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Posidonia, a marine plant

Posidonia oceanica is not an alga, but a marine flowering plant that lives only in the Mediterranean Sea.  It forms meadows, real underwater forests that can be found at depths of up to 40 meters.

 Unlike some algae that attaches itself  with crampons, Posidonia oceanica is a magnoliophyte, a plant made up of roots (up to 70 cm long),  horizontal rootstalks called rhizomes, leaves (about 1 cm wide and up to 80cm long), discrete green flowers and small fruits that look like olives.

Like terrestrial plants, Posidonia loses its leaves mainly in autumn and winter.  The fallen leaves are carried towards the beach by the wind and waves.  Once on the beach, these leaves associated with fragments of  the plant’s horizontal root system (rhizomes) and sand  form clusters called Posidonia “banquettes”.


Posidonia flowers are green and appear in autumn. Both male and female, they are grouped in inflorescences, like the ears of terrestrial wheat crops, at the top of a 10-30 cm long stem. Posidonia spends most of its energy to develop and defend itself. This is why it does not bloom every year. It also seems that flowering requires  high summer temperatures exceeding 25°C


The matte is a very compact entanglement formed by rhizome and roots, the interstices of which are filled with sediment.

Sea balls

The origin of the fibrous sea balls (egagropili) has long remained mysterious.  We know nowadays, that this sea balls are fragments of rhizomes mixed with Posidonia fibers and sand and rolled by waves giving them this round form. 

Fruits and rhizomes

Flowers can produce fruits.  These fruits look like green olives and take 6 to 9 months to ripen before breaking off and floating on the surface of the water, usually occurring  between May and July.  They contain a seed, which, in a favorable environment, will germinate to produce new sprouts.

The rhizomes, buried in the sediment, develop in a horizontal or vertical direction depending on the space available: horizontal, when the seagrass is able to gain ground, and  vertical when it reinforces its burial under the sediment.  These developments are very slow: only 1 to 3 cm per year!

Crédit photo : GIS Posidonie