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Water plant growing guide
Thalia geniculata is a tall, banana-leaved marginal plant for ponds and wetlands, with arching stems of small purple flowers. It is raised from seed in wet mud and then grown in shallow water, but it spreads readily, so plant it where it cannot escape into natural waterways.
Each small fruit holds a single seed, 5-10 mm long, smooth and brown, grey or black. The fruit wall traps an air space that makes it float, which is how thalia spreads along waterways. Remove any remaining fruit wall or bract before sowing, so the seed sinks and contacts the mud.
Not established for this species. No published trial shows that thalia seed needs scarifying. If seeds have not sprouted after several weeks in warm, wet conditions, lightly nick or sand a few of them on the side away from the scar to test whether the coat is holding them back.
Not needed. The species is tropical and frost-tender and has no cold requirement.
Optional. Seed is sown into saturated mud anyway, so a short soak in clean water before sowing simply starts it off wet.
Keep the seed pot warm and out of cold draughts. In India, sow in the warm season; in cooler countries sow indoors in spring on a warm windowsill or heat mat. Frost kills the plant, according to PROTA.
Press seed into the surface of wet mud and cover with a thin layer of soil or fine sand, just enough to hold it in place.
A heavy, wet soil: garden loam or aquatic compost without added fertiliser or peat that floats. The Florida Native Plant Society lists sand, loam and organic soils as all suitable.
Saturated at all times. Stand the pot in a tray of water with the water level at or just below the soil surface. Do not submerge the seed pot deeply until seedlings are established.
Bright light. Wetland seedlings stretch and weaken in shade.
No reliable figure has been published, and the Hawaii-Pacific Weed Risk Assessment notes that seed dormancy is unknown. Expect emergence to be uneven and keep the pot wet and warm for several months before discarding it.
When seedlings have a few leaves, move each into its own pot of heavy soil and top with gravel to stop the soil washing out. Gradually lower the pots deeper as plants grow: PROTA gives moist soil to water up to 25 cm (10 in) deep for cultivation.
Grow in full or part sun. Thalia forms a short rhizome and, once established, reaches 2-3.5 m (6.5-11.5 ft), with very large leaves, so give it a large container or a planting basket at the pond margin.
Thalia spreads by floating seed and rhizome pieces. A 2024 Weed Risk Assessment for Hawaii rated it high risk, Brisbane City Council lists it as an emerging environmental weed and pest vegetation, and it is a weed of rice fields in Africa and Cuba. Do not plant it beside streams, canals or paddy fields, and never dump plants or pond water into natural water.
Grow thalia in a pot or planting basket in a garden pond rather than in open mud, and cut off old flower stems before fruits drop if you do not want seedlings.
In Florida, where it is native, dense stands give cover to water birds.
Once you have a plant, rhizome division is a quicker way to get more than seed.
Sow alligator flag seed in warm, wet mud for a tall, tropical-looking marginal plant for a contained pond.
From wet soil at the margin to water up to about 25 cm (10 in) deep. Keep young plants shallower and lower them as they grow.
No published trial shows that they do. Sow untreated seed into warm, wet mud first; if some seeds stay dormant for many weeks, nicking the coat of a few is a reasonable test.
It can be outside its native range. It is listed as an emerging environmental weed in south-east Queensland, rated high risk in Hawaii and is a rice-field weed in parts of Africa and Cuba. Keep it in a contained pond and check local rules.
Not frost. In mild climates it dies back and regrows from the rhizome; in cold climates overwinter the pot in a frost-free place.
