DESCRIPTION
Arum is a genus of 26 accepted species of tuberous, seasonally dormant perennial herbs in the family Araceae — the genus that gives the family its name, and the one from which a good many other aroid genera were carved. It ranges from the Azores and the Canary Islands east to western China, and from southern Scandinavia and the British Isles south into North Africa and the Levant.
Every species spends part of the year as a bare tuber underground. Above ground the plant is a handful of cordate, sagittate or hastate leaves on sheathing petioles and a single inflorescence: a spathe rolled into a closed basal tube around the flowers, opening above into a limb, with a naked spadix appendix standing clear of the tube. Leaf shape and color vary so freely within one colony that the first leaf of the season and the last can look like different plants.
The name is much older than the genus. Linnaeus took up a word already in general use, recorded as a generic name in 1542 and, long before that, in classical Greek, where both Pliny and Dioscorides described more than one kind of aron. Arum maculatum alone is credited with more than a hundred English common names, nearly all of them drawn from the look of the plant in flower.
Recorded for Arum maculatum in England, Wales and Scotland by 1886. Most fall into a few families: paired names for the two spadix colors, clergy names for the hooded spathe, animal names for the berries, and a large group of frankly sexual names for the appendix.
DISTRIBUTION
Wholly Old World, and almost wholly temperate: Arum is the one genus of Araceae centered on Europe and the Mediterranean. The range runs from the Atlantic islands — the Azores, Madeira and the Canaries — east across the Mediterranean basin and the Caucasus into Central Asia, reaching Xinjiang, Tibet and the western Himalaya; north to Sweden, Denmark, Ireland and Great Britain; and south through Morocco, Algeria, Tunisia and Libya, and down the Levant into Iraq and Iran.
Beyond those 57 native places the genus is naturalized in 21 more — most of them states of the USA on both seaboards, along with New Zealand, Argentina, Norway and a handful of northern European countries where it does not occur wild.
ECOLOGY
Arum keeps the Mediterranean calendar rather than the tropical one. Across most of the range the rain arrives in autumn and winter — falling as snow further north — and the summer is dry, so the plants make their growth in the cool half of the year and retreat into the tuber when the wet weather stops. Southern species start into growth in autumn or early winter and either flower at once or run on until spring; the northern woodland species wait for spring. In woodland the limiting factor is not water at all but summer shade, and the tuber simply sits out the closed canopy until autumn leaf-fall lets the light back down. The charts below draw the genus’s measured envelope across its whole native range; the extremes in them belong to the continental and desert margins — the Central Asian and Mesopotamian species — and not to the woodland plants most growers know.
Arum favors heavy, nutrient-rich, basic soils, and for all the size of its range the genus occupies only five kinds of place. The commonest is garrigue — low, spiny, often aromatic Mediterranean scrub on soils derived from limestone — where the tuber usually sits buried beneath a boulder or a low shrub. Taller maquis holds a few plants, and deciduous woodland holds several more, among them Arum maculatum; variants on that woodland turn up as coniferous forest margins in the Crimea and as evergreen laurel forest on Tenerife. The fourth habitat is stream- and lake-side marsh, and only two species are regularly found in it — Arum hygrophilum in the Middle East and Arum euxinum in northern Turkey. The fifth is rocky hillside almost bare of woody plants, which several species use and none is confined to. There are no truly alpine Arum: Arum jacquemontii climbs highest, to over 3,000 m in the Himalaya, and Arum idaeum comes nearest in character, growing close to the snow line in boulder rubble on Crete alongside Anchusa caespitosa and Crocus oreocreticus.
Araceae · Form
Morphology
A buried tuber, a few leaves and one inflorescence a year — a small vocabulary, used with enough variation that a colony can look like several species at once.
The tuber is the character that carries the classification — even a plant with no flower on it can be placed to section if the tuber can be seen.
