African oxygen weed (Lagarosiphon major)

Common Names: curly waterweed, African elodea, oxygen weed, South African oxygen weed, Elodea crispa (aquarium trade name, not a botanical synonym)

Description:

African oxygen weed (Lagarosiphon major) is a fully submersed aquatic plant from southern Africa, widely sold in the aquarium and water garden trade as "oxygen weed," "curly waterweed," or "Elodea crispa." It is not yet established in the wild in North America but is listed as a federal noxious weed because it has devastated lakes in New Zealand, Ireland, and across Europe, where dense underwater mats crowd out native plants and disrupt aquatic habitat [1]. MISIN tracks it because the Great Lakes region is identified as high-risk habitat, and aquarium release is the most likely pathway of introduction [4].

Habit:

In temperate climates, African oxygen weed grows most actively from late fall through early summer ? while most native aquatic plants are still dormant. By the time native plants resume growth in spring, African oxygen weed may already form a dense canopy just below the surface [1]. Look for it in clear, still water in late winter and early spring, when it stands out as one of the only green plants actively growing.

The plant looks like a dark green bottlebrush underwater. Slender, brittle stems can grow up to 15 feet long, trailing along the bottom and rising toward the surface. Small leaves ? roughly ? to ? inch long ? are packed tightly in a spiral staircase pattern up the stem, and each leaf tip curves distinctly downward. Tiny pinkish-white flowers occasionally float at the water surface on hair-thin stalks during summer [2].

At-a-glance cues:

1. Leaves spiral up the stem like a staircase ? not in rings ? unlike all native waterweeds [1]
2. Each leaf tip curls noticeably downward, especially visible in clear water [1]
3. Dense, dark green bottlebrush appearance at stem tips [2]
4. Tiny pinkish-white flowers floating at the water surface on hair-thin stalks (when present in summer) [1]

Leaves:

1. Arrangement: alternate, spirally arranged along stem ? NOT in whorls; this is the key feature distinguishing it from all similar species [1]
2. Shape: linear to narrowly lance-shaped
3. Size: ??? inch (5?20 mm) long, about ? inch (2?3 mm) wide
4. Margins: minutely toothed; tip strongly recurved (curled downward)
5. Venation: 3-veined
6. Color: dark green
7. Leaves densely crowded near stem tips, giving a bottlebrush appearance [2]

Stems:

1. Habit: fully submersed perennial; roots in soft substrate, stems extend toward the water surface
2. Length: up to about 15 feet (5 m) in deep, clear water [1]
3. Diameter: slender, about ? inch (3?5 mm)
4. Branching: sparsely to freely branched; branching increases near the surface
5. Texture: smooth, brittle ? fragments very easily [5]
6. Color: green to reddish-green depending on light

Flowers:

1. Only female flowers produced in invasive populations ? no male plants exist outside the native range, so seeds cannot form [1]
2. Flowers tiny (about 1/100 inch, 0.25 mm), pinkish to clear-white
3. Each borne singly on a slender, translucent stalk up to about 2? inches (6 cm) long rising from an upper leaf axil; floats at the water surface [2]
4. Flowering season: primarily summer

Fruit and seeds:

1. No seeds produced in invasive populations ? only female plants are present, so pollination cannot occur [1]
2. In the native southern Africa range, small fruits and seeds are produced after pollination by male plants

Habitat:

African oxygen weed grows in clear, still or slow-moving freshwater ? lakes, ponds, reservoirs, canals, and slow river sections. It does best where sunlight reaches the lake bottom and can grow from the shallow margins to depths of about 20 feet (6.5 m) in very clear water. Fast-moving or turbid water limits its growth [1].

The plant is native to southern Africa, including South Africa, Lesotho, Zimbabwe, Zambia, and Botswana [2]. It is invasive and widespread in New Zealand, the United Kingdom, Ireland, and much of western and central Europe, where it was introduced through the aquarium and water garden trade [1]. It is not yet established in the wild in the United States or Canada [6].

The Great Lakes region is considered high-risk habitat. Deep, clear, cool inland lakes ? common throughout Michigan and the upper Midwest ? closely match the conditions where African oxygen weed has thrived in New Zealand and Europe [4].

Reproduction:

African oxygen weed cannot produce seeds anywhere outside its native range. Only female plants were introduced worldwide, and without male plants nearby, pollination is impossible. The plant spreads entirely by stem fragments [1].

Fragments as small as half an inch can root and grow into a new plant [5]. Stems break easily and drift with water currents, settling wherever they come to rest. Between water bodies, the most common way fragments travel is on boat hulls, propellers, trailers, anchors, fishing gear, and waders ? often too small to see. Waterfowl can also carry fragments on their feathers and feet.

