Brazilian elodea (Egeria densa)

Common Names: Brazilian waterweed, South American waterweed, Anacharis

Description:

Brazilian elodea (Egeria densa Planch.) is a submersed aquatic plant native to Argentina, Brazil, and Uruguay that has spread widely across the United States since the late 1800s, arriving through the aquarium and water garden trades. Its bright green, bottlebrush-shaped stems grow entirely underwater, forming dense mats that shade out native aquatic plants and disrupt the ecology of lakes, ponds, and slow-moving rivers [1]. MISIN tracks Brazilian elodea because established populations exist in Michigan waterways and new introductions continue through aquarium dumping and contaminated boating equipment [2].

Habit:

Brazilian elodea is a perennial aquatic plant that stays green and keeps growing while most native plants are dormant in winter. It grows most vigorously from spring through early fall, and summer is the easiest time to confirm identification ? the distinctive large white flowers float right at the surface, held up on slender stalks, making the plant unmistakable when blooming. Look for dense, bright green underwater mats in still or slow-moving water, from small garden ponds to large lake coves. When a stem is lifted from the water, it has a characteristic "bottlebrush" shape: small leaves packed tightly in rings around every inch of a hollow, brittle stem.

Stems are slender and fragile, often growing from the bottom toward the surface and reaching up to 9?15 feet (3?5 m) in deep water. Each leaf is roughly the size of a thumbnail ? about 1 inch (2.5 cm) long and ? inch wide ? with fine serrations along the edges you can feel with a fingernail. Adventitious roots sprout from nodes along the stem, and double nodes (two closely spaced nodes separated by a very short internode) are the only points that can produce a new plant after a stem breaks apart [1].

At-a-glance cues:

1. Leaves in whorls (rings) of 4?6 around the stem, packed tightly and bright green ? more leaves per ring than any native waterweed
2. Showy white 3-petaled flowers, roughly ? inch (2 cm) across, floating at the water surface in summer ? much larger than any native waterweed's flowers [1]
3. Underside of the leaf's central vein is smooth to the touch ? no spines or teeth (key difference from hydrilla)
4. Stems brittle and hollow, breaking easily into fragments at double-node points, each capable of growing into a new plant [1]

Leaves:

1. Arrangement: in whorls (rings) of 4?6 (most commonly 4?5) evenly spaced around the stem
2. Shape: oblong to strap-shaped; base rounded, tip pointed
3. Margin: finely toothed (serrulate) ? teeth best seen under a hand lens or felt with a fingernail
4. Venation: single midrib; underside of midrib is smooth (no teeth or spines)
5. Surface: bright green, slightly translucent
6. Size: approximately ??1? inches (2?3 cm) long, about ? inch (5?7 mm) wide

Stems:

1. Habit: herbaceous, fully submersed perennial; rooted at bottom or free-floating after fragmentation
2. Structure: round, hollow, branching; brittle and fragments easily under physical disturbance
3. Color: pale to medium green
4. Nodes: double nodes (two closely spaced nodes) occur at intervals of roughly 6?12 regular nodes; only double nodes regenerate into new plants after fragmentation [1]
5. Length: typically 1?2 feet (0.3?0.6 m) in shallow water; up to 9?15 feet (3?5 m) in deeper water
6. Roots: slender, pale, adventitious roots emerge from nodes along the stem

Flowers:

1. Color: white
2. Petals: 3, broadly rounded; noticeably larger than the 3 small green sepals
3. Size: approximately ? inch (2?3 cm) across ? larger and more showy than any native waterweed's flowers
4. Arrangement: solitary; held on a very slender, elongated stalk that lifts the flower to float at the water surface
5. Bloom time: late spring through summer
6. Sex: only male plants are present in United States populations; pollination and seed set do not occur here [1]

Fruit and seeds:

1. Fruit: small berry-like capsule; occasionally present where both male and female plants co-exist (native range in South America only)
2. Seeds: not produced in United States populations ? all US plants are male, so no fertilization occurs [1]
3. Dispersal: no seed dispersal pathway in the US; all spread is through vegetative stem fragments

Habitat:

In its native South America ? central Brazil, coastal Argentina, and coastal Uruguay ? Brazilian elodea grows in slow-moving rivers, oxbow lakes, and still ponds alongside other aquatic vegetation [1]. In the United States it has established across at least 14 major river basins, from California and the Pacific Northwest to the Gulf Coast, the Mid-Atlantic states, New England, and the Great Lakes region. The species has been recorded in the US as far back as 1893.

The plant grows in still or slow-moving freshwater: lakes, ponds, reservoirs, rivers, streams, and irrigation ditches. It tolerates a wide temperature range ? roughly 37?95 ?F (3?35 ?C) ? and roots at depths down to about 23 feet (7 m), though it forms the densest mats in 2?10 feet of water. It can grow in nutrient-rich or relatively clear water and tolerates moderate turbidity. In Michigan, any lake, pond, or slow-moving river is potential habitat; boat landings connected to infested water bodies are the highest-risk entry points for new populations [3].

Reproduction:

In the United States, Brazilian elodea reproduces entirely through vegetative fragmentation ? no seeds are produced because all US plants are male [1]. A stem fragment as short as two inches, as long as it contains a double node, can root and grow into a new plant. This makes even a tiny piece of plant material a potential new infestation.

Fragments detach naturally as crowded colonies are disturbed by wind and wave action, then drift downstream or across a lake. Boats and trailers are the most important pathway for moving fragments between water bodies: a wet propeller, anchor, or bilge pump can carry viable pieces for miles. Fishing gear, waders, and nets can carry fragments too. Waterfowl carry pieces on their feet and feathers between nearby water bodies. Aquarium hobbyists and water gardeners who discard plants ? sold in pet stores as "anacharis" ? into a yard, storm drain, or nearby stream are responsible for many new introductions [2].

