Amorphophallus

Too much can show on the leaf edges. Scott Hyndman of Vero Beach, Florida (USDA zone 10a), wrote that he over-fertilized his large Amorphophallus paeoniifolius in a 150-gallon (about 570-liter) pot, especially while the new tuber was growing: the leaf edges turned yellow, then brown and crumbly, which he attributed to high soluble salts, and the foliage looked as though the plant was about to die, yet it produced a healthy tuber that was about to bloom when he wrote on May 5th, 2001.

Lobin and colleagues’ Amorphophallus titanum protocol at Bonn used liquid feed every two weeks during active growth, reduced it in winter and stopped when leaves began to wither and shortly before the spathe opened. It is a single-species botanic-garden regime, not a general dose for the genus, but it reinforces the seasonal rule: feed the leaf that is rebuilding the tuber, then stop as growth closes.

Root-zone temperature

Soil temperature matters too, at least in young plants: Ron McHatton, who at one time grew about 4,000 Amorphophallus titanum from seed, found it important for keeping them growing. The optimum seemed to be near 80°F (27°C), and the plants tended to overheat if the soil went much higher. He added that wet potting mix can fall to 50°F (10°C) through evaporation when the air is 60°F (16°C) (November 27th, 2007).

Hardiness reports

A few species are the exception. Marco Motta’s Amorphophallus Resting Table lists five as frost resistant (Amorphophallus albus, Amorphophallus bulbifer, Amorphophallus konjac, Amorphophallus yuloensis and Amorphophallus yunnanensis) and three more as under test (Amorphophallus coaetaneus, Amorphophallus dunnii and Amorphophallus henryi). In a message to the Aroid-L mailing list on February 19th, 1997, Tony Avent reported that Amorphophallus bulbifer had survived 0°F (−18°C) in the garden at Plant Delights Nursery in Raleigh, North Carolina (USDA zone 7), for relatively short periods.

Growers’ reports on the frost-resistant species come with limits. Steve Marak wrote to the Aroid-L mailing list that winters in northwest Arkansas (USDA zone 6) reached about −20°F (−29°C) every five years or so, for short periods, and that he left his Amorphophallus konjac in the ground all year (May 7th, 2003). In 2007 he wrote that he had grown it outdoors there for more than 20 years (zone 7 for the last seven or eight of them), but that the other clones he acquired were less vigorous outdoors than the first he grew, and a couple disappeared (August 14th, 2007). Victor Soukup wrote from a suburb of Cincinnati, Ohio (zone 6), in 1997 that Amorphophallus konjac had not survived outdoors there several years earlier, and in 1999 that it would overwinter there with proper siting and winter protection as necessary (January 15th, 1997; April 20th, 1999). Marak’s one outdoor Amorphophallus bulbifer had come through about three years without special treatment, including the ice storm of January 2009, though the area had seen nothing below 0°F (−18°C) for about nine years (September 30th, 2009).

One of the three species Motta lists as under test, Amorphophallus coaetaneus, has a first-hand report: two plants that Russell Coker set in the ground in Mobile, Alabama (zone 8b), and covered with oak leaves once frost had cut them back came through what he called the coldest, wettest winter in recent memory, with several nights in a row in the 20s°F (about −7 to −2°C), and returned the next spring, the first emerging with a flower and both carrying leaves almost two months later (June 28th, 2010). Wilbert Hetterscheid had written in 2006 that the species is not hardy at all (December 29th, 2006).

Dormancy

Most species signal the end of the season on their own: the leaf yellows and collapses, and no new growth follows. From that point, reduce watering and then stop altogether — a dormant tuber sitting in wet substrate is at risk of rot. What happens to the tuber next depends on the species. Marco Motta’s Amorphophallus Resting Table records it for 113 of them:

Watering does not always stop with the leaf. Wilbert Hetterscheid noted that Amorphophallus polyanthus, one of the four in the Evergreen row above, has a strong tendency to flower immediately after leaf senescence sets in, sometimes while old leaves are still more or less present, so he advised not withholding water from a mature plant when the leaves start to turn yellow, then keeping it dry after flowering, although now and then he found plants thriving again quite soon after flowering (February 22nd, 2010).

Each species page shows its own entry (and the Potting tab has the full table), with Motta’s notes where he made them: some species that rest in soil are also quite drought resistant, and a few should be kept in soil only while young. Repot into fresh substrate when the new bud emerges, and resume watering once active growth is visible. The length of the rest varies by species and by individual — some tubers wake in weeks, others sit for many months. Seedlings may skip the rest at first: Allan, a grower in Canada, raised about 30 Amorphophallus hewittii from seed and lost only a couple; the seedlings kept making leaves for about two years before going dormant for the first time, and the rest has so far lasted only about two to three months (April 18th, 2007).

Not every grower pots tight, and the sources on this page differ:

Bulbils and offsets

In bulbil-bearing species such as Amorphophallus bulbifer, the tubercles at the leaflet junctions do the work on their own — each one a ready-made plant. Not every plant of such a species bears them: Wilbert Hetterscheid wrote that in Amorphophallus coaetaneus some clones develop bulbils in the leaf axis and others do not, and advised leaving them on the leaf until it has died back completely before planting them (December 15th, 2006).

Offsets are not universal. Wilbert Hetterscheid wrote that Amorphophallus titanum does not produce offsets, which he defined as true side tubers that separate from the mother tuber on their own in their first year or at the start of their second, and that the extra tubers sometimes seen are splits, adventitious buds developing into new tubers (May 22nd, 2006), though Doug Ewing, greenhouse manager at the University of Washington, had reported Amorphophallus titanum plants that offset, occasionally yielding an extra tuber or two (September 10th, 2002). Handling differs too: Hetterscheid advised keeping the offsets of Amorphophallus albus on the mother tuber until it is replanted and separating them just before planting, leaving attached any that resist (December 11th, 2006; October 27th, 2005), while Ron McHatton removed all of his offsets for years, stored them detached with little loss, and found that more of them sprouted than when they stayed on the parent tuber (October 27th, 2005).

A measured leaf-cutting trial

Lobin and colleagues tested ten Amorphophallus titanum leaf cuttings at Bonn in a peat-and-sand mix with bottom heat. Five failed to root; tuber tissue formed in about three months and the first leaves followed about six months later. The result demonstrates the route for that species but also shows that “readily” is not universal: most of the thinner cuttings, about 20 mm, survived, while all of the thicker ones, 60 to 70 mm, died.

Pathogens and vectors

Lobin and colleagues identified root-knot nematodes, mostly Meloidogyne incognita, as a major cause of short life in cultivated Amorphophallus titanum in warm conservatories. Dong and colleagues found Dasheen mosaic virus dominant in sampled cultivated Amorphophallus in Yunnan and also documented orthotospoviruses; western flower thrips dominated their nearby thrips traps, but the study did not demonstrate transmission from those insects to the crop. Visible stippling, distortion or mosaic therefore calls for diagnosis rather than an assumption that every mark is mite or thrips feeding.