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The Cultivation of Amorphophallus
by Wilbert Hetterscheid and S. Ittenbach
The information here in is adapted from W. Hetterscheid and S. Ittenbach, 1996, "Everything You Always Wanted to Know About Amorphophallus, but Were Afraid to Stick Your Nose Into!!!!!", Aroideana 19: 7-131.
Please also see the additional note/update at the bottom
The following information is based on experience by the first author, built up during about 10 years of cultivating Amorphophallus species, at first only a few specimens in a windowsill, but today a research collection of more than 400 individuals, representing some 130 species. Much of today's cultivation is carried out by the skilled greenhouse personnel of the Leiden Botanical Garden (Netherlands), but a fair number of difficult plants are still under the personal care of the first author. A number of African species are grown in the Bonn Botanic Garden (Germany). It has never been the goal to find out exactly what the perfect treatment of every individual species is. It would result in too many different guidelines and there just isn't enough opportunity to spend much time on all individual species in such a large collection.
The early years were times of trial and error, since nothing relevant was known about cultivating Amorphophallus. Most botanical gardens did possess one or two out of three well known species, viz. A. bulbifer, A. konjac (usually under the name A. rivieri Durieu ex Carr.), or A. paeoniifolius. That is not much of a surprise because these three are rather easily cultivated and readily available through tropical markets and even sold by specialized bulb growers. The more lucky botanical gardens got hold of specimens of A. titanum, but they were always imported as mature plants and usually died after flowering. That is why Kew Gardens may be mentioned as having made an astounding achievement at the end of the last century in bringing an A. titanum into flower starting from seed in a period of ten years. The trial and error method worked quite well for Amorphophallus because the majority of species are not very difficult to grow, provided a minimum of conditions are met. These are roughly a rich soil, the use of additional fertilizer, shading against direct sunlight, a minimum temperature of 22 oC during the daytime and 19 oC during the night, a well defined resting period, and a severe regimen against pests, preferably aimed at prevention. With these requirements, roughly 80 percent of all Amorphophallus species can be grown successfully. Below are some more precise ideas and observations, but it must be kept in mind that every grower may work out better circumstances for one or more species. On a smaller scale of cultivation, such ideas can be tested better.
Temperature: What has been said above about temperatures holds true for some 98 percent of the species. A notable exception is A. kiusianus from southern Japan. This species does not appreciate particularly high temperatures in cultivation and the leaf dies back quickly when the temperature rises well above 25 oC. Similarly, A. konjac seems to thrive best between 20 oC and 25 oC, although it is more resistant than A. kiusianus. Amorphophallus bulbifer seems to tolerate a rather large range of temperature and thrives well between 15 oC and 30 oC.
Soil: Roughly there are two types of soil needed. Both should be rich in organic matter and trace elements, but one must be well-drained and must not contain loam, while the other may be heavier and enriched with loam. This division corresponds roughly with the two major climatic preferences of Amorphophallus species. Those of strongly seasonal climates can be grown in the heavier soil. This includes all species with elongate tubers, and those from Africa, China, Japan, India, Thailand, Indochina, the Philippines, Java (but not all!), eastern Indonesia, New Guinea and Australia. Also, all three Asian species with a large geographical distribution are used to such soils (A. muelleri, A. paeoniifolius and A. prainii). For easy reference this large group of species will hereafter be referred to as "Group I". Group II comprises nearly all species from Sumatra, Borneo (= Kalimantan, Sarawak, Sabah, and Brunei) and West Malaysia with the exception of the earlier mentioned widespread ones. Group II species must be kept in a well drained soil because their roots only survive long enough to do their job in a well aerated soil. When the soil becomes too dense (through excess water or too much loam) the roots will rot. The trouble is that the leaf does not die down but keeps on living, supported by energy from the tuber, which will finally be depleted, becomes weak and suddenly starts rotting. When the leaf then suddenly tips over, all (or most) is lost. Both groups require a rich soil, especially in trace elements, which may be added with a fertilizer (see below), and organic matter. Thus far, adding limestone has not proven to be mandatory for any of the species, not even those from limestone regions. However, since lime (e.g. dolomite) has a high content of trace elements, adding it to the soil is an improvement.
