because it might suppress some of the fast-growing and highly-competitive species and hence encourage biodiversity. Remember, Gerard said it ‘annoyed’ dry meadows. There is modern evidence that this approach to enhancing biodiversity usually works (Coulson et al 2001, Jefferson 2005). Horticultural companies have noticed this, and now offer packets of seeds with the appropriate warning about the short seed longevity. I ordered my seeds, and I’ve sown them in a little grass patch (after scarifying the ground with a rake, according to instructions). The seeds came with plenty of advice and so I hope that my garden sward, now dominated by fast growing grasses Dactylis glomerata, Festuca rubra and Lolium perenne, will soon become biodiverse.

A grassland very rich in R. minor. Photo: Chris Jeffree

How does it work? To steal nutrients from the host, the water pipeline (xylem) of the hemiparasite must be connected to that of the host, and the hemiparasite’s tissues must be somewhat ‘dry’ to provide the gradient for water to move from host to hemiparasite. Then, water and dissolved nutrients will flow. Most ordinary plants close their stomata at night to conserve water but Press et al (1988) discovered that hemi-parasitic plants do not. Their stomata stay open, they loose more water, and thus a water-gradient is maintained and any fast-growing host is starved of nutrients and thus likely to be at least ‘annoyed’ and probably stunted.

This reduction in the host’s vigour is detested by farmers in many parts of North America, where the plant is deemed to be a weed by the US Department of Agriculture. See this link for one person’s entertaining account of Rhinanthus as a weed https://www.fsa.usda.gov/Internet/FSA_File/stnr_nh_06172011_weed.pdf

References

Coulson S.J., Bullock J.M., Stevenson M.J. & Pywell R.F. (2001) Colonization of grassland by sown species: dispersal versus microsite limitation in responses to management. Journal of Applied Ecology, 38, 204-216.

Jefferson R.G. (2005) The conservation management of upland hay meadows in Britain: a review. Grass and Forage Science, 60, 322-331.

Press MC, Graves JD and Stewart GR (1988) Transpiration and Carbon Acquisition in Root Hemiparasitic Angiosperms Journal of Experimental Botany, Volume 39, Issue 8, August 1988, Pages 1009–1014, https://doi.org/10.1093/jxb/39.8.1009

Stace C (2019) New Flora of the British Isles. C&M Floristics.

John Grace

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