Hydrothrix gardneri

Hydrothrix gardneri is an aquatic plant from eastern Brazil. There it grows in various types of stagnant to slow-flowing waters. In nature, H. gardneri is an annual plant.
H. gardneri belongs to the Pontederiaceae and is thus closely related to e.g. Eichhornia and Heteranthera, which is hard to believe if you only consider the plant's appearance. The delicate H. gardneri only grows submersed.
In 1976, H. gardneri was introduced into the aquarium hobby in Europe by Josef Bogner. He found it in fish ponds near the town Icó in the Brazilian state Ceará. This aquatic plant is only kept by few hobbyists and is rarely found in trade.

It forms thin stems with short lateral shoots that look like leaf whorls. The stems ramify readily.
Its thin leaves are thread-like or even hair-like, with an oval cross-section, curved, 2 to 4 cm long and light to dark green. There is a leaf sheath at the leaf base.

When cultivated in an aquarium, H. gardneri is rather short-lived, however, the population can be maintained by sowing the seeds, as it develops submersed cleistogamous flowers on its nodes, which means that the flowers remain closed and pollinate themselves. If the stems float on the surface, the flowers open up, showing six yellow petals that fall off after a short time. After flowering (even if they didn't open), transparent up to 3.5 mm long podlike fruit appear, containing 18 to 40 seeds. After the fruit have ripened, the plant dies off. When the plants are cultivated in a greenhouse or under daylight conditions this happens in autumn.
You can collect these fruit and sow the seeds controlledly under water. If you don't, the seeds just come to lie on the aquarium substrate, and if they are not eaten sometimes they germinate spontaneously after some time.

H. gardneri is a fast grower and does not have especially high demands, however, it grows best in soft to moderately hard water and likes the light intensity rather on the strong side. Temperatures of 23-25(-28) °C have proven favourable. Under less favourable conditions the plant grows undulate leaves, If the light is insufficient, its internodes grow out long, the leaves turn brown. If CO2 and nutrients are in ample supply and under strong light the plants develop a lush, dense foliage.
H. gardneri can be propagated by top or lateral shoot cuttings that are replanted in the substrate, just like most other stem plants. However, it seems as if propagation through cuttings does not prevent it from flowering and developing fruit and dying off afterwards. Reports of first-hand experience about its duration in the aquarium under various conditions might be very helpful.

Hydrothrix gardneri is a decorative, delicate green stem plant with fine leaves. A group of this plant in the middleground contrasts nicely against plants with a different texture, leaf size or colour.
As it is relatively short-lived and has to be propagated from seed, H. gardneri will probably remain a rare plant in the hobby, however, cultivation is worthwhile due to its beauty and its interesting flower biology. Its threadlike thin leaves make it look like Eriocaulon setaceum, which is - other than H. gardneri - very difficult to care for.