Egeria densa
Common name
Egeria
Family
Hydrocharitaceae
Flora category
Vascular – Exotic
Structural class
Herbs - Monocots
NVS code
The National Vegetation Survey (NVS) Databank is a physical archive and electronic databank containing records of over 94,000 vegetation survey plots - including data from over 19,000 permanent plots. NVS maintains a standard set of species code abbreviations that correspond to standard scientific plant names from the Ngä Tipu o Aotearoa - New Zealand Plants database.
EGEDEN
Brief description
Submerged, bottom-rooted perennial plant in the oxygenweed group. It is a large leafy plant, the leaves being 10 to 30 mm long and 2 to 5 mm wide with minutely serrated margins, which occur in whorls of four to five. Egeria produces flowers (male only), which are conspicuous during the summer months floating on the water surface. Flowers are to 20 mm across, with 3 white petals.
Distribution
Widely naturalised in North Island and Marlborough, few sites elsewhere in South Island.
Habitat
Submerged in moderate flowing to still water bodies, usually in moderately to highly enriched water bodies. Prefers sandy or silty sediments.
Features
Submerged, bottom-rooting perennial, growing to 5 m. Stems slender, brittle, much-branched, buoyant, 3 mm diam. Leaves in whorls of 4-6 (occ 3 near base), linear, 15-30 x 4 mm, dark green. Flowers on surface, 3 petaled, 20 mm diam, white with 9 yellow stamens, Nov-Jan. Only male plants found in NZ, no seed set.
Similar taxa
Canadian pondweed (Elodea canadensis) and lagarosiphon (Lagarosiphon major). Egeria can be differentiated from the others by its larger size and conspicuous white flowers. Canadian pondweed almost always has leaves arranged in whorls of three. Lagarosiphon has leaves that curl downwards and are not arranged in whorls.
Flowering
Spring/summer. When the plants flowers can reach the sruface of the water.
Flower colours
White, Yellow
Fruiting
Only male plants in NZ.
Life cycle
Egeria propagates from stem fragments containing lateral buds that give rise to new plants. Egeria does not produce seed in New Zealand, with only male flowering plants being present.
Year naturalised
1946
Origin
South America, naturalised throughout tropical and temperate areas of world.
Reason for introduction
Ornamental aquarium plant
Control techniques
Plants can be physically removed from the lake or waterway using SCUBA or snorkel divers for small scale infestations, or using mechanical diggers. Although the potential for contamination of other sites by mechanical equipment is a signficant concern. There are a number of manipulations to the habitat that in theory can control egeria (e.g., shading, bottom lining, water drawdown) but there are significant limits to their practical application, rendering them site (or waterbody) specific. The only chemical product registered for aquatic use in New Zealand that is efficacious on egeria is diquat. Diquat is a relatively fast acting contact herbicide, which interrupts the electron transport system in plant photosynthesis and causes the destruction of cell membranes and desiccation. At present the introduced grass carp (Ctenopharyngodon idella) are the only biocontrol agent available to manage egeria. Grass carp have been successfully used for the eradication of egeria from Parkinson’s Lake.
Tolerances
The plant is slightly tolerant to shade and frost, intolerant to drought and highly tolerant to poor drainage. Physical damage and grazing will result in regrowth from buried stem apicies and fragments.
Attribution
Factsheet prepared by Paul Champion and Deborah Hofstra (NIWA).
References and further reading
Champion et al (2012). Freshwater Pests of New Zealand. NIWA publication. http://www.niwa.co.nz/freshwater-and-estuaries/management-tools/identification-guides-and-fact-sheets/freshwater-pest-species.
Johnson PN, Brooke PA (1989). Wetland plants in New Zealand. DSIR Field Guide, DSIR Publishing, Wellington. 319pp.
Coffey BT, Clayton JS (1988). New Zealand water plants: a guide to plants found in New Zealand freshwaters. Ruakura Agricultural Cente. 65pp.
Popay et al (2010). An illustrated guide to common weeds of New Zealand, third edition. NZ Plant Protection Society Inc, 416pp.
Kasselmann C (2003). Aquarium plants. Krieger Publishing company, Florida, 518pp.; WSDE (2001). An aquatic plant identification manual for Washington’s freshwater plants. Washington State Department of Ecology, 195pp.
Aston, H (1977). Aquatic plants of Australia. Melbourne University Press, 367PP.
Hofstra D, P Champion, (2006). Management options assessment for Egeria densa. NIWA client Report HAM2006-160.
Hofstra D, P Champion, (2006). Organism Consequence Assessment Egeria densa. NIWA Client Report: HAM2006-058g.