Ageratina riparia (Regel) R.M.King & H.Rob., Asteraceae

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Present on Pacific Islands?  yes

Primarily a threat at high elevations?  no

Risk assessment results: 

Reject, score: 26 (Go to the risk assessment (Australia)).
Medium-high risk (Go to the risk assessment (United States)) (PDF format).

Other Latin names:  Eupatorium riparium Regel

Common name(s): [more details]

English: William Taylor, creeping croftonweed, mist flower, river eupatorium, spreading mistflower

French: Iapana malgache, abessouris, faux orthosiphon, jouvence, orthochifon, yapana malbare

Hawaiian: hāmākua pāmakani

Habit:  herb

Description:  "A herb of 40-60 cm height, sometimes exceeding 1 m, with numerous spreading or ascending stems.  Leaves have long petioles, are opposite and simple, lanceolate to elliptic, and toothed at the margins of the upper half.  The leaf blades are 3-12 cm long and 0.8-3 cm wide.  Flower-heads are 5-6 mm in diameter, subtended by green bracts, and contain 15-30 white or cream florets and are arranged in terminal clusters at the ends of branches.  The corolla is 3-3.5 mm long.  Fruits are dark brown achenes of c. 2 mm length and have a pappus of white hairs.  The plant has a short and thick root stock"  (Weber, 2003; p. 29).

"Spreading subshrubs from a creeping rootstock; stems sprawling, 5-10 dm long, sparsely puberulent. Leaves lanceolate to elliptic, 4-8 (-12) cm long, 1-1.5 cm wide, margins sharply serrate, petioles 0.5-1.3 cm long. Inflorescences lax; involucral bracts 4-5 mm long, puberulent; corollas white, 3-3.5 mm long. Achenes black, 1.5-2 mm long, puberulent on the angles" (Wagner et al., 1999; p. 255).

Habitat/ecology:  In Hawai‘i, dry disturbed habitats to mesic and wet forest, 3-1,200 m (Wagner et al., 1999; p. 254). “Mistflower rapidly invades disturbed areas and tends to spread along gullies and river banks. It is shade tolerant and can become a dominant understory plant. Chemicals from the leaf-litter suppress the growth of other plants e.g. native species, giving mistflower a further competitive advantage.” (Northern Territory of Australia Agnote No. 628).  "Grows very densely and overtops groundcovers, is long-lived, quick-maturing, and produces a large number of highly viable, well-dispersed (probably short-lived) seed. Drooping stems can also take root in wet sites wherever they touch the ground. Tolerates deep shade and damp, damage and grazing, salt, and most soil types, but is limited by dry conditions and frost. Poisonous, so usually shunned by livestock"  (Weedbusters New Zealand).

Propagation:  Seed. “Spread of seeds can occur by wind, running water, contamination of agricultural produce, and seeds sticking to clothing, animals, vehicles and machinery. Existing colonies increase in size and density by layering, thus forming a mat of interwoven stems” (Northern Territory of Australia Agnote No. 628).

Native range:  Mexico; cultivated in tropics (GRIN).

Presence:

Pacific
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia (Pacific offshore islands) Norfolk Islands	Norfolk Island	introduced invasive	Ziesing, P. (1997) (pp. 30-31)
Australia (Pacific offshore islands) Norfolk Islands	Norfolk Island	introduced invasive	Orchard, Anthony E., ed. (1994) (p. 7) "A serious weed in areas of degraded forest, and an invader of the margins of undisturbed forest, smothering ferns and seedlings". Vouchers cited: G. Uhe 1100 (K), R.M. Laing (CHR), the Administrator of Norfolk Is. (NSW), J.D. McComish 96 (NSW), P. Ralston 312 (A, K)
State of Hawaii Hawaiian Islands	Hawai‘i (Big) Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 255)
State of Hawaii Hawaiian Islands	Kaua‘i Island	introduced invasive	Herbst, Derral R./Wagner, Warren L. (1996) (p. 9) Voucher cited: Flynn et al. 5279 (BISH, PTBG, US)
State of Hawaii Hawaiian Islands	Lāna‘i Island	introduced invasive	Oppenheimer, Hank L./Bartlett, Randal T. (2002) (p. 3) Voucher cited: Oppenheimer H109924 (BISH)
State of Hawaii Hawaiian Islands	Maui Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 255)
State of Hawaii Hawaiian Islands	Moloka‘i Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 255)
State of Hawaii Hawaiian Islands	O‘ahu Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 255)

