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Sea Knotgrass

Polygonum maritimum

Please keep in mind that it is illegal to uproot a plant without the landowner's consent and care should be taken at all times not to damage wild plants. Wild plants should never be picked for pleasure and some plants are protected by law.
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Plant Profile

Flowering Months:
Polygonaceae (Dock)
Life Cycle:
Maximum Size:
50 centimetres tall
Beaches, saltmarshes, sand dunes, seaside.

Pink, 5 petals
Pinkish-white flowers clustered together in groups of 1 to 4. The flowers are borne in the leaf axils where they meet the stems. Just like the similar looking Ray's Knotgrass (Polygonum oxyspermum), the flowers are large for a knotgrass.
The fruit is an achene.
UK's rarest knotgrass species and the only one which is always perennial. Sea Knotgrass is a prostrate plant with slightly leathery, broadly oval leaves. The margins of the leaves are down-turned.
Other Names:
Beach Knotweed, Knotgrass.
Frequency (UK):
Rarely seen  

Similar Species

Other Information


Polygonum maritimum, also known as sea knotgrass or beach knotweed, is a perennial herb that is native to coastal areas in Europe and Asia. It is typically found on sandy beaches, dunes, and salt marshes. The plant has small, green leaves and white or pink flowers that bloom in the summer. It is considered an invasive species in some areas and can be difficult to control once established.


Sea Knotgrass (Polygonum maritimum) is a species of flowering plant that belongs to the knotweed family (Polygonaceae). It is a perennial herbaceous plant that grows in coastal areas, particularly in sand dunes, shingle beaches, and salt marshes. This plant is commonly found in the northern hemisphere, including Europe, Asia, and North America.

Sea Knotgrass has a low-growing habit, reaching up to 50 cm in height. Its leaves are small, narrow, and fleshy, with a blue-green color that helps them to retain moisture in the harsh coastal environment. The plant also has small, pinkish-white flowers that bloom from July to October. The flowers are hermaphrodite, meaning they have both male and female reproductive organs, and are pollinated by wind and insects.

One of the most remarkable features of Sea Knotgrass is its ability to adapt to the harsh conditions of the coastal environment. The plant has developed several strategies to survive in this challenging habitat, including:

  1. Salt tolerance: Sea Knotgrass can tolerate high levels of salt in the soil and the air, which is essential in coastal areas where salt spray from the sea can be a significant stress factor for plants.

  2. Drought tolerance: The plant has a shallow root system that allows it to quickly absorb moisture from the soil during rain events. It can also withstand long periods of drought by reducing its water loss through transpiration.

  3. Wind resistance: Sea Knotgrass has a prostrate growth habit and flexible stems that can bend and sway with the wind, reducing the risk of damage from strong coastal winds.

  4. Sand burial tolerance: In sand dune environments, Sea Knotgrass can tolerate being buried under several centimeters of sand. This ability allows it to survive in shifting sand dunes and to colonize new areas as sand moves.

In addition to its ecological importance, Sea Knotgrass has several uses in traditional medicine and cuisine. In some cultures, the plant is used to treat digestive problems and respiratory illnesses. The young leaves and shoots of the plant are edible and can be cooked or eaten raw. They have a slightly salty and sour flavor, which makes them a popular ingredient in salads and soups.

Overall, Sea Knotgrass is an impressive plant that has adapted to the challenging coastal environment. Its ability to thrive in harsh conditions makes it a valuable species for coastal ecosystems and a source of food and medicine for human populations living in coastal regions.

Blog continued...

Sea Knotgrass is an important plant for the stabilization of coastal ecosystems. Its shallow root system helps to anchor sand dunes and prevent erosion, while its fleshy leaves provide a valuable source of organic matter for the soil. The plant also serves as a habitat and food source for a variety of coastal animals, including insects, birds, and small mammals.

Despite its ecological and cultural importance, Sea Knotgrass is under threat from a variety of human activities, including coastal development, overgrazing, and pollution. In many parts of the world, the plant is considered a vulnerable or endangered species, and conservation efforts are needed to protect its habitat and ensure its survival.

One way to protect Sea Knotgrass and other coastal plants is to establish protected areas and promote sustainable land use practices. This can include limiting development in coastal zones, regulating grazing and other forms of land use, and promoting eco-tourism as a means of supporting local communities while preserving the natural environment.

Another important step is to raise awareness of the importance of Sea Knotgrass and other coastal plants. This can involve education campaigns targeting local communities, tourists, and policymakers, as well as research and monitoring programs that provide scientific data on the plant's distribution, habitat, and ecological role.

In addition to its ecological and cultural significance, Sea Knotgrass also has potential applications in biotechnology and phytoremediation. Biotechnology researchers are interested in the plant's ability to tolerate salt and drought, which could be used to develop crops that can grow in arid and saline environments. Phytoremediation scientists are also studying Sea Knotgrass as a means of removing pollutants from contaminated soils and waters.

Sea Knotgrass is a promising plant for phytoremediation because it can accumulate heavy metals and other toxic compounds in its tissues without being harmed. This ability makes it a potential candidate for the remediation of contaminated sites, including former industrial sites and landfills.

Furthermore, the plant's salt tolerance could be used to develop crops that can grow in saline soils, which are abundant in many parts of the world. These crops could provide a sustainable source of food and fuel for populations living in arid regions, where traditional crops often fail due to soil salinity and water scarcity.

Overall, Sea Knotgrass is a valuable plant with multiple potential applications in biotechnology, phytoremediation, and sustainable agriculture. Protecting and conserving this plant is essential for maintaining the health and productivity of coastal ecosystems, as well as promoting the wellbeing of human populations.

Distribution Map

Reproduced by kind permission of the BSBI.

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