This is a pretty garden plant, cultivated in Britain since 1571, and now frequently seen in the wild in hedgerows, waysides and waste ground. I’ve seen it especially in allotment gardens, not quite ‘wild’ but tolerated by plot-holders because its bright flowers lift one’s spirit at a busy time of year. In Scotland it shuns the west but it has crept up the eastern side of the country.
Dense stand of Honesty in full flower. Image: Chris Jeffree.
It belongs to the cabbage and mustard Family, the Brassicaceae. It comes from South-East Europe and is naturalised in all those parts of the world that have been colonised by Europeans. Its introduction to North America is said to have been by the Pilgrims.
Despite its Latin name annua it grows as a biennial, seeding freely and even considered a ‘weed’ by some people. The flowers are purple, with quite large (petals 15-25 mm) and sometimes white. In its first year it produces a large over-wintering storage root, said to be edible. Its growth the following spring is rapid as it draws upon the reserve of carbohydrates.
The plant was described by Linneaus in 1737 and he must have thought it to be an annual. Somewhat later, the German botanist Conrad Mönch (1744 – 1805) renamed it Lunaria biennis. However, the taxonomic Principle of Priority states “The first formal scientific name published for a plant or animal taxon shall be the name that is to be used”. Taxonomists uphold this Principle, even when the name is clearly wrong.
Close-up photograph to show the distribution of hairs. Image: Chris Jeffree.
It grows easily from seeds, and flower-arrangers love it for the highly decorative seed cases, flattened like a coin and often called the Money Flower. The seed case (a ‘silique’) is somewhat transparent, earning the common name ‘Honesty’ because it openly reveals its wealth of seeds. The surface of this structure becomes silvery and eye-catching, rather like the moon (earning the Latin name Lunaria). You may see it in a florist’s window.
The seed cases give rise to the many plant names that have been used over the centuries. Old plant names are not only fun, they can shed light on the origins and peoples’ perceptions of the plant. Alice Coats, in her book Flowers and their Histories, points out that many names were used for this plant at the time Gerard wrote his Herbal (1597) and suggests that the date of introduction into Britain must therefore have been before 1571 (the date given in modern botanical books). The names she quotes are: Bolbonac, Lunarie, White Sattin, Sattin-flower, Pennieflower, Silverplate, Prick-song-wort, Judas pence, Shillings, Two-Pennies in a Purse, Money in both Pockets. I especially like the French name Herbe aux Lunettes, the Spectacle Plant. However, in the on-line version of Gerard’s Herbal I cannot find these names. I suspect she was referring to the later 1636 much-revised version of the book.
Detail of flower. Flowers are 4-6, scentless, visited for nectar by butterlies and long-tongued bees and for pollen by smaller insects. They can be cross-pollinated but also they are self-pollinated. Image: Chris Jeffree.
Recent interest in this species has been sparked by the oil contained in the seeds. Like many members of the Brassicaceae the whole plant is rich in chemical compounds (which is why we like or possibly dislike the flavour of cabbages, turnips and mustard). One of the most widely grown members of the Family is Oil-seed Rape, Brassica napus. It provides oil for cooking and is a source of biofuel-oil and lubricants. In Britain, it was widely grown a few years ago, but now it has become less popular with farmers as it is subject to attack by pests and diseases which require applications of expensive pesticides. A possible successor is Lunaria annua, which grows quite well, even as far north as Aberdeenshire (Walker et al 2003).
The fruits are broadly elliptical, oblong or almost circular, 30-45 x 20-25 mm. Seeds are uniform, winged, 5-8 mm. Image: Chris Jeffree.
It turns out that L. annua produces perfectly satisfactory oil and has caught the attention of the oleochemical industry. The seed yield is said to be up to 2 tons per hectare, compared to the average of 3.1 tons from Brassica napus. The trouble is, a biennial crop is inconvenient, not fitting well in the schedule of the modern farm. In the UK, Honesty would have to be planted in the late Spring and be harvested in July of the following year. Brassica napus on the other hand is an annual. Research is underway in the Netherlands to find (or breed) genotypes of Honesty that have a shorter life cycle (Dodos et al 2015).
White form, growing at an Edinburgh allotment site with Green alkanet (Pentaglottis sempervirens) and Nettle (Urtica dioca). Image: John Grace.
Like most of the Brassicaceae, all parts of the plant are rich in sulphur-containing compounds called glucosinolates. When the plant is chewed the glucosinolate comes into contact with an enzyme called myrosinase to release allyl isothiocyanate the ‘mustard oil’ that gives the pungent taste to mustard, radish, horseradish and wasabi. This so-called ‘mustard oil bomb’ defends the plant against herbivores. As you might expect, in the evolutionary arms race there are some herbivores that can diffuse the bomb. The diamondback moth (Plutella xylostella) can do it, and is a worldwide pest of the Brassicaceae.
Honesty below-ground, showing the storage root. Attribution: Qniemiec, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
There may also be opportunities to develop Honesty as a medicinal plant. A group of research chemists in Serbia point to its antioxidant, antigenotoxic and anti-inflammatory properties, and they discuss the likely pharmaceutical value of various products (Katanic Stankovic et al 2022). The seed-oil consists mainly of long-chain fatty acids of which erucic acid (44%) and nervonic acid (23%) are dominant. They are used as industrial lubricants, but nervonic acid is also used to produce a drug to help repair damaged brains and ease multiple sclerosis. Nervonic acid 24-carbon chain, and is found naturally in animal brains (first isolated from shark brain).
Meanwhile, at least one major cosmetic company advertises the oil for use in skin cream.
Lunaria annua, distribution in the British Isles. From BSBI/Maps.
References
Clapham AR et al. (1987) Fora of the British Isles. Cambridge.
Dodos GS et al (2015) Renewable fuels and lubricants from Lunaria annua L. Industrial Crops and Products 75, 43−50. doi: 10.1016/ j.indcrop.2015.05.046
Katanic Stankovic JS et al (2022) The qualitative composition and copmparative biological potential of Lunaria annua L. extracts. Kragujevac Journal of Science44, 75–89.
Walker RL et al (2003) Adaptation potential of the novel oilseed crop, Honesty (Lunaria annua L.), to the Scottish climate. Industrial Crops and Products,18, 7-15
©John Grace
Botany in Scotland 