Billygoat weed (Ageratum conyzoides, L. 1753)

Identity

Billygoat weed (Ageratum conyzoides L., 1753) is an accepted species in the genus Ageratum, in the Asteraceae family. Crush its hairy foliage and the common name becomes clear: it releases a pungent, goatlike odor. ^{4 1}

Regional names include goatweed, whiteweed and tropical whiteweed in English, baume blanc and herbe à femme in French-speaking regions, and mentrasto in Brazil. These names refer to the same plant but do not reliably establish its identity. Odor and flower color vary with climate, while the scientific name remains consistent across continents. ^{2 9}

This weed has no recognized ornamental value. Its close relative Ageratum houstonianum, bedding ageratum, is the member of the genus commonly cultivated in France. Confusing them can lead to pointless weeding: pulling bedding ageratum from a patio border will not stop Billygoat weed returning in a nearby vegetable bed. ⁷

Recognition

Upright, branching stems give Ageratum conyzoides its annual herbaceous habit. Usually 30 cm to 1 m (1–3.3 ft) tall, it can reach 1.2 m (4 ft) in rich soil and full sun. Fine white hairs cover the stems and leaves; the opposite, oval leaves grow up to 7.5 cm (3 in) long. The plant is slightly aromatic. ^{7 4}

Terminal clusters hold flower heads measuring 5–8 mm (0.2–0.3 in) across, each containing 30–50 tiny mauve, bluish, pink or white florets. Each floret produces one brown achene, topped by a scaly pappus rather than conspicuous barbed bristles. ^{7 3}

Inspecting the flower bracts provides the most reliable distinction. Those of Ageratum conyzoides bear only a few scattered hairs; its relative Ageratum houstonianum has dense glandular hairs and shorter floral styles. This visible detail separates the two without relying on flower color or common names. ⁷

Useful taxonomy

Botanical references accept the name Ageratum conyzoides L. POWO links it to a long list of synonyms accumulated over more than two centuries, including Ageratum album Steud., Ageratum hirsutum Poir., Ageratum ciliare L., Eupatorium conyzoides (L.) E.H.L.Krause and Carelia conyzoides (L.) Kuntze. ¹

Repeated descriptions under different names reflect a variable species encountered by botanists in different parts of the world. The practical consequence is that a nursery label in Asia or Latin America may identify a different plant if it omits the accepted scientific name. POWO's accepted name provides a steadier reference than local horticultural or common names. ¹

Habitat and range

Tropical America, including Central and South America and the Caribbean, forms the original range described here; POWO centers the native range around Mexico. Agriculture and the seed trade subsequently carried the species into almost every tropical and subtropical region. ^{1 8}

Disturbed ground gives Ageratum conyzoides many opportunities to establish: around buildings, along roads, in overgrazed pastures, at forest edges and even on coastal dunes. It occurs from sea level to 2,500 m (8,200 ft) in elevation. ^{8 7}

Several regions assign it a clear weed status. South Africa lists it as noxious; it is invasive in Kenya, Tanzania and Uganda and threatens Himalayan plant communities in India. It affects more than 40 crops and pastures and acts as an alternative host for crop pathogens and nematodes, extending its effects beyond competition for light. Across the Indian Ocean it is established on Réunion, Madagascar, Mauritius, the Comoros and the Seychelles. These regional classifications should not be read as the rules applying everywhere. ^{7 5 4}

Horticulture itself contributed to this worldwide spread. In several regions the plant arrived as a garden introduction or as a contaminant in seed lots of other ornamentals, then escaped from beds into surrounding open habitats. ⁷

Life cycle and reproduction

Germination through flowering takes less than two months. The seeds germinate mainly in light, a response known as positive photoblastism, and often lose viability after twelve months. ⁷

Traveling achenes attach to livestock and wildlife fur, clothing and agricultural machinery. The scaly pappus of Billygoat weed grips less strongly than the barbed bristles of some other weedy Asteraceae, yet still enables long-distance dispersal. Isolated patches can appear several kilometers from the nearest population without deliberate sowing. ⁷

Uses and risks

Traditional medicine uses the whole plant across much of Africa for fever, rheumatism, headaches, colic, burns, uterine disorders and pneumonia. In Kenya, it is also used in remedies for asthma and to stop bleeding. These are records of traditional use rather than treatment recommendations. ⁷

Documented toxicity remains a concern despite those uses. Ageratum conyzoides contains liver-toxic pyrrolizidine alkaloids, including lycopsamine, found even in herbal teas sold in Brazil without clear toxicological oversight. A mass poisoning was reported in Ethiopia after consumption of grain contaminated by the plant. ^{10 9}

PFAF records no food use for this species. No contact toxicity is reported from routine garden handling; the main concern is repeated ingestion through herbal tea or contaminated fodder. ⁸

Varieties, look-alikes and related plants

Horticultural cultivars are not recognized for Ageratum conyzoides. Its most frequent look-alike is Ageratum houstonianum, widely grown in French gardens for blue or violet flower heads. Bedding ageratum has densely hairy, glandular flower bracts, compared with only a few scattered hairs in A. conyzoides. Its floral styles measure 1–2 mm (0.04–0.08 in), compared with about 5 mm (0.2 in) in the wild species. ⁷

Toothed leaves and small colored flower heads may also resemble certain weedy Bidens or Erigeron. Fruit shape, specifically an achene with a scaly pappus rather than barbed bristles, and the hairiness of the bracts remain the most useful field characters. ³

Main sources

• Plants of the World Online / Kew, Ageratum conyzoides L.
• INPN, Ageratum conyzoides L., 1753
• Pl@ntUse French, Ageratum conyzoides
• Wiktrop, Ageratum conyzoides L.
• Global Invasive Species Database / IUCN, Ageratum conyzoides
• EPPO Global Database, Ageratum conyzoides (AGECO)
• EAfrinet / Lucidcentral, Ageratum conyzoides (Billygoat Weed) factsheet
• Plants For A Future, Ageratum conyzoides
• Wikipedia (EN), Ageratum conyzoides
• Revista Brasileira de Farmacognosia, pyrrolizidine alkaloids in medicinal tea of Ageratum conyzoides