The Genus Amorphophallus (Araceae)

The Genus Amorphophallus (Titan Arums)

Geography and Habit
The genus Amorphophallus (commonly known as "Titan Arums") is made up of some 180 herbaceous species at present. The majority of Amorphophallus species is located in subtropical and tropical lowlands of forest margins and open, disturbed spots woods throughout Asia. Few species are found in Africa (e.g. Amorphophallus abyssinicus, western to eastern Africa), Australia (represented by a single species, namely Amorphophallus galbra, Queensland, North Australia and Papua New Guinea), and Polynesia respectively. Few species such as Amorphophallus paeoniifolius (Madagascar to Polynesia) serve as a food source throughout the Asian region for their tubers and have been cultivated in the tropics for a long time. A complete list of Amorphophallus species including many photos of Titan Aroids is available from the website of the International Aroid Society.

As in other related genera within the broad Aroid family (Araceae), such as Arisaema, Dracontium, and Typhonium, most species develop a large, subterranean round to elongated tuber or fleshy rhizome, which serves as a storage organ and may gain up to 60kg and more in the tallest species, such as Amorphophallus hewitii (Sarawak, eastern Malaysia), Amorphophallus titanum (Sumatra, Indonesia), and Amorphophallus gigas (Sumatra, Indonesia). In most species a single, long-stemmed, umbrella-like, dissected leaf is developed each growing cycle, resembling almost a small tree in the giant members of this genus. In certain species, such as Amorphophallus bulbifer (widespread throughout India) a secondary leaf may develop under favorable growing conditions.

On maturity plants produce a usually single inflorescence, composed of a sometimes very colorful outer spatha (pseudo petal) and an inner spadix, which may be shorter or protruding the spatha, usually in a different color. The latter hosts the quite small male and female flowers at its base. On the day of opening the female flowers are receptive and the inflorescence produces a sometimes very, rather unpleasant strong scent, which is very attractive for its main pollinators, such as flies, and certain beetles. On the second day the female flowers are not receptive anymore, whereas the male flowers release their pollen. After successful pollination the female flowers develop into some colorful berries, which may contain 1 to rarely 2 or 3 seeds each. In most species berries are bright orange to scarlet, whereas some may be bluish-lilac (Amorphophallus kiusianus, southern Japan, eastern China, and Taiwan) or rarely whitish-green to yellowish-green. The ovate to elliptic seeds are embedded in bright colored flesh in most species and are usually dispersed by birds.

Cultivation
In culture most species are easily grown in any humus-rich, rather organic, well drained and well aerated substrate. The majority of subtropical Amorphophallus species can be grown at a minimum of some 20°C (optimal 25°C to 30°C) during summer in a semi-moist soil in a partially shaded spot with protection from direct sunlight, especially during hours at midday. Few tropical species, such as the famous Amorphophallus titanum from Sumatra require a higher average temperature of at least 25°C throughout both their growing cycle and dormancy period. Few species have a prolonged growing cycle which exceeds 12 months, or are even evergreen. However in most species the single leaf will start to yellow and to wither in autumn. At this point plants should be kept absolutely dry at a minimum of some 10°C. The tuber of immature specimens may increase its weight threefold within a single season.

Most tubers stay dormant for some 3 to 7 months. Thus they are preferably moved to a warmer spot with beginning of spring, the increase in temperature during daytime stimulates in most Amorphophallus species new growth. As soon as the new leaf appears from the center of the tuber, plants shall be watered carefully again. Most Amorphophallus species flower before the leaf unfolds, whereas some may rest for the entire year.

As soon as roots emerge, which is in most species primarily from the upper part of the tuber, it shall be planted deep enough so that the roots may develop freely. Three times the size of the tuber is a suitable depth for most species.

Several species vegetatively propagate as well, either by stoloniferous tubers (as in Amorphophallus albus, China and Amorphophallus konjac, southern China and Vietnam), rhizomatous offsets (Amorphophallus krausei, (Myanmar, northern and western Thailand, and Yunnan Province in China), small to medium sized bulbils formed between leaflets (Amorphophallus bulbifer and Amorphophallus muelleri (western Thailand, Andaman Islands in India, Sumatra, Java, Timor, and Sulawesii in Indonesia), and small adjacent tubers (as in Amorphophallus odoratus, syn. Amorphophallus dunni, south-eastern China and Amorphophallus paeoniifolius). These shall stay attached to the main tuber until they can be separated easily and repotted as a new individual.

