Can I grow this in a home aquarium?

No, it requires specific marine conditions and vast space that cannot be replicated in a home tank.

Is it a type of seaweed?

No, it is a seagrass, which is a flowering plant with roots, stems, and leaves.

Plant guide

Neptune grass.

Posidonia oceanica

Pronounced po-zi-DOH-nee-uh oh-shee-AH-nee-kuh

Also known as Mediterranean Tapeweed, Neptune Grass, Species code: Po

Neptune grass is a vital seagrass species that forms massive underwater meadows in the Mediterranean Sea. It plays a crucial role in coastal protection and carbon sequestration. This plant is strictly marine and cannot be grown in a home garden setting.

At a glance

Care level: Difficult
Plant type: Seagrass
Light: Full sun in shallow marine waters
Indoor light: Bright indirect
Water: Every 0 days
Mature size: 1 meter leaf length
Growth rate: Slow
Hardiness: USDA 9 to 11
Bloom time: Autumn

Care

This plant requires a stable marine environment with specific salinity and water quality parameters. It is not suitable for cultivation in standard home aquariums. Watch for signs of water pollution or temperature fluctuations which can cause rapid decline.

Watering: This is an aquatic plant that must remain submerged in seawater at all times.
Feeding: It obtains nutrients directly from the marine water column and sediment.
Pruning: Pruning is not necessary or recommended for this wild species.
Repotting: This plant is not suitable for container cultivation.
Soil: Marine sediment
Soil pH: 8.0 to 8.4
Temperature: 10 to 28°C
Humidity: 80 to 100%

Safety

People: There is no known toxicity to humans, though it is not edible.
Children: No specific risks exist for children, but it should remain undisturbed in its natural marine habitat.
Edibility: This plant is not for human consumption.

How to identify it

Long ribbon like green leaves
Forms dense underwater meadows
Produces small, inconspicuous greenish flowers
Fibrous brown root structures

Height: 0.1 to 1m
Spread: 1 to 10m
Leaves: bright green, dark green, evergreen
Flowers: greenish, 1 to 2cm
Fruit: brown, Spring

Problems to watch for

By symptom

Plant problems by symptom

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Growing it

Propagation: seed, rhizome division
When to plant: spring

This plant is a foundational species that supports entire marine communities rather than garden companions.

Similar species

Cymodocea nodosa · It is another common seagrass found in the Mediterranean.

Science

Family: Posidoniaceae
Genus: Posidonia
Order: Alismatales
Class: Liliopsida
Native range: Mediterranean Sea
Conservation: This species is protected under international law due to its vital role in Mediterranean ecosystems.

It grows in dense underwater meadows that stabilize sandy seabeds and provide essential shelter for marine life.

Value to wildlife: nursery for fish, carbon sequestration, coastal erosion control
Uses: coastal protection, marine habitat restoration, carbon storage

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 9/9
Soil reaction: 8/9
Light: 8/9
Temperature: 6/9
Moisture: 9/9
Nutrients: 2/9

Taxonomy resolved against GBIF.

Questions people ask

Can I grow this in a home aquarium?: No, it requires specific marine conditions and vast space that cannot be replicated in a home tank.
Is it a type of seaweed?: No, it is a seagrass, which is a flowering plant with roots, stems, and leaves.

Where the name comes from

The genus is named after Poseidon, the Greek god of the sea, reflecting its aquatic nature.

Good to know

It forms vast underwater meadows that are thousands of years old.
The plant produces oxygen and helps keep Mediterranean waters crystal clear.
It is often called the lungs of the Mediterranean.
Dried leaves are sometimes found on beaches and are known as sea balls.

Keep your neptune grass growing

This page is a general guide. Save your plant in Pocket Botanist and you get care reminders timed for this plant, plus a care history that shows what worked.

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This guide was generated with AI on September 26, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.