Fertilizing: Group I species, when grown in a rich and heavy soil, do not need fertilizer very frequently, i.e. once in a month may be more than enough. Group II species however must be fed extra when the soil is indeed loose and contains only organic matter. However, do not give a concentration any higher than what is recommended for the fertilizer. Excess fertilizer may well kill off all roots. The ideal fertilizer is one with a high phosphate content (e.g. 15-30-15), which stimulates tuber growth. In the end the health of an Amorphophallus is measured by the increase of the size of the tuber every season, especially in young to submature plants. The tuber may increase its weight threefold per season.
Planting: The top of an Amorphophallus tuber must always be well below the soil surface, since the roots emerge from the top and must secure the tuber to carry the long leaf or inflorescence. For this purpose the first roots grow very fast and horizontal. After they have reached full length, they start contracting and so secure and stabilize the tuber and the developing leaf. This is necessary because the developing leaf depletes most or all of the old tuber, so its base tends to "hang loose" in an empty hole where the tuber first was. This entire structure is very unstable and has a great need for the contractile roots. Those species producing elongate tubers need very deep pots to accommodate the strong vertical growth. When the pot is not deep enough, the base of the tuber will get cramped against the bottom and become deformed. This however does not usually impair the health of the tuber. The only problem may be that excess water that is often found in the bottom of the pot may cause local rotting of parts of the tuber base. The diameter of the pot must be at least twice that of the tuber that is planted, for obvious reasons.
Watering: Species of Group I are quite resistant to excess water with the exception of those with elongate tubers. The latter species usually show slight superficial rot, even when they have not been overwatered. These small spots usually dry well when the tuber is taken out during dormancy. When the spots are large, they may be scraped off superficially and inspected for deeper rot. Group I species are also quite resistant to dry conditions and they will stand neglect fairly well for a longer period (to ca. 3 weeks). Group II species are sensitive to excess water which is why they need to be grown in very well-drained soils. Since most roots of Amorphophallus plants amass in the bottom of the pot, the soil there must be especially well prepared to transport water. Do not use a thick layer of coarse grit because then the roots will be unable to extract water and nutrients and will subsequently die. Adding bark to the bottom layer of the soil is a useful alternative.
Dormancy: Except for a few evergreen species (e.g. A. arnautovii, A. pingbianensis), all Amorphophallus have a dormancy period in nature. All species of Group I show the strongest tendency to dormancy in cultivation, while those of Group II may on occasion skip dormancy altogether, but not as a rule. Dormancy may take place in two parts of a species' growing cycle. When a leaf dies down after a regular growing season the tuber will invariably take a resting period. This period may take 3 to 7 months, depending on the species, after which a leaf may emerge. When the plant is mature enough to flower the resting period is usually considerably shorter and may take no longer than a month or even less at times. During this "after leaf" dormancy, tubers of Group I species can be stored dry but not seedling tubers or thin rhizomatous offsets. Group II tubers must be left in the soil because they are usually prone to desiccation. A second type of dormancy may set in after flowering. This only holds true for non- evergreen Asian species, which in nature invariably refuse to develop a leaf after flowering and effective pollination. Therefore, of these species fruiting plants are never found with leaves. In cultivation a similar behavior is observed with most species. But sometimes, when the inflorescence is taken off, a plant may develop a leaf after all. Since this behaviour cannot be predicted, the tuber is best left in the soil after the withered inflorescence has been removed. Watering must be lessened and the pot must be well observed for renewed growth. If this happens, the plant must be watered as usual; if not, it must be left to dry out until the next season. African species all develop a leaf in the same season as flowering, with or without pollination and fruit set. Most develop a leaf shoot directly alongside the peduncle, others take a short resting period. This leads to an important warning: when the old inflorescence of an African species is cut away, be sure not to cut the new leaf shoot which may be very close to the peduncle! Remove the cataphylls first to locate the new shoot before cutting the peduncle. The African Savannah species pose a special problem (e.g. A. aphyllus, A. dracontioides) because it seems that they need a special treatment during dormancy. So far none of these species have been brought to flower, even though plants have reached full size. Probably they need extra heat during resting, imitating the harsh conditions on the savanna during the dry period.