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia Australia (continental)	Australia (continental)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
Australia Australia (continental)	Queensland	introduced invasive	Queensland Herbarium (2002) (p. 1)
Indonesia Indonesia	Indonesia (Republic of)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
Mexico Mexico	Mexico (United Mexican States)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Owen, S. J. (1997)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Webb, C. J./Sykes, W. R./Garnock-Jones, P. J. (1988) (p. 205) "Forest margins and clearings, waste places, damp banks and streamsides".
South America (Pacific rim) South America (Pacific rim)	Perú (Republic of)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized

Indian Ocean
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
La Réunion (France) La Réunion Island	La Réunion Island	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
La Réunion (France) La Réunion Island	La Réunion Island	introduced invasive	Lavergne, Christophe (2006) "Très envahissant"
La Réunion (France) La Réunion Island	La Réunion Island	introduced invasive	Baret, Stephane/Rouget, Mathieu/Richardson, David M./Lavergne, Christophe/Egoh, Benis/Dupont, Joel/Strasberg, Dominique (2006) (p. 758)
Mauritius Mautitius Islands (Mauritius and Rodrigues)	Mauritius Island	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized

Comments:  A problem species in Mauritius (John Mauramootoo, pers. com.). A declared noxious weed in Hawai‘i and the Northern Territory, Australia.

Control: 

Physical:  Hand pull or dig out seedlings and small plants.

Chemical:  Treat freshly cut stumps with 2,4-D or dicamba.

"1. Weed wipe (all year round): glyphosate (333ml/L) or metsulferon-methyl 600g/kg (2g/L). 2. Spray (spring-summer): glyphosate (20ml/L + penetrant). 3. Spray (spring-summer): metsulferon-methyl 600g/kg (20g/100 L (spraygun) or 3g/10L (knapsack)). Add penetrant in winter. Spray lightly, not to run off. Avoid water contamination"  (Weedbusters New Zealand).

Biological: "Hawaii's fight against Ageratina riparia has been "one of the most successful [biological control programmes] undertaken anywhere in the world" (Morin et al. 1997). A biological control program was initiated in 1955 by the Hawaii Department of Agriculture. Over the years about 35 insects, most from Mexico, were collected as potential biocontrol agents, and 18 were studied in quarantine. A tephritid fly Xanthaciura connexionis was released in 1955 and 1960, but did not become established. In the 1970s, the fungus Entyloma ageratinae, the gall fly Procecidochares alani, and the plume moth Oidaematophorus beneficus were introduced. Within 10 years, mist flower was successfully controlled by these agents, and formerly infested pastures were returned to productive use. The fungus appears to be the most important of the biocontrol agents, followed by the fly, then the moth. Because of their effectiveness in Hawaii, these biocontrol agents were considered for release against mist flower in New Zealand"  (Biological control in Hawai‘i)

"Biological control attempts on mistflower have included the introduction of a gall-fly (Procecidochares alani) to Queensland, which has had little impact due to the attack by native parasites. However the recent introduction of a white rust fungus (Entyloma ageratinae) to New Zealand has proven to be highly specific to Ageratina riparia, with early indications it will provide useful control" (Northern Territory of Australia Agnote No. 628).

"Mist flower, a low-growing perennial with tiny white daisy-like flowers, was accidentally introduced to Hawai‘i in 1925. By 1972 it had occupied 52,000 ha of rangeland.  A plume moth (Oidematophorus beneficus), a gall wasp (Procecidochares alani), and a smut fungus (Entyloma ageratinae), were introduced to attack this aggressive weed in the mid-1970s.  Biological control of mist flower in Hawai‘i has been an outstanding success. Of the three agents the fungus was the most effective and it achieved total control of the plant in wet areas within eight months, and in dry areas within 3-8 years. The plant has remained under control ever since. Dense stands of mist flower have been reduced to isolated patches of sickly, stunted plants on dry rocky outcrops, where there is little competing vegetation, and the rangeland has been returned to productive use.

Mist flower has increasingly become a problem in northern New Zealand. A feasibility study showed that infested areas of New Zealand were likely to be suitable for the mist flower agents, so the smut fungus and the gall fly were released in New Zealand in 1998 and 2001 respectively.  Both are establishing and spreading rapidly and it looks promising that successful control of the plant will be achieved here too" (Landcare Research, New Zealand).