Water onion

Crinum thaianum

Also known as Thai onion plant, Onion plant, Giant onion plant, Water lily onion

FreshwaterBulb / tuberBulb offsets

Water onion (Crinum thaianum) — Ito Y, Barfod A · CC BY 4.0

Care at a glance

Light: Low light
CO2: No CO₂ needed
Growth rate: Medium
Placement: Background
Type: Bulb / tuber
Temperature: 72–82°F (22–28°C)
pH: 6.0–8.0
Hardness: 4–18 dGH
Max height: 39.4 in (100 cm)
Spread: 11.8 in (30 cm)
Nutrient uptake: Root feeder
Growth region: Submersed or emersed
Difficulty: Easy
Origin: Southern Thailand (endemic)
Family: Amaryllidaceae

Overview

This is a big, forgiving plant for large tanks: bury only the lower third of the bulb so it does not rot, give it a rich substrate and plenty of room, and it tolerates temperature swings and a wide range of light. It grows steadily rather than fast and does not require CO2, though feeding the roots keeps it strong. The fibrous leaves are usually left alone by herbivorous fish and cichlids, so it doubles as a rugged background or specimen plant.

In the wild

Crinum thaianum is endemic to a handful of coastal-plain rivers and streams in southern peninsular Thailand, where it grows as an emergent bulb rooted in the bed with long ribbon leaves streaming in the flow. The wild population has collapsed from habitat loss, dredging and over-collection, and the species is now listed as Endangered on the IUCN Red List even though it remains widely cultivated for the hobby. In the aquarium it grows submersed, with mature plants able to reach the surface and grow emersed.

Good to know

It is a bulbous species in the amaryllis family (Amaryllidaceae), so like its relatives it is a true bulb rather than a stem or rosette plant. Its tough, ribbon-like leaves can reach 1.5 metres or more in the wild while staying only a couple of centimetres wide. Mature bulbs can flower with fragrant, lily-like white blooms carried to the water surface.

Did you know

Wild Crinum thaianum is now protected under conservation efforts in Thailand after river dredging wiped out most of its natural range, making it one of the few common aquarium plants that is genuinely endangered in the wild.