Hydrilla (Hydrilla verticillata)

Common Names: hydrilla, water thyme, Florida elodea, waterthyme, oxygen weed

Description:

Hydrilla (Hydrilla verticillata) is a fast-growing underwater plant from warm parts of Asia, Africa, and Australia. It was brought to the United States in the 1950s through the aquarium trade and discarded into Florida canals [2]. It now grows in lakes, ponds, and rivers across much of the southern and eastern United States, where it forms thick mats that crowd out native plants and choke swimming, boating, and fishing [3][5]. MISIN tracks hydrilla because it is one of the most aggressive water weeds in the country and is listed as a federal noxious weed [3].

Habit:

Hydrilla is a rooted underwater plant with long, thin, branching stems that can reach more than 25 feet to grow from deep water all the way to the surface [5]. Its small, pointed leaves grow in rings (whorls) of four to eight around the stem [1][5]. The leaf edges are saw-toothed and rough enough to feel, and one or more tiny teeth sit on the midrib (the central vein) on the underside of each leaf ? this midrib tooth is the single most reliable way to tell hydrilla apart from its look-alikes [5]. Under good conditions it can grow up to an inch a day [5].

Hydrilla is easiest to spot from mid- to late summer, when its stems reach the surface and branch into dense floating mats over the water [5]. In late summer and fall the plant also forms small, pale, potato-like tubers in the mud and dark, scaly buds in the leaf joints ? the structures that let it survive winter and start new plants [3][5].

At-a-glance cues:

1. Small, pointed leaves in rings of four to eight around the stem ? most native look-alikes have only three [1][5]
2. Leaf edges are saw-toothed and feel rough; one or more small teeth sit on the midrib on the leaf's underside [5]
3. Long, thin stems branch heavily near the surface and form dense floating mats by late summer [5]
4. Small, pale, potato-like tubers buried in the mud and dark, scaly buds (turions) in the leaf joints [3][5]

Leaves:

1. Arranged in rings (whorls) of four to eight around the stem [1][5]
2. Small and narrow with a pointed tip, usually up to about three-quarters of an inch long
3. Edges are clearly saw-toothed (serrated) and rough to the touch [5]
4. One or more sharp teeth on the midrib on the underside of the leaf ? the most reliable single identification trait [5]
5. Green to reddish-green and somewhat see-through

Stems:

1. Slender, heavily branching, and soft (not woody)
2. Can grow more than 25 feet long, reaching from deep water to the surface [5]
3. Branch densely near the surface to form a thick canopy [5]
4. Rooted in the bottom mud; the plant also makes underground tubers and stem buds called turions [3]

Flowers:

1. Tiny and easy to miss
2. Female flowers are white to translucent with three small petals and float at the surface on thread-like stalks
3. Male flowers break loose and float across the surface to reach the female flowers
4. Some US plants carry only male or only female flowers (dioecious); others carry both on one plant (monoecious) [2]
5. Bloom in summer

Fruit and seeds:

1. Hydrilla spreads mostly without seeds; plants that carry both flower types can set small seeds in summer [2]
2. Its main reproductive structures are tubers ? small, pale, potato-like buds buried in the mud ? and turions, dark scaly buds that form in the leaf joints [3][5]
3. Tubers stay alive in the mud for many years and survive drying, freezing, and being eaten and passed by waterfowl [3][4]
4. A single sprouted tuber can produce hundreds of new tubers in one growing season [5]

Habitat:

Hydrilla grows in fresh water ? lakes, ponds, slow-moving rivers, canals, and ditches. It roots in the bottom and can grow in water more than 20 feet deep because it thrives in low light, which lets it start growing earlier and deeper than most native plants [4]. It tolerates a wide range of water conditions, including slightly salty water (up to about 7 parts per thousand) and both nutrient-poor and nutrient-rich lakes [4].

Hydrilla is native to warm regions of Asia, Africa, and Australia [1]. In the United States it is established across the South and East and is spreading north, now reported in more than two dozen states plus Guam and Puerto Rico [2].

Reproduction:

Hydrilla spreads most often when small pieces of stem break off and travel to new water. Fragments hitch a ride on boats, motors, trailers, and in livewells, and a piece only an inch long can root and start a whole new infestation [2][5]. Waterfowl also move its tubers and buds from one water body to another, and it can arrive as a hitchhiker on aquarium or water-garden plants [2][3].

Once established, hydrilla reproduces mostly by growing new plants rather than by seed. It makes tubers in the mud and scaly buds (turions) in the leaf joints ? a single plant can produce hundreds of tubers, and they can stay alive in the sediment for years [4][5]. The plant grows fast, up to an inch a day, and quickly fills the water column [5].

