Giant taro (Alocasia macrorrhizos, (L.) G.Don 1839)

Identity

The giant taro (Alocasia macrorrhizos (L.) G.Don, 1839), also called giant alocasia or elephant ear, is a perennial in the genus Alocasia, part of the arum family, Araceae. POWO accepts this name and keeps the species in Alocasia; the Missouri Botanical Garden places its native range in Australia, Malesia and Papuasia. ^{1 2}

The botanical name refers to a bulky rhizome (macro- + rhiza), not to the leaves. In a container the plant commonly tops 1.5–2.5 m (5–8 ft), and it reaches 3–4 m (10–13 ft) in the tropics. ³ The scientific name is a safer guide than any common one: in the nursery trade, "elephant ear" and "giant taro" are generic tags for several large alocasias, which makes buying the right plant harder than it should be. ⁴

Recognition

On adult plants, the arrow- to heart-shaped leaves often exceed 1 m (3.3 ft) in length, held on thick leaf stalks that rank among the longest in the genus Alocasia. ^{3 4}

To tell Alocasia macrorrhizos from true taro (Colocasia esculenta), look at where the leaf stalk joins the blade rather than at leaf size. In Alocasia, it meets the blade at the notch and the leaves point upward; in Colocasia, the attachment sits closer to the center and the blade hangs toward the ground. With age, the stacked leaf-stalk bases form a fibrous pseudotrunk that can reach 1 m (3.3 ft) tall without ever turning woody. ^{5 2}

Early morning offers another clue: a row of droplets beads along the leaf margin. This guttation, driven by root pressure in humid air, is not a disease; it shows the potting mix is well supplied with water. ⁶

Useful taxonomy

The name accepted by POWO keeps the authority (L.) G.Don: Carl Linnaeus published the basionym Arum macrorrhizon in 1753, and George Don moved the species into Alocasia in 1839. The synonyms you are most likely to meet are: ^{1 7}

• Colocasia macrorrhizos (L.) Schott
• Caladium macrorrhizon (L.) R.Br.
• Arum macrorrhizon L.
• Alocasia rapiformis (Roxb.) Schott

Alocasia macrorrhizos var. brisbanensis is a trap: that name actually belongs in the synonymy of a separate species, Alocasia brisbanensis, not in that of Alocasia macrorrhizos. Mixing the two up leads to misidentified plants at the garden center. The stable reference remains the name accepted by POWO, kept apart from look-alike varieties and trade names. ¹

Habitat and range

Central Malesia (Borneo, Sulawesi, the Moluccas) marks one end of the native range, which continues through the Philippines and New Guinea to Queensland, in the Murray Islands group of the Torres Strait. ^{1 2}

There the species grows along forest edges and riverbanks, in lowlands up to about 500 m (1,640 ft), on moist soils rich in organic matter. That liking for saturated ground explains why Alocasia macrorrhizos tolerates periodic flooding in cultivation. ^{6 4}

Grown for centuries for its edible corm, the species has been carried well beyond its native range, into the Pacific and the Indian subcontinent. In the United States, several alocasias have naturalized in Florida and around the Gulf Coast, where their status shifts from ornamental to potentially invasive depending on the area. ⁴

Life cycle and reproduction

Growth is fast in warm, humid conditions: during the growing season a new leaf regularly outsizes the previous one within a few weeks, then growth slows sharply below 18 °C (64 °F). ^{6 3}

Flowers follow the aroid pattern, a spadix wrapped in a pale green to cream spathe about 20–25 cm (8–10 in) tall. Indoors they appear only now and then; mature plants in a favorable climate bloom more often. ²

In cultivation, propagation is mostly by dividing rooted basal offsets, less often by sowing fresh seed kept at about 24 °C (75 °F). An offset separated without roots of its own establishes slowly and stays fragile for the first few weeks; cold, soggy mix encourages it to rot. ⁶

Uses and risks

As food, the corm and stem bases are eaten cooked in several Southeast Asian and Pacific cuisines, provided they are cooked long enough to break down the calcium oxalate crystals. Raw, the plant stays irritating and unfit to eat. ⁶

All fresh parts contain insoluble calcium oxalate raphides. When the plant is chewed or swallowed, the ASPCA reports oral irritation, excessive drooling, vomiting and difficulty swallowing in cats, dogs and horses; contact with the sap can also cause dermatitis. ⁸

Indoors or in a conservatory, the plant is grown mainly for ornament, for the sheer volume and texture of its foliage. In frost-free climates, roughly USDA zones 10 to 12, it can also fill a shady bed or edge a pond, thanks to its tolerance of waterlogged soil. ^{4 3}

Varieties, look-alikes and related plants

Not every nursery label points to a stable cultivar. Variegata refers to leaves marbled with creamy white, often unstable depending on light; Stingray has a blade narrowed to a point that suggests a stingray; Borneo Giant is sold for a form with noticeably broader foliage, which does not always match the species itself. ^{5 4}

Common look-alikes include:

• true taro (Colocasia esculenta), with drooping leaves and a more central, peltate attachment
• Alocasia brisbanensis, a distinct species despite a similar synonym
• Xanthosoma sagittifolium, another large aroid with an edible corm, whose main veins are less prominent

Since the trade name "elephant ear" covers plants from several different genera, the scientific name is a more reliable way than the label to pin down the exact species.

Main sources

• Plants of the World Online / Kew, Alocasia macrorrhizos (L.) G.Don
• GBIF Backbone Taxonomy, Alocasia macrorrhizos
• Royal Horticultural Society, Alocasia macrorrhizos
• Missouri Botanical Garden Plant Finder, Alocasia macrorrhizos
• NC State Extension Gardener Plant Toolbox, Alocasia, Giant Taro
• Plants For A Future, Alocasia macrorrhizos
• ASPCA, Alocasia toxicity entry
• University of Wisconsin Extension, Elephant Ears: Colocasia, Alocasia and Xanthosoma