A single fragment reaching a new pond in spring can form a large mat by late summer. The plant does not fully die back in winter; semi-dormant shoots persist under ice, giving Brazilian elodea a head start over native aquatic plants each spring. New, small populations are easiest to find in late spring through early summer, before growth has spread enough to be obvious from shore [1].

Impact and Damage:

For homeowners with a pond, lake, or water feature, Brazilian elodea is a direct nuisance: dense mats tangle boat propellers and fishing gear, clog pump intakes, and make swimming or wading unpleasant. Heavily infested shorelines see reduced property values and restricted access to water [1].

Brazilian elodea functions as what ecologists call an ecosystem engineer ? it physically restructures the underwater environment in ways that ripple through the food web [4]. Dense underwater canopies block sunlight from reaching the bottom, eliminating habitat for native submersed plants. The mats slow water movement, trap fine sediment, and create low-oxygen zones near the bottom that harm fish and invertebrates [1]. Studies of invaded waterways document sharp declines in native aquatic plant diversity after Brazilian elodea becomes dominant [4].

Economic costs can be severe. In Brazil, infestations in hydroelectric reservoirs required removal of approximately 48,000 cubic meters of plant material from a single facility. Nationally, aquatic invasive plant management costs states and municipalities tens of millions of dollars each year in prevention, monitoring, and control ? costs borne partly by lakefront property owners through permit fees and special assessments.

Similar species:

Hydrilla (Hydrilla verticillata) is also an invasive aquatic plant ? and a higher federal concern ? that can be mistaken for Brazilian elodea in dense underwater mats. The clearest way to tell them apart: hydrilla produces small tubers (potato-like nodules) on its roots and rhizomes that Brazilian elodea never produces. Look underwater along the roots ? if you see white, pea-sized tubers, it is hydrilla. Hydrilla also has tiny teeth visible along the underside of the leaf's central vein; Brazilian elodea's midrib underside is smooth. The southern form of hydrilla additionally has spines on the leaf surface itself.

Canadian waterweed (Elodea canadensis) is a common native waterweed frequently found in the same lakes and streams as Brazilian elodea. The simplest distinction is the number of leaves per whorl: Canadian waterweed has exactly 3 leaves per ring, while Brazilian elodea has 4?6. Canadian waterweed's leaves are also somewhat darker green and slightly smaller.

Western waterweed (Elodea nuttallii) is another native bottlebrush-shaped aquatic plant with a similar growth form. Its leaves are paler green, narrower, and more limp than Brazilian elodea's, and occur in whorls of 3 or 4. Western waterweed's flowers are tiny and barely visible at the surface ? very different from Brazilian elodea's large, showy white blooms.

African oxygen weed (Lagarosiphon major) is another invasive aquatic plant in the same plant family (Hydrocharitaceae) that can look similar underwater. The key difference is how the leaves attach: African oxygen weed's leaves spiral continuously up the stem in a staircase pattern, while Brazilian elodea's leaves are in distinct rings (whorls) of 4?6 at each node. African oxygen weed's leaves also curve distinctly downward at the tip [5].

Monitoring and rapid response:

Signs to watch for:

1. Dense bright green underwater tangles visible from a dock or boat in summer ? nearly impenetrable mats just below the surface in still or slow-moving water
2. White three-petaled flowers about ? inch across floating at the surface on hair-thin stalks from late spring through summer; these are far larger and more noticeable than the flowers of any native waterweed
3. A "bottlebrush" shape when a stem is lifted from the water, with leaves packed in tight rings of 4?6 all the way along the stem
4. Stems that snap and fragment easily when handled ? treat every broken piece as a potential new plant

What you can do:

1. For a small patch in a garden pond or water feature, carefully hand-remove all plant material during early summer before it sets more fragments; seal every piece in a trash bag and put it in the garbage ? never compost or dump near any waterway, ditch, or storm drain [2]
2. After any water activity, drain, rinse, and dry your boat, trailer, waders, and all fishing gear completely before moving to a different water body; remove every visible plant fragment before leaving the landing
3. Never release aquarium or water garden plants ? including those sold as "anacharis" ? into any natural water, yard drainage, or storm drain; this is the primary way new populations start [2]
4. Natural area crews should prioritize small outlier populations ? isolated plants or patches away from the main infestation ? because eliminating these prevents spread to new locations; revisit sites at least twice during the growing season, as regrowth from surviving fragments is common
5. Avoid mechanical harvesting of large, established infestations ? cutting creates fragments that start new growth; hand-removal is safer for small, contained patches where all material can be bagged and removed

When to call a professional:

1. Any infestation larger than a small garden pond warrants contacting your local Michigan DNR office or a licensed aquatic invasive species manager before attempting treatment
2. In Michigan, most aquatic plant management in navigable waters requires a DNR permit; contact them first to avoid violations
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. IUCN Global Invasive Species Database. Egeria densa. iucngisd.org
2. Minnesota Department of Natural Resources. Brazilian elodea (Egeria densa). dnr.state.mn.us
3. Michigan Department of Natural Resources. Invasive Species: Brazilian elodea. michigan.gov/invasives
4. Yarrow MM, Marin VH, Finlayson M, Tironi A, Delgado LE, Fischer F (2009). "The ecology of Egeria densa Planchon (Liliopsida: Alismatales): A wetland ecosystem engineer?" Revista Chilena de Historia Natural 82:299?313. semanticscholar.org (156 citations on Semantic Scholar)
5. IUCN Global Invasive Species Database. Lagarosiphon major (African oxygen weed). iucngisd.org
6. Plants of the World Online (POWO). Egeria densa Planch. ? taxonomic synonymy. powo.science.kew.org

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