Sea Pea, Lathyrus japonicus, at Shingle Street (29th July 2017).

Last month we were given a lift to Shingle Street in Suffolk, a tiny hamlet at the mouth of the River Ore. It must be bleak in winter, but on a sunny day at the end of July it was a lovely place to visit. We saw several species of butterflies, including a couple of Clouded Yellows that swept past on a south-westerly wind. Most impressive, apart from the sheer amount of sky and fresh air, were the plants. These included big clumps of Sea Kale, Crambe maritima, and large patches of Sea Pea, Lathyrus japonicus.

Sea Pea, Lathyrus japonicus, is a a member of the Pea family (Fabaceae). It is long-lived perennial herb that grows on shingle beaches and, occasionally, blown sand, where it is one of the first plants to colonise. Good places to see it include Shingle Street and Orfordness in Suffolk and Rye Harbour in Sussex. It is usually in flower from late May until late July. The flowers are followed by seed pods, green at first, ripening to dark brown. By late summer the plants start to die back, leaving dry stems without leaves, which can persist through early winter. Stems show above the ground again from April.

Sea Pea is a very distinctive plant. Its trailing stems grow up to 50 to 80 centimetres (cm) long and bear waxy, glaucous green pinnate leaves, five to ten cm long, with two to five pairs of leaflets. The terminal leaflet is often replaced by a twining tendril. The large flowers are produced by plants that are three or more years old. They have a dark purple standard petal and paler purple wing and keel petals. They are produced in racemes of up to twelve flowers. There are some lovely photos on the NatureGate and Seasonal Wild Flowers websites. The flowers are pollinated by long-tongued bumblebees such as Bombus pascuorum, B. hortorum and B. lapidarius.

Bombus pascuorum (Common Carder-bee) on Sea Pea flowers.

Sea Pea’s stronghold in the British Isles is East Anglia but there are isolated colonies in Angus on the east coast of Scotland and even on Unst in Shetland (see map). It is classified as being of “Least Concern” in the Vascular Plant Red Data List for Great Britain. R.E. Randall has written about its past and present status and distribution in the British Isles in in Watsonia vol. 11, pp 247 – 251 (1977).

Lathyrus japonicus is also found on the coasts of Denmark and Norway and, in Sweden and Finland, on coasts by the Baltic Sea, the Gulf of Finland, Gulf of Bothnia and beside Lakes Ladoga and Oneda in Russia. It also grows in parts of Greenland and Iceland, in Eastern Siberia through Kamchatka to Japan (on sand dunes) and on the western coast of North America from northern California to Alaska and on the east coat from Newfoundland to Long Island, the lower St. Lawrence River and the Great Lakes (see p28 of “Shingle Biodiversity and Habitat Distrurbance” by E. J. Low). It is now extinct in northern France.

Lathyrus japonicus is a tough plant, especially once it is fully grown. Although seedlings are susceptible to drought, both mature plants and seedlings are frost resistant. A covering of 15 cm of shingle can kill younger plants, but older ones can produce axillary shoots on their stems, which can grow almost vertically up through as much as 40 cm of blown sand (Low p31). Mature plants can also regenerate from short sections of rhizome after storms. The seeds are light and buoyant and are viable for at least five years, meaning that they can be dispersed on the tides. Visitor pressure may be one of the biggest threats to the plant – repeated human footfall can damage the plant – but at Shingle Street it seems to be thriving.

Stock Doves and Wood Pigeons sometimes eat Sea Pea seeds and may help to disperse them – this probably accounts for a big increase in the plant at Rye Harbour in the 1960s. Sea Pea is also one of the foodplants of the micro moth Pima boisduvaliella, which flies on warm summer evenings. The weevil Bruchus loti emerged from seeds collected at Rye Harbour in 2003 (Low p32).

Sea Pea seeds have been eaten by humans, especially in times of famine, including at Aldeburgh in Suffolk.The Plants for a Future website says that the seeds are said to be safe and very nutritious in small quantities, but should not comprise more than 30% of the diet. The note of caution is well advised. The seeds contain the neurotoxin oxalyldiaminopropionic acid (ODAP), which can cause lathyrism, a condition involving degenerative changes to the spinal cord, leading to paralysis and lack of strength in or the inability to move the lower limbs.

Lathyrism is more usually associated with the Grass Pea, Lathyrus sativus, a tasty and nutritious crop grown for food for humans and livestock in East Africa and parts of Asia. Advice from Kew Gardens is that Grass Pea is “harmless to humans in small quantities” but it should not be eaten in larger amounts over an extended period of time. Some Western Asian varieties of Grass Pea have been bred to contain lower amounts of ODAP, but the chemical is thought to be of benefit to the plant, helping it cope with drought and waterlogging.

It is because of Sea Pea’s fairly restricted distribution and the slight risk of lathyrism from eating the seeds that I have included it in the “Ornamental” rather than “Edible” category on my blog.

Update 18th March 2018:

When we visited Shingle Street, we noticed the line of shells on the beach. I  have just discovered why they are there: “Shingle Street shell line inspired by friends’ cancer treatment“.