Once established in a lake, the plant extends outward along the bottom each season. Mechanical disturbance creates more fragments and accelerates spread rather than controlling it ? making early detection and prevention far more effective than attempted removal [5].

Impact and Damage:

African oxygen weed has no known direct health risks to people or pets, but it causes serious harm to water bodies and the recreation and wildlife that depend on them.

Dense underwater mats block sunlight from reaching the lake bottom, eliminating the native aquatic plants that would otherwise grow there. At the surface, mats foul boat propellers, tangle fishing lines, and close off swimming areas. Rotting vegetation washed onto shores is a common problem wherever the plant is established. In New Zealand, dense growth has blocked intakes at hydroelectric facilities [1].

Despite being marketed as an "oxygen plant," dense mats consume more oxygen than they produce during the night and during decomposition, creating low-oxygen conditions near the bottom that harm fish and invertebrates. Water pH beneath dense stands also rises sharply during daytime, further stressing aquatic life.

For natural area stewards, the plant's winter growth is especially concerning: it establishes its canopy before native aquatic plants resume growth in spring, shading them out before they have a chance to compete [1].

Similar species:

The most reliable way to distinguish African oxygen weed from similar aquatic plants is leaf arrangement. African oxygen weed leaves spiral up the stem like a staircase, with one leaf attaching at each point and rotating around the stem. All common look-alike species have leaves that grow in distinct rings (whorls) around the stem [7].

Canadian waterweed (Elodea canadensis) and Nuttall's waterweed (Elodea nuttallii) are both native to the Great Lakes and look similar from a distance, but their leaves grow in whorls of three with flatter, less recurved tips. Brazilian waterweed (Elodea densa) has broader leaves in whorls of three to six with clearly visible teeth along the edge. Hydrilla (Hydrilla verticillata) also has whorled leaves, often with a small tooth on the underside of the midrib, and produces distinctive underground tubers and dormant buds that African oxygen weed does not [7].

If you are unsure, gently hold a stem and trace where the leaves attach: a rotating, one-at-a-time spiral pattern strongly indicates African oxygen weed.

Monitoring and rapid response:

Signs to watch for:

1. Dense, dark green underwater growth in clear, still water ? most visible in late winter and early spring when native aquatic plants are dormant [1]
2. Stems with leaves arranged in a tight spiral staircase pattern, each leaf tip curling downward [1]
3. Plants sold or labeled as "oxygen weed," "curly waterweed," or "Elodea crispa" at aquarium shops or water garden retailers
4. Tiny pinkish-white flowers floating on hair-thin stalks at the water surface in summer [2]

What you can do:

1. Never release aquarium or pond plants into any natural waterway, drainage ditch, or storm drain ? wrap unwanted plants in newspaper and put them in the trash
2. Before moving your boat, trailer, or water gear to a new water body, remove all visible plant material, drain all water, and allow everything to dry completely ? fragments invisible to the naked eye can start a new infestation [5]
3. Check kayaks, canoes, waders, anchors, and fishing gear for plant fragments after every outing
4. Natural area crews should add early spring aquatic plant surveys to their monitoring calendar, when African oxygen weed is actively growing and native plants are still dormant ? making it easier to spot

When to call a professional:

1. If you find what you think is African oxygen weed growing in a natural water body, do not attempt to remove it ? cutting or disturbing the plant creates fragments and will spread it further. Contact your state natural resources agency or invasive species program immediately
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Federal noxious weed: African oxygen weed is listed as a federal noxious weed under the U.S. Plant Protection Act. Importing it into the United States or transporting it across state lines without a USDA APHIS permit is prohibited [3]. If you encounter it for sale, report it to USDA APHIS.

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 Invasive Species Specialist Group. "Lagarosiphon major." Global Invasive Species Database. iucngisd.org
2. Royal Botanic Gardens, Kew. "Lagarosiphon major (Ridl.) Moss." Plants of the World Online. powo.science.kew.org
3. USDA PLANTS Database. "Lagarosiphon major (Ridl.) Moss ? African elodea." LAMA15. plants.usda.gov
4. U.S. Fish & Wildlife Service. "Ecological Risk Screening Summary: African Elodea (Lagarosiphon major) ? High Risk." fws.gov
5. NIWA. "Lagarosiphon, be gone!" Freshwater Update 83 (April 2020). niwa.co.nz
6. USDA National Invasive Species Information Center. "Oxygen Weed (Lagarosiphon major)." invasivespeciesinfo.gov
7. Invasive Species Ireland. "Curly Waterweed (Lagarosiphon major)." invasivespeciesireland.com

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