The next time you go for a splash to the beach take a moment to look for this little plant. It is easily found at almost any seaside. It grows on the upper beach, not the dunes, and it forms rough cushions of stiff green shoots no more than 20 cm tall, which arise in springtime from a tough underground rhizome system. The rough cushion structure slows the wind speed near the ground, and so the wind-borne sand and salt particles fall around it to form ‘embryo dunes’. It also likes to grow on shingle beaches. For most species this position near the sea would be an extremely aggressive environment. Several factors are hostile for plants. The sand surface dries out easily and so there is a desiccating effect; the sand particles are abrasive; the substrate is mobile (shifting in storms); sea spray is saline; and humans are quite likely to step on it, or unceremoniously remove it along with any accompanying plastic flotsam using bucket and spade to create a ‘clean’ space for sunbathing. Thus, any plant growing on the upper beach must be xerophytic (tolerating drought), halophytic (tolerating salt), mechanically strong (leaf surfaces that do not get abraded when struck by speedy sharps of sand). Enter Honckenya !
It’s the only species in the genus. It belongs to the pink family, the Caryophyllaceae, but you’d never think so unless you had a really close look. It does have the opposite pairs of leaves in common with its relatives the carnations, campions and stitchworts, but the leaves are stiff and succulent, and the plant lacks the showy flowers of most members of that family. You could be forgiven for thinking it has no flowers at all, as sometimes its petals are reduced to tiny stubs.
Unlike many seaside plants, it rarely comes inland (contrast with scurvy-grass Cochlearia danica, discussed in a previous blog, a plant often found on salted motorways, click https://botsocscot.wordpress.com/2021/05/16/plant-of-the-week-17th-may-2021-scurvygrass-cochlearia-officinalis-and-c-danica/). This apparent avoidance of motorways may be because Honckenya’s seed capsules are large, like little peas, adapted for flotation in the sea, too heavy for wind dispersal and not rough enough to stick to fur, clothes, tyres of cars or footwear. Its distribution is described as ‘circumpolar’: three main subspecies occupy different parts of a great circumpolar circle that includes Japan, North America and Northern Europe. It reaches Spain but hardly penetrates the Mediterranean.
Its special interest lies in its sexuality. Earlier it was noted as being dioecious, i.e. having separate male and female plants, rather like its ‘cousin’ Silene dioica. Only about 5% of all flowering plants are dioecious, but that includes many familiar ones. Gardeners will know that asparagus and holly are dioecious, and so is sea beet and stinging nettle. But in Honckenya there is some ambiguity. Clapham, Tutin and Warburg (1981) mysteriously state that it is ‘more or less’ dioecious. Its sexual mystery was unravelled by Japanese Botanists, Takahiro Tsukui and Takashi Sugawara in 1992. They first observed two flower types, both honey-scented and both visited by small bees, hover-flies, flies and ants: (i) male-type flowers with stamens and (ii) inconspicuous female-type flowers with long styles, non-functional anthers and a much-reduced length of petals. This latter type, as might be expected, developed the seed capsules. However, sometimes a third type was seen (iii) the male-type developing seed capsules – these individuals were clearly hermaphrodites. Thereafter, everyone called Honckenya ‘sub-dioecious’.
Evolutionary biologists have speculated on how the dioecious trait evolved – in fact the subject has attracted much interest among some of the most deep-thinking theoreticians (Lloyd 1974, Bawa 1980). The usual assumption is that dioecious plants evolved from hermaphrodite ancestors, and dioecious types are favoured by a number of selection pressures such as (i) inbreeding is prevented (ii) specialisation is a good idea (as in human affairs), as more resources are needed to sustain both male and female functions together. I have the impression that, in general, dioecious species are more frequent in stressful environments where resources are scarce – but it would be a big task to test the hypothesis.
In the case of Honckenya, it seems that evolution has been ‘caught in two minds’ or that we are witnessing a gradual transition from perfect hermaphrodite flowers to those which are either male or female.
Most of the recent work on Honckenya has been to describe the physiology of the two types, male and female. Julia Sanchez-Vilas, now at Cardiff University, examined plants from the coast of Galicia. She found that the plants do indeed differ in the allocation of resources and photosynthesis especially when they are water-stressed. Female plants have a higher efficiency of water use (Sanchez-Vilas and Retuerto 2009).
In some conditions the species can spread very quickly. Phillipp & Adsersen (2014) report its extremely rapid colonisation of a tiny volcanic island of Surtsey which emerged from the Atlantic off the Icelandic coast in 1963. The species was first recorded on the coast in 1967 and has increased exponentially, now reaching the top of two calderas.
As I write this, I’ve just returned home from a trip to find female plants at North Berwick. I was sure Honckenya was there in 2019 along with Cakile maritima. Both seem to have vanished. They may have fallen victim to the habit of local Councils to ‘clean’ the beach. This process involves the use of a tractor to dredge up the vegetation. I’ve seen it in action at Girvan on the Ayrshire coast and it does not please me. I’ve found plenty of Honckenya elsewhere on the Ayrshire coast, often on shingle. Male flowers seem to predominate, but there are females too. I am guessing the some of the males are in fact hermaphrodite. I’ll revisit them in the weeks to come and then I’ll know for sure.
It can be eaten. The ever-optimistic organisation Plants for the Future states:
“Young shoots – raw or cooked and used as a potherb. A delicious flavour, they are rich in vitamins A and C. They are at their best before the plant flowers. The leaves can also be fermented and used like sauerkraut. In Iceland the plant is steeped in sour whey and allowed to ferment. The resulting liquor is said to taste like olive oil and is used as a beverage. Seed – used as a garnish or ground into a powder and added to flour. Very fiddly to harvest”.
The piece I tried was rather tasteless, but had ‘crunch’ and might go well in a salad.
Bawa KS (1980) Evolution of dioecy in flowering plants. Annual Review of Ecology and Systematics 11, 15-39.
Clapham AR, Tutin TG and Warburg EF (1981) Excursion Flora of the British Isles, 3rd edition. Cambridge.
Lloyd DG (1974) Theoretical sex ratios of dioecious and gynodioecious angiosperms. Heredity 32, 11-34.
Phillipp M and Adsersen (2014) Colonization of an empty island: how does a plant with a plastic gender system respond? Biogeosciences 11, 6657–6665. doi:10.5194/bg-11-6657-2014.
Sanchez-Vilas J and Retuerto R (2009) Plant Biology ISSN 1435-8603 Sex-specific physiological, allocation and growth responses to water availability in the subdioecious plant Honckenya peploides. Plant Biology ISSN 1435-8603.
Tsukui T and Sugawara T (1992) Dioecy in Honckenya peploides var. major (Caryophyllaceae). Botanical Magazine, Tokyo 105, 615-624.