The Genus Aponogeton
by Birgit McKinnon, CAS
originally published in The Calquarium Volume 39, Number 6, March 1997
The genus Aponogeton is distributed widely through tropical Asia as far east as India and as far north as South Korea. The most popular aquarium species (e.g. A. crispus, A. undulatus, and A. natans) come from the Indo-Malayan area. Another suitable aquarium species, A. elongatus, is found in Australia. Aponogetons are also found throughout Africa but these species are not generally suitable for the aquarium because of their large size and the long resting period they require. In Europe, these species are used in outdoor ponds where their requirements can be met. Suitable aquarium species are also found on Madagascar, notably A. ulvaceus and A. madagascariensis.
Aponogetons differ from most other aquarium plants in that they originate from tubercles. These are starchy bulbs in which the plant stores energy reserves to sustain it during the growing season. A healthy tubercle can live in clean, washed sand for several months, growing leaves and even flowering. This leads aquarists to believe that a rich substrate is not required. But without nourishment, after several months, the plant stops growing, and on inspection of the tubercle it is found to be much smaller, half-empty or even rotting.
Plants of this genus are propagated mainly from seeds. Two exceptions are A. undulatus and A. rigidifolius. In the former case, adventitious tubercles form on the end of the flower stalks. They soon develop leaves and roots and are shortly thereafter released by the disintegration of the flower stalk. A. rigidifolius is the only member of the genus to possess a rhizome rather than a tubercle. This rhizome develops offshoots which can be divided from the main plant.
Once a healthy growth of leaves has been produced, a flower stalk is often seen. These are stalks with clusters of small white, pink, yellow, or purple flowers that rise above the water. The length of the flower clusters on the end of the stalk can vary from one to four inches. The species from Asia and Australia form a single flower spike on the end of the flower stalk while those species from Africa and Madagascar form two to five flower spikes on each stalk.
In nature the pollen is carried by insects, but to ensure pollination in the aquarium a soft brush is used to transfer pollen from flower to flower. The flowers open first from the bottom of the spike and the whole spike will take about three to four days to open completely. Brushing the open flowers daily will distribute pollen to all receptive flowers. Many Aponogeton species are self-fertile, which means that pollination can take place between flowers on the same plant or even flowers from different species. For this reason hybrids are easily formed and, in fact, many Aponogetons offered for sale are hybrids. Other species are self-sterile and must be pollinated from flowers from a different plant.
Once pollinated, large seeds are produced. These may be from 50 mm to 75 mm in diameter and have small pockets of air encased in the outer skin. When the seeds are ripe, they fall from the flower stalk and float away from the parent plant. Aided by the wind, waves and current, they can travel quite some distance and in this manner are distributed in nature. After about 24 hours, the seed coating splits open and the seed sinks to the bottom where it germinates if conditions are appropriate. Small roots grow within a few days, allowing the plantlet to anchor itself, absorb nutrients and grow.
In the aquarium, the seeds are best gathered before they fall off on their own so that they may be given suitable conditions for growth. Seeds should be sown on a bed of washed sand and transferred to the aquarium when the leaves are about 25 mm long and the roots are about 50 mm long. Good seed development is also produced when the seeds are kept in a bottle of clear water until they reach the aforementioned size.
In nature, Aponogetons live a cyclical existence. Typically there is a period of vegetative growth from March to May, followed by a flowering period from May to June. Seed production takes place from June to July and building up of reserves in the tubercle occurs from August to October. After this time, the tubercle sheds its leaves and goes dormant until the next growing phase. In the aquarium this is best achieved by not heating the aquarium and allowing the temperature to drop to 8C to 10C. In February or March the temperature is raised to 18C to 20C and this increase in temperature stimulates the growth of new leaves. Unfortunately, aquarium temperatures rarely drop low enough to induce dormancy by the simple unplugging of the heater. Once the plant begins to shed its leaves, it may be removed from the aquarium and placed with water in the crisper of the refrigerator for 2 to 3 months. A return to the aquarium should produce vigorous growth. Of course, sometimes all that is recovered is a bag of mush. An exception is A. undulatus which is able to achieve it’s resting period in the aquarium and will cyclically grow, reproduce and die back without a reduction in temperature.