Vegetative Propagation: Most Group I species readily produce offsets every year of vegetative growth. These offsets may still be attached to the main tuber and should be severed when the mother tuber is replanted for a new season. In other species the offsets are already loose when the plant is dug up after a season's growth. Usually less than 100 percent of the offsets grow into new plants. There is always a percentage of inert offsets that may linger on for years without sprouting and finally die. Every once in a while however, such a latent offset may start growing again, so never throw offsets away until they are definitely dead. Keep slim, rhizomatous offsets in soil during resting (e.g. A. hohenackeri, A. krausei). Offsets that are left on the main tuber will usually not develop into new plants but are often (though not always) devoured when the tuber dissolves to nourish the developing leaf. Some species produce bulbils on the leaves. These are of two main types. One type is produced entirely on the surface of branching points of the leaf (epiphyllar bulbil, e.g. in A. bulbifer and A. muelleri). The other type is formed when the entire branching point of a leaf transforms into a bulbil and is dislodged when the leaf rots away (intercalary bulbil, e.g. in A. angulatus, A. manta, and A. sparsiflorus). A third type has been found only in A. yuloensis, which is half epiphyllar and half intercalary. Some species produce seed vegetatively, without pollination (apomictic seed). These are A. bulbifer, A. muelleri and A. kiusianus. The first two species exhibit a genome with 39 chromosomes and can thus not form normal haploid gametes for sexual reproduction. Vegetative propagation through tissue culture has succeeded in several occasions as witnessed by the literature. Cell suspensions have been prepared from tuber parts as well as from leaf parts (e.g. in A. titanum). The authors have no first-hand experience with this process.
Sexual Propagation: Pollinating plants of different clones in cultivation is usually successful. The pollen must be put on the stigmas on the first day of flowering, as early as possible. The stigmas are very sticky and no pollen will fall off. On the day that pollen is released the stigmas are no longer receptive to pollen. Pollination within one and the same clone has been successful in a limited number of cases (e.g. with A. atroviridis, personal communication with J. Banta) but usually leads to only very partial seed-set (two selfings in A. curvistylis yielded only three viable seeds). Hybrids between different species have been raised in Leiden but not released to the public because of fear of spreading unnatural variation into existing collections, which may lead to taxonomic misnomers. The successful crosses were: A. longituberosus x albispathus, and A. odoratus x yunnanensis.
Growing from Seed: Fresh seed of Amorphophallus usually germinates quickly (between one and three weeks). Notable exceptions so far observed are A. henryi and A. kiusianus. The seeds may be taken from the flesh of the berries but must then not be stored dry. The seed coat of Amorphophallus is rather thin and there is no endosperm layer to protect the embryo from desiccation. The first leaf is already three- or five-parted (in A. prainii often to 7-parted) and soon new leaves will emerge. This may go on for more than a year before the first real resting period starts. The young tubers are best left in the soil until they are bigger and more resistant to drought (only Group I tubers may be stored dry!). Sow the seeds in a typical sowing-soil (poor in nutrients and slightly acidic). Difficult seeds may be forced to germinate in sphagnum. Once dry, Amorphophallus seeds cannot be rehydrated successfully.