There are two kinds, and they behave as differently as they look. The horizontal, rhizomatous tuber grows the way a bearded iris does: a growing point at one end throws the main shoot, the new tuber forms there later in the year as the top dies back, and buds along the length of the old tuber make their own shoots and their own tubers through the season. Those detach in time and set up as independent plants, so a horizontal-tubered species spreads into wide, open colonies.
The vertical, discoid tuber is closer to a tuberous begonia: a flat disc with one central growing point, sitting either flat with the growing point uppermost or on its side. Each new tuber forms on top of the last. The buds around the base of the main shoot mostly stay dormant unless that shoot is damaged, and when they do grow they usually stay attached — so a discoid-tubered species builds a compact, congested clump instead of a spreading sheet.
The roots come in two kinds as well. At the start of the season a few thick, unbranched contractile roots go straight down and anchor a tuber that has spent four or five months loose in the soil. Only once it is secure do the fine feeding roots follow, and those spread into a dense mat rather than driving downward — useful where the rain is heavy and brief, and where the soil worth having is in rock crevices and on stony slopes. At the end of the season the feeding roots wither and the tough core inside the contractile roots shortens, pulling the new tuber back down to about the depth its parent started at. That is what keeps it out of the summer heat.
Fig · The two tuber types
Two flowering modes, and a vegetative plasticity wide enough that the first leaf of the season and the last can look like different species.
The shoot
Two or three cataphylls sheath the growing point and protect the shoot underground and as it comes through. In subgenus Arum the shoot starts in autumn, winter or spring depending on the species; each leaf emerges from the petiole-sheath of the one before it, and about two months in, the inflorescence begins to form inside. It expands and pushes out of the terminal leaf's petiole-sheath as that leaf matures — so the flower arrives at the end of the growing season.
Subgenus Gymnomesium, which holds only Arum pictum, inverts the whole calendar. The inflorescence comes before the leaves, terminally from the previous year's shoot, and it flowers in autumn as the new growth period opens rather than as it closes. The difference in flowering habit is the reason the subgenus exists at all.
The leaf
Leaves are few to many, more or less erect, on free to moderately clasping petioles that are terete to D-shaped in section and carry a closed wing-like or open sheath at the base. The blade runs from cordate through sagittate to hastate, with posterior lobes that are usually well defined and occasionally almost absent. At the extremes sit the weakly lobed leaves of Arum euxinum and the hastate ones of Arum italicum, and every intermediate can be found.
The variation is worth taking seriously before naming anything. Within a single colony leaf shape and size range widely; in many species the first leaf out differs from those that follow, sometimes greatly, and so does the last; and a species with two growth flushes produces two quite different-looking sets of leaves in one year. Color moves as freely — pale green in Arum hygrophilum to deep green in Arum orientale, with silver or gray blotches, black speckles, purple tinting or silvery gray veins according to species, and petioles that may be plain green, deep purple, or striped in paler green, purple or greenish brown.
How much that variation can mislead is on record. Arum italicum was long split into subspecies on leaf shape and on vein-associated silver-gray variegation — the form most favored in gardens. Wider study of wild populations found intermediates everywhere, the supposed northern and southern types not separating on either character, and the variegation turning up in plants it was said to be absent from; the subspecies were sunk into one.
Fig · Spring-flowering and autumn-flowering
A tube, a limb and a bare club — three parts that do three different jobs, and between them the most tightly organized inflorescence in the family.
The spathe
The spathe is a bract of unusual shape, divided into a basal tube and an apical limb. The tube is generally cylindric with free margins and a constricted mouth that limits what can get in; in Arum creticum alone it is open and goblet-like. Inside, it may be greenish white, white or yellow, with or without a horizontal purple band around the middle; a few taxa are purple in the upper half and white in the lower, and in Arum euxinum, Arum hygrophilum and Arum purpureospathum the whole interior is purple.