Winter Storage
Both smaller corms with a diameter of 3cm ø and less as well as rhizomatous corms with a length of 7cm and less usually do not have sufficient nutrient and/or water reserves, in order to be overwintered under fully dry conditions. They should be kept potted up in winter in a very lightly moist substrate at a minimum of some 15°C. The optimal temperature range are some 20°C to 25°C for these plant sizes. Corms should be covered approx. 2cm in height with substrate. Pot sizes of 1 up to 2 litres are sufficient for these plants.

Under warmer conditions they may start new growth in winter already. If this should happen, keep plants in full sun until mid April at a minimum of some 20°C and careful watering. The substrate should become never completely wet, but it should stay only slightly moist. Starting from late spring, i.e. in the middle/at the end of April, plants should be placed to a partially shaded spot, in order to avoid leaf burns by the more intensively sun exposure in summer. The majority of Amorphophallus species inhabit woods in nature and consequently are never exposed to the full equator sun despite their subtropical to tropical origin !

Larger tubers over 3cm ø in diameter or a length of over 7cm can either be unpotted and dry-stored just like dahlia tubers or potatoes in a dark and cool place (minimum some 10°C, e.g. in the cellar). Alternatively they can stay in their pots at higher temperatures during their dormancy period in winter. In the latter case all plants, which obviously start to grow should be taken care of as described above.

Certain species (in particular Amorphophallus konjac) will automatically bloom towards in mid winter or by the end of winter despite relatively cool winter conditions of perhaps only some 10°C. The flowering induction depends on numerous environmental factors and can usually only be influenced to a minor extent from outside. In contrast to a new leaf shoot, an inflorescence will quite rapidly emerge from the center of the corm at even low temperatures (approx. some 10 days in Amorphophallus konjac). To ensure that the inflorescence unfolds completely, the corm should be potted up and kept in a warm spot in full sun during winter. After the inflorescence will have collapsed and withered, a new leaf will grow some 4 to 12 weeks later. This will usually happen under more favorable climatic conditions for the plant toward end of spring/at the beginning of summer in Central Europe. This leaf will collect new energy during the growing period in summer and store nutrients inside the corm for the following blooming period. It is important that in this phase plants shall not be repotted under all circumstances, since during its growing period the old tuber will dissolve completely and be replaced by an entirely new one. Any repotting would possibly lead to growing disturbances and/or a total loss of the corm caused by rot.

Sowing
Most Amorphophallus are easily raised from seed. Seeds are potted some 1 cm deep in the same soil mix as mentioned above, which is kept only slightly moist. The optimal temperature is some 25°C at day and some 20°C at night. Hence most seedlings occur in a period of some 2 to 6 weeks, some may take a couple of weeks more. If seeds are sowed in autumn or winter, rarely some seeds of various species will enter dormancy and will not germinate before following spring/summer.

Most seedlings of Titan Arums show a prolonged growing cycle in comparison to older and mature specimens within the same species. Thus pots should kept warm in partial shade throughout the year as long as seedlings are “green”, i.e. as long as they have a leaf. Several seedlings may make not only one, but two or more leaves before they enter dormancy for the first time. In general young dormant tubers from seedlings shall not be kept completely dry, as they may desiccate easily. They prefer a slightly moist soil at a minimum of some 20°C.

Pests and Diseases
Especially dormant tubers are sometimes prone to fungal or bacterial infections and rotting, especially in moist soil at low temperatures. If an infection is detected (parts of the tuber start to get slimy or mashy), an effort of rescue may be started by generously cutting away all infected parts. The rest of the tuber shall hence be dusted with some power fungicide and kept absolutely dry for several days before being repotted in a rather dry soil. In general and as a prevention measure against fungal infections older plants should not be irrigated during dormancy as long as the new leaf does not emerge.

A second pest which may especially infect plants in growth are nematodes (small almost transparent worms some 2mm to 4mm in length). All dormant tubers shall be repotted every other year and shall be inspected for these insects. If they are detected, an all-purpose systemic pesticide should be applied as directed for at least two growing seasons.

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Note: Information on Arisaema (Cobra Lilies or Jack -in-the-Pulpit), a genus which is closely related to Amorphophallus within the aroid family (Araceae),are available on this page.

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