Pests: The two major primary pests of Amorphophallus in cultivation are nematodes and root mealy bugs. Root mealy bugs are similar in appearance to mealy bugs and they are very resistant to most superficial chemical pesticides. No biological control mechanism yet exists. Therefore, the best method is using a systemic pesticide with a broad activity spectrum. Unfortunately such pesticides are usually rather unhealthy for humans. The most effective has proven to be Temik, as well as the partially systemic pesticide Vydate, but must be given in small doses because leaf damage may occur. Both pesticides can also be used preventively. Nematodes can only be effectively fought off with strong methods like using Temik. The infections are easily recognized in their early stages as small to medium-sized, hemispheric warts/bumps on the tuber surface, not associated with root scars or accessory buds. When cut, they show a dirty pale grayish center, consisting of cells that are greatly enlarged and contain a great amount of water. In the middle of this bump resides the female nematode filled with young specimens. The female nematode is dissolved and releases the young ones, who feed on the watery tissue of the bump. After this, the bump implodes and becomes a crater, which is attacked by other secondary pests (acarids, fungi, bacteria). The released nematodes reinfect the newly developing tuber and destroy it, or damage it beyond rescue. All bumps on an infected tuber must be cut away and the tuber then put in a Temik solution. After that it must be left to dry and must be regularly inspected for desiccation or new infections. When planted, the soil must be mixed with Temik before planting. This treatment must be repeated for two or three seasons, after which the infection is usually conquered. Do not make an effort to save an individual of a species of which you have enough duplicates. Better to throw the infected tuber away. Heavily infected tubers show large craters and rotting parts. When the top of the tuber is still intact, it may be saved. When the top is gone and there are no accessory buds, the tuber is beyond rescue. A heavily infected tuber must be cleaned and the largest scars sealed with a fungicide (e.g. Topsin) in order to prevent infection and desiccation. Tubers that have been in wet soil too long may start rotting at the base of the roots. This type of rot spreads through the tuber with tremendous speed and is usually detected too late. Cut away all rotting parts, seal off with Topsin and pray! The same holds for occasional bacterial infections. These are recognized by the flesh of the tuber turning into a slimy substance. Cut away the bad parts and treat the scars with a bactericide. When the scar becomes dry, seal off with Topsin. Prevention is the rule in controlling pests in Amorphophallus. It is therefore recommended that tubers always be inspected for pests after the growing season. Be sure to put tubers of Group II species back in the soil after examination.
Additional Note from Wilbert Hetterscheid
Hi John and other peoples, I cannot confirm your observation. In fact I have noticed quite the opposite. In my collection, the ones that need continuous soil and/or watering always have roots spread all over the tuber that usually are healthy when the leaf has already died down. The ones that need dry conditions and have lost their leaves, are usually also without roots or have dead or dying roots. My experience and that of others written to me, have changed my views on dormancy issues in Amorphophallus quite dramatically over the years since my publication with Ittenbach in Vol. 19 of Aroideana. Generally it seems that species from (West-)Central Africa, West Malaysia, Borneo, Java, Sumatra, S. & SE China and N. Vietnam need to be kept in soil all the time. They tend to have rather short dormancies or nearly none at all. They are from very wet areas (everwet tropical). During growth they can hardly be ""overloaded"" with water during watergift. Species for which this works good are: angolensis angulatus annulifer bangkokensis baumannii beccarii borneensis boyceanus brachyphyllus bufo coaetaneus costatus croatii decus-silvae discophorus dzuii eburneus eichleri elegans excentricus (is also quite drought resistant) galbra gigas glossophyllus (also quite drought resistant) hayi hetterscheidii hewittii hirsutus hottae infundibuliformis interruptus (also quite drought resistant) julaihii koratensis (when young) lambii manta myosuroides ochroleucus opertus palawanensis (can also be stored dry) pendulus prainii preussii rhizomatosus rugosus sagittarius scaber (only when young) sinuatus sparsiflorus spectabilis staudtii subpedatus tinekeae titanum tonkinensis tuberculatus venustus verticillatus zenkeri Unfortunately, this behaviour makes them the more difficult species to grow (except the ones from N. Vietnam, S. & SE China and A. hottae). The ones that have a prolongued dormancy are usually the stronger ones in cultivation and need less humid conditions. Then the smallest of species also appreciate not to be bone dry but they must also be watered carefully and not become too wet. They too tend to have hardly any appreciable dormancy and flower immediately after leaf shed or alongside mature leaves: myosuroides obscurus ongsakulii polyanthus pusillus serrulatus I think Tony Avent and Alan Galloway may have plans to publish on this. In case the IAS board decides to reprint vol. 19, I will add a text on this subject as introduction. Of course it may be that under local circumstances at individual people's places, plants may have a deviating behaviour. But this is merely a general directive. Cheerio, Wilbert
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