Plant of the week – January 31st 2022 – Winter Aconite – Eranthis hyemalis

The winter aconite, Eranthis hyemalis (L.) Salisbury, is a beautiful, diminutive winter flowering plant. It is not really an aconite and was originally interpreted as a type of hellebore, to which it is closely related, (other common names include winter hellebore, winter wolf’s bane). It is one of the earliest plants in the British flora to flower and may help the bees to cope with winter warming.

It is a neophyte, introduced to Britain, from Europe.  Exactly when, why, how and by whom are now lost in the mists of time but according to the Biological Records Centre “E. hyemalis was introduced as a garden plant by 1595”.  The first report of it in the wild was by Sir William Jackson Hooker (1785 –1865) in East Suffolk in 1820. He was an English botanist and botanical illustrator who became the first director of Kew Gardens in 1831. By the time of his report, he might have known it as Eranthis hyemalis, but originally the species was described by Carl Linnaeus as a hellebore.

Eranthis hyemalis is a tuberous herbaceous perennial flowering plant. It is among the earliest flowering plants in the British flora, first appearing from late December onwards, depending on seasonal temperatures. It is quite responsive to temperature, its flowers opening on bright days when the temperature reaches about 10^oC. The flowers last several weeks, remaining closed at night and in dull, wet weather, but opening fully on warmer, sunnier days.

It is not more than about 15cm in height. The leaves are all basal, arising from a tuberous rhizome, which is about 1cm in diameter at maturity with several conjoined, rounded lobes from each of which one or two unbranched flower stems arise, each with a solitary, upward-facing flower.  Immediately beneath the flower, three palmately-lobed bracts form a whorl that encloses and protects the flower as it rises backwards from the soil. There are no other stem leaves. The plants are entirely glabrous, without any waxy bloom, glandular or clothing hairs, the bracts are a dark glossy green with contrasting dichotomously branched veins.

The genus Eranthis is now placed in Jussieu’s buttercup family Ranunculaceae, but Linnaeus in his Systeme Sexuale, which was based solely on the number and arrangement of the sexual organs,  placed it in class 13, Polyandria: ‘flowers with many stamens inserted on the receptacle’. In his Species Plantarum (1753) 557, Linnaeus described specimens of winter aconite obtained from Italy as a species of hellebore, naming it Helleborus hyemalis. The very same type specimens were later renamed Eranthis by Salisbury 1807, who wrote “This plant appears to me a distinct genus, nor can I find any affinity whatever in it, to that in which it has hitherto been placed,” (referring to Helleborus) “beyond the common characters of the order; for the same Nectaria occur in several others.” The perianth is typically composed of two whorls of three petaloid tepals that are interpreted as sepals, so collectively the yellow tepals are the calyx.  The tepals of the inner whorl are slightly narrower than those of the outer, but are otherwise almost identical. They are all bright yellow as soon as they emerge from the ground. They lack the varnish-like gloss that is present on the petals of buttercups and lesser celandines, but the petals nevertheless reflect yellow light very strongly (see Plant of the Week Lesser celandine).

The stamens are numerous and there are 3 to 12 (but commonly 6) separate, short-stalked multi-seeded carpels called follicles. Between the inner whorl of sepals and the stamens there are approximately 6 tubular organs that secrete nectar.  These are in the position normally occupied by petals but are almost unrecognizable as such because they are heavily modified to secrete nectar rewards containing glucose, fructose and sucrose for pollinators. The identity of these organs was discussed at length by Linnaeus, Jussieu and by RA Salisbury (1807), because there is considerable overlap and indeterminacy between the types of organs in this and related genera in the Ranunculaceae. RA Salisbury, the authority for the name Eranthis, agreed with Linnaeus that the nectaries were additional structures, not petals. He rejected the idea, strongly argued by Jussieu, that they be raised to “the proud and exclusive dignity of the Petals”.  He was keen to refute the doctrine of the French School, with whom issues sometimes crackled with international tension. Plus ça change . . .

More than a century later, EJ Salisbury (1919) referred to these organs as “nectar leaves” and Clapham et al. (1962) as “nectaries”, avoiding any controversial reference to petals, but in most recent accounts, ranging from Bentham and Hooker (1945) to Stace edition 4 (2019) they are referred to as modified petals.

As mentioned above, the flowers are already formed underground and emerge out of the soil backwards, protected from damage by the enfolding bracts, as shown in the images below.

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Life cycle

Winter aconites are very easy to propagate from ripe seed if it is sown immediately into a place that will not be disturbed for about four years. Seeds will germinate in the spring following sowing and produce simple elliptical leaves that you might simply dismiss as weed seedlings.  First year seedlings will normally appear two to three weeks later than first flowering date of mature plants.

