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Gypsywort is an emergent perennial herb up to approximately 1m tall. It lacks the characteristic minty smell of similar species. Stems are square, leaves toothed and up to 3 x 8.5cm. Flowers small, white to pale pink/purple, and borne summer-autumn. Seeds (nutlets) are minute, and borne summer-autumn.
It occurs in the margins of lakes, rivers, ponds and other water bodies, in drainage ditches, damp pasture and waste land. Some salinity tolerance (co-occurs with spartina in estuarine marshes overseas, but this habitat is likely of marginal suitability). Gypsywort is know to have some shade tolerance, this makes it more adept at invading established native vegitation along river banks and lake margins.
Seeds are highly buoyant, and can be dispersed by water movement. Seed is also dispersed as a contaminant on machinery or footwear, and following ingestion and defecation by livestock. Seed can still grow after floating for 15 months. Gypsywort also spreads by a creeping root system or stolons.
Seeds can be dispersed by water movement, as contaminant on machinery or footwear, and following ingestion and defecation by livestock.
Lake Te Werahi is closed due to risk of spread of the serious pest plant, gypsywort (Lycopus europaeus).