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Indian Swampweed

(Hygrophila polysperma)

Photo of Indian swampweed
Photo credit: USDA APHIS, Bugwood.org

A fast-growing submerged plant that is on the federal noxious weed list in the United States. It is commonly grown in the aquarium trade and could pose a threat to waterways if released.

Overview

Other names for this plant include:
  • Common names: Miramar weed, dwarf hygrophila, hygro
  • Scientific names: Justicia polysperma, Hemidelphis polysperma
Ecological threats:
  • Very fast rate of growth, at one lake in Florida - spread from 0.1 to 10 acres in one year.
  • It can invade freshwater lakes, reservoirs, ponds, marshes and ditches making boating, fishing and almost all other water activities difficult.
  • Degrades water quality by blocking the air-water interface and greatly reducing oxygen levels in the water, impacting underwater animals such as fish.
  • Greatly reduces biological diversity: vegetation mats block sunlight, preventing the growth of submerged and emersed plant communities and also alter animal communities by blocking access to the water and/or reducing plants the animals depend on for shelter and nesting.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for Indian swampweed was based upon this literature review [PDF] developed by the department.

Identification

Leaves: grow opposite along the stems. They are rounded, hairy and up to three inches long with pointed tips. They range in color from green to brown to reddish.

Flowers: bluish-white to white, and have two lobes on the upper lip and three lobes on the lower lip. They grow from the axils where the leaves meet the stems.

Fruits & seeds: Narrow capsule 6-7 mm long; 20-30 tiny flattened-round seeds.

Stems: Square stems can grow up to six feet long.

Similar species: This plant could easily be confused with other aquatic species such as Ludwigia repens, Alternanthera philoxeroides and H. lacustris.

Distribution

Currently, there have been no reports of Indian swampweed in Wisconsin. Have you seen it? Send us a report.

Control

Mechanical: Very small populations can be controlled by pulling. Physical removal should be completed before the flowering and seed set. This plant can spread from fragments so care must be taken to collect and destroy all parts of the plant.

Chemical: Registered aquatic herbicides can provide some small control of Indian swampweed, though it shows greater resistance to herbicides than many other aquatic invasives. The application of aquatic herbicide requires a permit.

Photos

View Indian swampweed pictures in our photo gallery!

Resources

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