In the following years the young plants produce progressively more complex leaves, and they should flower in the fourth spring (occasionally in their third) after sowing. The seeds ripen in early March and are shed green. If sown immediately and left undisturbed until the following spring, germination rate is high, but dried seeds may not germinate easily. Tubers are best transplanted in the green because viability is greatly reduced if they are allowed to dry out.

Habitat and distribution

Eranthis hyemalis is a plant of deciduous woodlands, an ephemeral that exploits the brief absence of a shading leaf canopy during the early spring before the trees come into leaf. It grows very well in full sun or semi shade, ripens its seeds quickly and disappears below ground for the summer just as the first trees burst their buds.

The native distribution of Eranthis hyemalis includes southern and eastern Europe, from Italy to Bulgaria and Turkey (BRC Online Atlas), SE France, former Yugoslavia and Lebanon/Syria (POWO).

In the British Isles, its distribution is predominantly southern and eastern, indicating a preference for continental climates with colder winters and warmer, drier summers. It also seems to prefer areas of chalk and limestone.  It is virtually absent from Western Scotland, the Scottish islands and Ireland.

Uses

Winter aconites are excellent garden plants, suitable for naturalising in woodland or beneath specimen trees. They will grow happily in short grass in semi-shade, giving much-needed very early season colour. The leaf-like bracts form a dense canopy after flowering as the follicles ripen, (see picture above) but are gone by early May, clearing the ground for other successional species.

The pollen and nectar are valuable early-season sources of food for bees, the pollen providing proteins for brood development and the nectar providing sugars for energy. Very early-flowering species such as this, Lamium album and L. purpureum and Petasites pyrenaicus may become increasingly important resources for bees as winter climate warms. The species has earned recognition from the Royal Horticultural Society as a Bee Plant and has gained their Award of Garden Merit.

Despite its name,  winter aconite does not contain the virulently toxic alkaloid aconitine that is a speciality of monkshood and delphiniums. However, all parts of the plant and especially the rhizomes, contain in addition to lectins, cardiac glycosides that produce an impressive range of toxic symptoms in humans and animals. These include bufanolides, a class of toxins named after toad venom (genus Bufo) from which they were first isolated. Admittedly it is improbable that your favourite treasure who has just refused to eat Brussels sprouts will be found in the garden munching on Eranthis as salad greens, but it is best to be forewarned..

Winter aconite rhizomes have been much studied as a rich source (up to 3% of soluble protein) of an N-acetylgalactosamine-binding lectin that is a powerful inhibitor of protein synthesis. Its claim to fame is that it is the first lectin ever to have been discovered in plants of the family Ranunculaceae (Cammue et al. 1985), although such compounds are known from many other plant families including the red kidney bean lectin PHA (PhytoHaemAgglutinin) and concanavalin A from the jack bean Canavalia ensiformis in the family Fabaceae and the infamous ricin from the castor oil plant Ricinus communis (Euphorbiaceae). The Eranthis lectin has been reported to have potent antiviral activity, including against the HIV virus, and its function in the plant may be to prevent infection by microorganisms, but it would probably also be effective against larger organisms such as nematodes and herbivorous arthropods.

References
Bentham G and Hooker JD (1930) Handbook of the British Flora. A description of the flowering plants and ferns indigenous to, or naturalised in the British Isles. L. Reeve and Co. Ltd., Ashford Kent.

Cammue BP,  Peeters, B and Peumans, WJ (1985) Isolation and partial characterization of an N-acetylgalactosamine-specific lectin from winter-aconite (Eranthis hyemalis) root tubers. Biochemical Journal 227, 949-955 doi:10.1042/bj2270949

Clapham AR, Tutin TG and Warburg EF (1962) Flora of the British Isles, 2nd edition. Cambridge University Press.

Plants of the World Online (2022) Eranthis hyemalis (L.) Salisb. https://powo.science.kew.org/taxon/711189-1

Royal Horticultural Society (2022) Eranthis hyemalis https://www.rhs.org.uk/plants/6540/i-eranthis-hyemalis-i/details

Salisbury EJ (1919) Variation in Eranthis hyemalis, Ficaria verna, and other members of the Ranunculaceae, with special reference to trimery and the origin of the perianth. Annals of Botany, 33, 129, 47-79.  https://www.jstor.org/stable/43236287

Salisbury RA (1807) (read 1805) XIII. Observations on the Genera of Trollius, Eranthis, Helleborus, Coptis, and Isopyrum. Transactions of the Linnean Society of London Volume 8, Issue1, March 1807, Pages 300-307. https://doi.org/10.1111/j.1096-3642.1807.tb00315.x

Stace, C. A. (2019). New Flora of the British Isles (Fourth edition). C & M Floristics, Middlewood Green, Suffolk, U.K. ISBN 978-1-5272-2630-2.

Chris Jeffree