Following are some of the more commonly available species of Aponogeton:
Distribution: Apnonogeton crispus originates in Sri Lanka.
Appearance: Leaves grow up to 20 inches long. Leaf edges are rippled and extremely wavy. Colour varies from dark to olive-green depending on light. Single-spiked flowers are white, self-fertile and easy to propagate, making this an excellent choice for the novice aponogetonist.
Care: Water, pH, and hardness values are not critical. Temperature 22C - 30C. Substrate should be rich with nutrients. Iron fertilizer is beneficial. Give a resting period when leaves begin to die back.
Light: 50 watts (and more) per 100 liters.
Propagation: Seeds. Fruit matures in two months.
Distribution:Aponogeton rigidifolius originates in Sri Lanka.
Appearance: Leaves grow to 24 inches in length and are dark green to brownish. Leaves are somewhat brittle, rough and hard with lightly wavy edges. Forms a rhizome rather than a tubercle so there is no resting period. Flowers are greenish, single-spiked and self-sterile.
Care: This is a rather delicate Aponogeton. It does not transplant well and requires soft water low in salt. Recommended values are 20 ppm to 50ppm carbonate hardness with a pH of 5.5 - 6.5. Temperature should be between 22C and 28C. In unfavourable conditions, leaves become spotty and curl downward. With CO2 fertilization it will tolerate somewhat harder water. Iron fertilizer is appreciated.
Light: 50 watts per 100 liters.
Propagation: Lateral shoots on the rhizome.
Distribution:Central and Northern Madagascar.
Appearance: Leaves are short-stalked, and wavy with a length of 60cm. Colour is bright green; reddish is very bright light. Flowers are double-spiked, yellow and generally self-sterile. A very decorative plant.
Care: Liquid fertilizer with iron should be applied. Grows well in moving water. Is not demanding with regard to water hardness. pH should be around 7.5. Temperature from 22C - 28C. In poor light it grows very tall and thin. Give resting period.
Light: 50 watts per 100 liters.
Distribution:India and northern Indo-China.
Appearance: Leaves are about 16 inches long with ruffled edges. In weak light the edges are almost smooth. Rarely produces flowers, rather an adventitious plantlet forms on the flower stalk. Adapts well to hard water.
Care: Does well in a substrate rich in nutrients and appreciates iron fertilizer. Water conditions not critical. Temperature from 22C - 28C.
Light: 50 watts per 100 liters.
Propagation: Adventitious plantlets form on the end of the flower stalk. These may be removed from the stalk once roots are formed or may be left on the stalk until the stalk deteriorates, setting free the plantlet.
Aponogetons are very decorative aquarium plants. With great variety in leaf shape and colour, they provide wonderful contrast when aquascaping. Growth is very fast and flowers are frequently produced. Many are quite undemanding and adaptable, making them valuable residents in our aquaria. With the (generally) low cost, I encourage you to try these plants. I’m sure you’ll enjoy them.
Water Plants in the Aquarium, Ines Scheurmann, Barron’s Educational Series Inc. 1987.
A Fishkeeper’s Guide to Aquarium Plants, Barry James, Tetra Press, Salamander Books Ltd. 1986.
Baensch Aquarium Atlas l, Dr. Rudiger Riehl and Hans A. Baensch, MERGUS-Verlag Hans A. Baensch, 1987.
Baensch Aquarium Atlas 2, Hans A. Baensch and Dr. Rudiger Riehl, MERGUS-Verlag GmbH Hans A. Baensch, 1993.
Aquarium Plants, Dr. Karel Rataj and Thomas J. Horeman, T.F.H. Publications Inc. Ltd. 1977.
Other aquarium clubs and non-profit organizations can use this, or other articles, in their own journals or web sites, provided that credit is given to the author, the Calgary Aquarium Society, and The Calquarium. In the case of a reprint in a hardcopy publication, two copies of the published work are sent to the Calgary Aquarium Society at its mailing address. And in the case of a reprint in an Internet publication, a link back to the original article site must be provided in a prominent location.