Tubers and turions mostly form in late summer and fall. Removing plants earlier in the season, before these structures form, helps keep the infestation from coming back. Because every broken fragment can grow into a new plant, cutting or chopping hydrilla tends to spread it rather than control it [5].

Impact and Damage:

For people who use the water, hydrilla is both a nuisance and a hazard. Its dense surface mats tangle swimmers ? drownings have been reported in heavy beds ? foul boat propellers, and make fishing and swimming nearly impossible [5]. Thick growth in a pond or along a lakefront lowers the value and usefulness of the property.

For natural areas, hydrilla pushes out native life. It forms single-species mats that shade out native underwater plants such as wild celery and coontail [2][6]. As the mats grow and later rot, they cause large daily swings in dissolved oxygen and nighttime oxygen crashes that stress and can kill fish [6]. The warm, stagnant water trapped under the mats can also favor mosquito breeding [3], and dense stands lower the size and condition of sport fish by making it hard for them to feed [2].

Hydrilla is costly to manage. It clogs irrigation channels, flood-control systems, and water intakes for drinking water and power generation [2]. Because of these impacts it is listed as a federal noxious weed [3].

Similar species:

Canadian waterweed (Elodea canadensis), a native plant, is easily confused with hydrilla because both grow underwater with leaves in rings around the stem. You can tell them apart by counting leaves in a whorl near the stem tip: Canadian waterweed has three per ring, while hydrilla usually has four to eight. Canadian waterweed also has smooth leaf edges, no teeth on the leaf's midrib, and never makes tubers.

Brazilian waterweed (Elodea densa) is another bushy underwater plant often mistaken for hydrilla. It has larger, showier white three-petaled flowers and usually four to six leaves per ring. Unlike hydrilla, it has no teeth on the underside of the leaf's midrib and does not make tubers.

African oxygen weed (Lagarosiphon major) also grows underwater with crowded leaves that can look whorled. Its leaves attach singly and curl sharply backward toward the stem, rather than sitting in neat rings of four to eight. It lacks both the midrib tooth and the tubers that hydrilla produces.

Southern naiad (Najas guadalupensis), a native plant, has fine, narrow underwater leaves that can look whorled at a glance. Its leaves actually grow in pairs across from each other, not in rings, and it has no midrib tooth and no tubers.

Monitoring and rapid response:

Signs to watch for:

1. Dense underwater growth or floating surface mats from mid- to late summer, especially near boat launches and shorelines [5]
2. Small, pointed leaves in rings of four to eight, with saw-toothed edges and a tooth on the midrib underneath [1][5]
3. Small, pale, potato-like tubers in the mud and dark, scaly buds in the leaf joints in late summer and fall [3][5]

What you can do:

1. Your most important step is to stop the spread: clean, drain, and dry your boat, trailer, motor, and gear before moving between water bodies, since a fragment an inch long can start a new infestation [2][5].
2. In a small private pond or along your own shoreline, you can carefully hand-pull or rake small amounts, taking care to collect every fragment. Bag the plants and dispose of them in the trash away from water ? never compost them near a waterway.
3. Do this earlier in the season, before tubers and turions form in late summer, so fewer survive to regrow [5].
4. Avoid running a mechanical cutter or harvester, which chops the plant into pieces that float off and start new patches [5].
5. Volunteer crews and land managers can target small, outlier patches first, return for repeat removal across a season, and use careful removal methods that avoid leaving fragments behind.

When to call a professional:

1. Large or well-established infestations, or any hydrilla in public or shared waters, call for a licensed aquatic land manager ? in-water control usually requires permits.
2. Because hydrilla regrows from tubers that survive in the mud for years, eradication takes a planned, repeated, professional program rather than a one-time effort.
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. USDA NRCS PLANTS Database. Hydrilla verticillata (L.f.) Royle. plants.usda.gov
2. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Hydrilla verticillata species profile. nas.er.usgs.gov
3. IUCN/ISSG Global Invasive Species Database. Hydrilla verticillata. iucngisd.org
4. Langeland KA (1996). "Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae), 'The Perfect Aquatic Weed.'" Castanea 61(3):293?304. semanticscholar.org
5. University of Florida IFAS Center for Aquatic and Invasive Plants. "Hydrilla: Florida's Worst Submersed Weed" (SS-AGR-400). ask.ifas.ufl.edu
6. U.S. Fish and Wildlife Service. Ecological Risk Screening Summary: Hydrilla (Hydrilla verticillata). fws.gov

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