The limb ranges from linear-lanceolate to broadly elliptic and from almost pure white through greens and purples to crimson — Arum creticum is pale yellow inside and out. Spotting is a warning to identifiers: in Arum dioscoridis it is a stable diagnostic feature, but in Arum maculatum and Arum italicum the pale greenish white spathe is only occasionally marked with small purplish black speckles, and the character is too inconstant to name a plant by. Plants with spotted spathes usually have speckled leaves to match.
The spadix
The spadix divides into a sterile appendix above and a fertile portion below. The appendix runs from narrowly cylindric to stoutly conic-cylindric, and its base either tapers gradually or narrows abruptly into a stipe — the presence or absence of that stipe is an important diagnostic character, and what it is for is not known. Below it, in most species, are two zones of bristles: the upper, above the staminate flowers, reads as modified stamens, and the lower, above the pistillate flowers, as modified pistils; intermediate structures at both boundaries support the reading. Whether the bristle bases are rough or smooth is consistent within a species.
The staminate flowers are crowded into a cylindric, oblong or globose zone, each of two to four fused anthers opening by a longitudinal slit in subgenus Arum or by a subapical pore in subgenus Gymnomesium. Anther color before dehiscence is diagnostic for many species and worth noting in the field. The pistillate flowers sit in a cylindric zone at the base, each a naked, sessile, oblong-ovoid gynoecium with a one-locular, many-ovuled ovary and a brush-like sessile stigma that exudes a droplet as it withers, leaving a dull purple to brown pit at the ovary apex.
At dehiscence most of the pollen falls to the bottom of the spathe-tube rather than staying on the anthers, and field observation suggests insects pick it up by crawling around inside the tube rather than by touching the anthers at all.
Scent, heat and the trap
The appendix is the scent organ, and what it emits ranges across the genus from foetid through oily to sweet, with some species scentless. What the smell does, what the heat does, how the tube holds insects overnight and how they get out again are the subject of a page of their own — several of the answers everyone repeats are wrong. See Arum reproduction.
Fig · The inflorescence, opened
A spike of orange-red berries held up for birds — with one species that does none of it.
The fruiting head is uniform across the genus: a cylindric cluster of juicy, few-seeded, pyriform to globose glossy berries, stigmatic remains sunken at the tip, orange-red and carried clear of the foliage in a way that reads as adaptation to bird dispersal. As the head matures the peduncle thickens and hardens; in the species with hidden inflorescences it also elongates markedly, lifting the spike into view, while in those that display the inflorescence at flowering there is little or no elongation because there is nothing left to do.
Each fruit holds two to five seeds in a clear, orange, viscous pulp that comes from the breakdown of the funicles, trichomes and placenta hairs. The pulp also carries calcium oxalate crystals in varying amounts — Arum pictum the most, Arum hygrophilum little or none. Seeds are moderately large and ovoid to globose, with a leathery reticulate testa over copious starchy endosperm and a straight embryo. The pattern on that testa varies subtly but distinctly between species and has been suggested as a way into the taxonomically difficult ones.
Arum pictum departs here as well as at flowering: its peduncle does not elongate, and its infructescence is a globular head of much drier, silvery lilac berries sitting at ground level. How they are dispersed is not known.
Germination
There are two strategies, and they track winter cold. Species that do not meet cold winters — Arum dioscoridis, Arum nigrum, Arum pictum — germinate, put up an eophyll and start a tuber more or less at once, the first leaf appearing above ground within days. Species that do meet cold winters — Arum cylindraceum, Arum maculatum, Arum idaeum — germinate with no sign of growth above ground at all, building a comparatively large tuber underground and producing no foliage until the second growing season.
Fig · The fruiting head
Araceae · Care
Cultivation
Arum asks less than most bulbous subjects — no perfect drainage, no rigorously policed rest and growth — and forgives the occasional lapse. Getting the calendar right matters far more than getting the details right.
Fertile, humus-rich and on the limy side — these are plants of basic soils, not of ericaceous mixes.
