Hordeum marinum (sea barley)

The UK is at the northern edge of the distribution of Hordeum marinum, where it grows from the south coast of Wales to its northernmost limit at the Wash. It is absent from Ireland and no longer occurs in Scotland. It is also found across the Mediterranean and parts of Central Asia on disturbed inland areas. This species has become extensively naturalised outside its native range.

Herbarium specimen of Hordeum marinum

Sea barley grows up to 10–40 cm tall with stiff, smooth stems growing singly or clustered loosely together. Stems grow either straight upwards or outwards, spreading from a bent base. Each stem contains 3 or 4 nodes (areas from which leaves emerge). Sheaths (the lower parts of the leaf that wrap around the stem) are smooth and rounded on their backs. The leaf blade is bluish-green and narrows to a fine point at the tip. Membrane-like ligules (outgrowths on the inner side of the junction between leaf sheath and blade) are less than 1 mm in length.

Flower heads are green or purplish, spike-shaped and bristled, 2.0–6.0 cm long and 1.5–3.0 cm wide. Spikelets (single units of the flower head composed of modified leaves and flowers) occur in groups of three, alternating on opposite sides of the flowering stem. The central spikelet contains a single bisexual flower and joins directly to the flowering stem. Lateral spikelets are sterile and borne on short stalks. On fruiting, spikelets fall from the flowering stem in groups of three. Flowering and fruiting occurs during the summer

There are two subspecies of sea barley. Hordeum marinum subspecies marinum is distinguished by the wide, wing-like base of the inner glumes (modified leaves at the base of spikelets) of its lateral spikelets. In contrast, H. marinum subspecies gussoneanum has a narrower inner glume that is not winged and a more variable chromosome number. Although H. marinum subspecies gussoneanum is not native to the British Isles, it is widely invasive in Britain and has been present in Guernsey for more than a hundred years. Seeds of H. marinum may colonise outside of their native range, carried as contaminants in imports of agricultural plants.

Sea barley is an annual that self-pollinates and reproduces solely by seed. It requires an open habitat, performing best in mud that begins to dry during spring and is hard by the middle of the summer. Flooding in winter may help disperse the seeds, as winter flood lines from the previous year are found to contain seed deposits. Seedlings are seen in both spring and autumn; however, it is not known if either of these populations contributes more significantly to persistence of the species.

The distribution of sea barley around the south coast of England has become patchy as populations at the limits of its distribution disappear. These local extinctions are due to loss of appropriate habitat through filling-in of brackish land, land improvement, conversion of marshland grazing sites and construction of sea defences. These habitat losses have led to sea barley being listed as Vulnerable in the Vascular Plant Red Data List for Great Britain 2005.

Another reason for its decline is that annuals such as Hordeum marinum are susceptible to competition from perennial species. Elymus pycnanthus, for example, may take over the saline habitats previously occupied by H. marinum, a problem that can be alleviated by the continuous creation of open areas, which are essential for the establishment and persistence of sea barley.

Sea barley is tolerant of high salt levels in the soil and also of the water-logging that commonly accompanies high salinity. These qualities have led to sea barley being proposed as a candidate for hybridisation with wheat (Triticum species), with the aim of producing plants that are more suited to the increasing salinity of arable land, which can result from climate change.

Kew is one of a number of institutions collaborating on the Adapting Agriculture to Climate Change project. This project seeks to combat the threat to global food security posed by climate change and will focus on collecting, protecting and preparing crop wild relatives for breeding programmes.

Hordeum marinum is an important wild relative of the major cereal grain Hordeum vulgare (barley). Previous studies of H. marinum have found that its saline tolerance makes it a possible candidate for hybridisation with wheat (Triticum species). It is hoped that introducing traits from wild relatives to crop species will produce hybrids adapted to new climates with different demands.

The Millennium Seed Bank Partnership aims to save plant life worldwide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.

Two collections of Hordeum marinum seeds are held in Kew’s Millennium Seed Bank based at Wakehurst in West Sussex.

See Kew’s Seed Information Database for further information on Hordeum marinum.

Sea barley is not currently grown at Kew, but other Hordeum species can be seen growing here in the Grass Garden.

Pressed and dried specimens of Hordeum marinum are held in Kew’s Herbarium where they are available to researchers by appointment. The details of some of these specimens, including some images, can be seen online in Kew’s Herbarium Catalogue.

Search Kew’s science databases for more information on Hordeum marinum

GrassBase – Hordeum marinum

The commelinids group at Kew

About the grass family (Poaceae)

Hordeum marinum is one of 50 species on the Botanical Society of the British Isles (BSBI) Threatened Plants Project list. The herbarium specimens of these species are being digitised as part of the UK Virtual Herbarium Project. Find out how you can help in this project by visiting herbaria@home.