Posidonia spp.

Mediterranean Sea and southern Australia

Overview

Posidonia spp. is a genus of about nine seagrasses in the family Posidoniaceae - true flowering plants, not algae - that grow submerged in shallow coastal seas. Posidonia oceanica is found only in the Mediterranean, while the other species grow around southern Australia. Plants spread by thick, fibrous rhizomes rooted in sand and sediment, forming dense underwater meadows from clusters of ribbon-like leaves. The leaves are flat, green, 0.4-0.6 inch (1-1.5 cm) wide and 8-60 inches (20-150 cm) long, growing in tufts of 4-8 and shed and replaced through the year, leaving fibrous sheaths on the rhizome. Small greenish flowers are borne in branched clusters within the leaf tufts, usually in autumn, and are pollinated underwater; floating fruits nicknamed sea olives ripen and drift to new ground. Growth is slow, and meadows expand only inches a year, with some clones thousands of years old. Posidonia spp. needs clear, fully marine water and light, and meadows decline where water is murky, polluted, or disturbed by anchoring and coastal works. Fibrous leaf remains form rounded balls that wash up on beaches.

Native Range

Posidonia oceanica is endemic to the Mediterranean Sea, while the remaining species occur in the temperate coastal waters of southern and western Australia. Plants grow on sandy and muddy seabeds in clear, sheltered, shallow water from the surface down to about 130 feet (40 m) where light allows.

Suggested Uses

Used mainly as marine habitat and for coastal protection rather than as a cultivated plant: meadows steady the seabed, slow erosion, store carbon, and act as nurseries for fish and shellfish. Dried fibrous balls and leaves have been used locally as packing, insulation, and soil mulch. Posidonia spp. is planted only in marine restoration and is protected in many areas.

How to Identify

Recognized as submerged meadows of flat, ribbon-like green leaves 0.4-0.6 inch (1-1.5 cm) wide growing in tufts from thick, fibrous rhizomes anchored in sand. Old leaf bases leave brown fibers on the rhizome, and these fibers wash ashore as rounded balls. The broad, tufted leaves and true roots and flowers separate Posidonia spp. from narrow-leaved seagrasses such as Zostera spp. and from rootless algae.

Appearance

Size & Dimensions

Height8" - 5'

Colors

Flower Colors

green

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Flowers form underwater in branched clusters within the leaf tufts, usually in autumn, though flowering is irregular and many years pass without it in Posidonia oceanica. Pollination occurs in the water, without insects. Floating fruits ripen over the following months and drift to colonize new seabed.

Detailed Descriptions

Foliage Description

green

Growing Conditions

Sun Requirements

SunFull Sun

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

Soil Types

sand

Water & Climate

Water Needs

High

Time to Maturity

Slow; meadows expand over decades

Care & Maintenance

Care Guide

Seagrass meadows are wild marine habitat rather than garden plants, though Posidonia spp. is grown in restoration projects. Plants need clear, fully marine seawater, steady light, and stable sandy seabed, and they decline where water turns murky from pollution or runoff. Anchoring, dredging, trawling, and coastal building damage and uproot meadows, which recover only over decades because growth is slow. Restoration uses transplanted rhizome cuttings and, less often, seedlings raised from drifting fruit. Meadows store large amounts of carbon and shelter many fish and invertebrates.