Outdoors
Two things matter: a fertile, humus-rich soil, and a sunny position that does not get too hot. Prepare by clearing every perennial weed from the site and working in plenty of bulky compost if the soil is short of humus. The weeding is not a formality — once established, a colony is best left undisturbed, and anything creeping through it will be there for years. Plant the tubers about 9–10 cm deep and 15–18 cm apart in a wide, flat-bottomed hole, with a generous layer of grit underneath if the soil is heavy. Mark the spot; the emerging shoots are easy to hoe through.
In pots and frames
Choose the richest mix you would use for bulbs. The monograph's own recipe is two parts moss peat, one part loam and one part 3 mm grit — grit, not sand — with dolomitic limestone to bring the pH to about 7.5 and base fertilizer at the rate recommended for John Innes No. 3. Pots suit these plants better than pans, because arums root deeply and a deeper pot holds moisture longer. Fill to about half depth over a piece of plastic gauze, set the tubers around 5 cm deep, cover to within 2 cm of the rim and top the rest with grit: it suppresses moss and liverwort, keeps the neck of the plant dry and finishes the pot properly.
Clay is the more forgiving material, since it makes overwatering much harder, though clay pots large enough for the bigger species get heavy and expensive; plastic works well if you understand what the species wants. In a larger glasshouse a free root-run bed beats both — a border dug to 45 cm and conditioned with grit and peat, or a timber or brick bed over about 4 cm of shingle filled with the pot mix. Everything grows more strongly in one, and the creeping rhizomatous species, which outgrow conventional containers quickly, benefit most.
Nitrogen early for the leaves, phosphate later for the tuber — the feed changes when the plant's job changes.
Container-grown arums will not perform without feeding. Either scatter a granular feed on the surface at the start of the growth season, which is convenient if the plants are to be left alone for long stretches, or use a liquid feed with every other watering, which gives you far more control over what the plant is getting and when.
That control is the point, because the plant wants two different things across one season. A high-nitrogen feed through the first three months of growth builds the leaves. Switching to a high-phosphate feed from just before flowering until dormancy builds the tuber, which is what next year's plant is made of — a large, well-ripened tuber is the whole return on the season.
Arums exhaust the soil in even a fairly large pot, so annual repotting into fresh mix is part of feeding them rather than an alternative to it. Seedlings are the exception to the schedule: give them half-strength liquid feed at every watering as soon as the first leaf shows, which gets the most out of the first season.
A comprehensive collection needs both a garden and a frame: the northern species want to be outdoors, and many of the best-looking ones cannot take frost.
Which species go where
Species from the northern part of the range only really succeed outdoors. The more southerly ones, which include many of the most attractive, need protection from frost, so most growers keep them in a cold frame or bulb frame — a glass-covered raised bed planted directly — or in pots under glass. If some of the tender species are to be tried outside, pick a well-drained sunny spot that does not catch early morning sun: frosted shoots survive a slow thaw far better than a bright morning, which ruptures the cells and collapses the plant.
Water and the season
In growth, do not let the plants dry out; a check at this stage is expensive and drying out simply pushes them toward dormancy. After flowering the leaves go over quickly, yellowing and withering. Reduce watering then, but do not stop it. Resist tidying: wait until the leaves are completely dead and brown, when they come away freely. Pulling a leaf that is not ready damages the new tuber and can take the growing point with it.
Dormancy
Dormant tubers must not get either too hot or too dry. In a frame, leave the side glass open through the rest for air movement; give the frame a thorough soaking in late August and put the glass back, which breaks dormancy and gets a root system established before the cold arrives. Pots want a warm, dry place out of full sun — plunged to half their depth in barely damp sand is better still, and keeps the tubers plump and reasonably cool. It is worth knowing that in the wild the species from seasonally arid country grow between rocks and under bushes, in shelter that holds a little soil moisture all summer; the tuber does not in fact tolerate prolonged heat and drought just because the climate is dry.
Winter
Through winter, check the plants over now and then for slug damage and for gray mold. When heavy frost is forecast, cover the frame two or three hours before nightfall and take the covering off as soon as possible after sunrise the next day.
