Curly waterweed

The leaves curl downward along the stem, which is the easiest way to tell this species from the similar Egeria densa (Anacharis). Lagarosiphon major comes from southern Africa but has naturalised in waterways across Europe, where it is often classified as an invasive species. It is also known as curly waterweed or oxygen weed.

The stems are thick and brittle, covered in small, curved leaves that point downward. Growth is fast under moderate light, and the plant reaches 50 to 60 cm quickly, forming dense stands. It is a cold-water species — it thrives best at 18 to 24 °C and is winter hardy, surviving near-freezing temperatures in outdoor ponds.

It does not need CO₂ injection and grows in soft or hard water across a pH range of 6 to 8. Moderate light is sufficient; under low light the lower stem goes bare and the plant unravels. Regular trimming keeps it compact and encourages side shoots.

In many regions, including parts of Europe and New Zealand, Lagarosiphon major is a controlled species and its sale or transport is restricted. Check local regulations before buying it. In the aquarium it is a reliable, low-maintenance background plant that outcompetes algae when given moderate light and regular trimming. Never release it into local waterways.