Oenanthe sarmentosa
Common names
American horsebane
Biostatus
Exotic
Conservation status
Not applicable
Category
Vascular
Structural class
Herbs - Dicotyledons other than Composites
Simplified description
Sprawling floating marginal perennial aquatic plant up to 1.5 m tall. Foliage is divided but less so than horsebane, stems are hollow and green in colour.
Flower colours
White
Detailed description
Biennial to perennial sprawling herb up to 1.5 m tall, with floating stems hollow, grooved, green, usually rooting at the nodes. Emergent leaves bipinnate, tripinnate on lower branches with segments up 10-35 mm long, segments lobed. Submerged leaves absent. Flowers in an umbel, either terminal or opposite a leaf 2-5 cm diameter, with 10-20 rays. Subtending bracts 1 or lacking. Flowers white c. 2 mm diameter. Fruit cylindric 2.5-3.5 mm long, prominantly ribbed.
Similar taxa
Horsebane (Oenanthe aquatica) and water celery (Apium nodiflorum). Horsebane has more divided leaves with shorter segments. Water celery has less divided (pinnate) leaves.
Distribution
Locally common in the vicinity of Greymouth, Westland.
Habitat
Stream edges and swamps.
Detailed taxonomy
Genus
Family
Endemic family
No
Ecology
Flowering
Summer
Fruiting
Summer-autumn
Year naturalised
1975
Origin
W. North America
Reason for introduction
Unknown, possibly a seed contaminant
Control techniques
Not controlled in New Zealand.
Life cycle and dispersal
Vegetative spread by stoloniferous growth and by floating seeds.
Wetland plant indicator status rating
Information derived from the revised national wetland plant list prepared to assist councils in delineating and monitoring wetlands (Clarkson et al., 2021 Manaaki Whenua – Landcare Research Contract Report LC3975 for Hawke’s Bay Regional Council). The national plant list categorises plants by the extent to which they are found in wetlands and not ‘drylands’. The indicator status ratings are OBL (obligate wetland), FACW (facultative wetland), FAC (facultative), FACU (facultative upland), and UPL (obligate upland). If you have suggestions for the Wetland Indicator Status Rating, please contact: [Enable JavaScript to view protected content]
OBL: Obligate Wetland
Almost always is a hydrophyte, rarely in uplands (non-wetlands).
Other information
Etymology
sarmentosa: Twiggy
Referencing and citations
References and further reading
Johnson PN, Brooke PA (1989). Wetland plants in New Zealand. DSIR Field Guide, DSIR Publishing, Wellington. 319pp.
Attribution
Factsheet prepared by Paul Champion and Deborah Hofstra (NIWA).