Division if you want two or three plants; seed if you want to see what a species actually does across a population.
Division
Dividing arums is straightforward, and the ideal moment is while the plants are dormant — which is also when a plant in the open ground is impossible to find. Mark the growing point with a cane during the season and the problem goes away. Lift, check the tuber for rot, replant. Potted stock is easier still: divide at repotting. If a plant has to be divided in growth, keep the division damp until it is back in the ground and water the parent well afterward; plants divided in growth often fail to flower, and if an inflorescence does come it is good practice to cut it off as soon as the spathe clears the petiole-sheath so the plant can put the season into establishing.
Established clumps eventually need lifting anyway. The signs are smaller leaves and a marked drop in flowering. Do it as soon as the plants have died down in early summer, lift the colony with a fork, and replant the largest tubers.
Seed
Seed takes patience — four to five years from sowing to flowering on average, three for Arum dioscoridis and Arum pictum. Clean it first, whether fresh or bought. If the orange-red pericarp has dried onto the seed, soften it by soaking for about an hour in tepid water and rub it off. It has to go for two reasons: it appears to contain a germination inhibitor, and the warm damp conditions that germinate the seed also rot the pericarp, which then takes the seedling with it. The pericarp is caustic and rich in raphides and can burn the hands badly — wear gloves for this.
Sow at once, in plastic pots rather than trays: seedlings begin forming a tuber immediately and a shallow tray cramps that badly. Fill as for mature plants, sow on the surface, cover lightly, top with grit and soak. Fresh seed germinates at the start of the next growth season; old or dry seed can take a year longer. Seedlings of the cold-winter species spend that entire first season underground as a tuber with nothing showing, so do not discard a pot for at least two years. Once the first leaf appears, give them all the light you can, keep them from drying out, and keep them under glass for the first year to let the young tubers reach a reasonable size. Repot about three to a 10 cm pot when the rest of the collection is done, and treat them as mature plants from then on. Anything destined for the open ground is best kept in pots for three growing seasons before it is planted out.
Robust plants, on the whole — the real risk is water, not insects.
Pests
Aphids are the most troublesome, infesting and distorting newly emerging leaves and inflorescences, and they are also the reason to take them seriously: they move virus around a collection. Red spider mite turns up less often and is much harder to shift once it arrives; a light infestation can be knocked back by spraying the undersides of the leaves with a fairly coarse jet of water, and keeping the air damp — the usual advice — does not work. Slugs and snails are more of a problem outdoors and in frames than under glass, and despite how toxic every part of the plant is to mammals they eat both new shoots and mature leaves; hand removal at night is the most effective control there is. If you do use bait, keep the pellets out of the leaf-axils, where they decompose in wet weather and rot the leaf tissue.
Diseases
Disease is rare in a well-grown collection, and the one that matters is wet rot of the tuber, caused by severe overwatering or by too much moisture during dormancy. On a dormant tuber, caught in time, cut the rotten areas away, leave it out a couple of days to dry off, and repot into almost dry mix. On a growing plant it is effectively terminal: by the time you see flaccid leaves and slimy petioles the tuber is gone. Remove it, and repot the unaffected tubers into fresh mix, disturbing them as little as possible. Leaves are rarely troubled by fungal disease, though botrytis can take them if a frame or glasshouse stays very humid for a long spell, as in prolonged fog.
Virus
Arums are relatively trouble-free here too, and a strongly growing plant rarely shows anything. Sickly ones — particularly where the roots have been damaged by overwatering — may show distorted, yellow-marbled leaves and small, malformed inflorescences. Destroy any plant showing those symptoms rather than nursing it, and if virus is suspected anywhere in a collection, control the aphids, which are how it travels.
Alocasia Timeline
An interactive timeline showing the year each Alocasia species was first described as Alocasia (species may previously have been classified in other genera)
Complete Bibliography
Every source cited across all species, cultivar and hybrid pages