This species had been described by Rataj in 1969 for the first time and survived the revision of Haynes & Holm-Nielsen. Echinodorus horizontalis is occuring in the north of South-America from Columbia, Venezuela, Ecuador, Peru and the Amazonian basin of Brasil. The name is derived from the blades set up horizontal. This position is typical for this species and isn’t occuring in this significant style at other species. Unfortunately E. horizontalis is nearly vanished in aquarium culture. Most of the plants sold under this name are belonging to other species.
I was glad, getting an original E. horizontalis from an older hobbyist, who had cultivated this line since 40 years. This was my first contact to this
species. In the meanwhile I visited Ecuador and could study this taxon in natural habitat.
Echinodorus horizontalis is growing in the understory of higher vegetation. Not only in the shadow, but also in full sunlight. It grows mostly as
emerged plants in cause of the water level is often only some centimeter. Like other amazonian waters the conductivity is very low and all types of
water (white water, clear water and black water) can be recognized. The substrate is clay in the underground and a thick level of rotten leaves lies
on it. In its natural habitat the emerged plants can reach up to 1 meter height. The largest blades I could find, reach up to 40 cm length and 25 cm
width! In submerged culture I never could see this size. The propagation in nature is mostly by adventitious plants at the last whorl of the inflorescence. But it could propagate also by seeds.
The blades have a round-oval form, the base is cordate and the apix acute. Normally the blades have 5-7 veins. High illumination generates red
colored leaves for the first days changing to bright green after some days. Just below the base of the blade the petiole has a sharp angle to set up the
blade in horizontal position. The blades grow up to 15 cm (6 “) with about 6,5 cm (2,5 “) of broadness.
Cutting off an emersed leaf and drying it between paper you can recognize beside the normal veins pellucid markings forming hexagonal rings like
honeycombs. These pellucid markings can be onlyfound at E. horizontalis and 3 other narrow related species, but these have other formed blades or inflorescences.
For correct determination the inflorescence is necessary. The rounded petioles of the inflorescences are much longer than the petioles of the leaves. Only 2 or 3 whorls are appearing, containing
only 2 or 3(4) flowers. If you are lucky, you can get only one or two adventitious plants per inflorescence, mostly at the upper whorl. This may be the reason, that E. horizontalis is rare. Fascinating is, that the flower
opens in the later morning hours only for 2 hours in maximum. After this, it closes completely and you could be of the opinion that the flower didn’t bloom. E. horizontalis opens its flowers even unter water. The
plant is self fertile. Normally the nutlets of Echinodorus have a hard cover, but E. horizontalis has soft coated nutlets.
Deciding for the authenticy of Echinodorus horizontalis are the cordate, horizontal positionated, pale-green blades with pellucid markings in form of honeycombs and inflorescences, longer than the leaves, having only 2 or 3
(4 ?) whorls. These characteristics must all be present, otherwise it is an other species, for example Echinodorus tunicatus
, having only short inflorescences (shorter than the leaves) with many whorls.
Echinodorus horizontalis is a typical jungle plant with less requirements of light, because it’s living in the undergrowth of trees and brushes. But in
aquarium culture full lightning is recommended. The shadow of the jungle is much more brighter than aquarium light! Although the plants don’t grow
very large, they require enough space because of their broad horizontal leaves. In nature E. horizontalis is occuring in water with less conductivity
(<100 µS), but in aquarium culture it can be cultivated even in water with higher hardness if there is enough carbon dioxide available. The substrate
should be fairly provided with nutrients, therefore additioning clay to the substrate is beneficially.
All in all Echinodorus horizontalis is one of the most beautyful aquarium plants. The horizontal positionated leaves are giving a pleasant contrast to
the other erect growing plants. What a pity that these plants are hardly to get in shops.
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