There are 21 invasive aquatic plants Michigan officials want residents to be aware of.
A particular one you should know is cylindro, a species of blue-green algae that can be toxic to humans and is especially dangerous for dogs because they are more likely to swallow the water.
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Below I’ll go over the aquatic plants that Michigan has listed online, how to identify them and what threat they pose to you or the ecosystem.
Update (Sept. 29, 2024): The state has added water-primrose to this list. More information can be found here.
- If you believe you have found one of these invasive aquatic plants, you can report it through the Midwest Invasive Species Information Network. Just click here and search for the plant.
African Oxygen Weed
This submerged aquatic plant has not been established in Michigan.
How to identify it:
- Submerged aquatic plant.
- Dark green, alternating leaves curl downward.
- Form dense mats up to 3 feet thick.
- Stems grow up to 20 feet long with ‘j’ shaped curve.
Where is it found?
It can be found in freshwater lakes and slow-flowing rivers. It likes cool water and abundant light.
Why is it a concern?
Dense mats of African oxygen weed decrease oxygen levels, increase pH levels, decrease sunlight generation, increase decomposition of dead plants and reduce biodiversity. Recreation is also limited in infested areas.
Brazilian Elodea
This bushy aquatic plant has not been detected in Michigan.
How to identify it:
- Bushy aquatic plant with dense whorls of bright green leaves.
- Generally 4 leaves per whorl.
- White, 3-petaled blooms float on the surface of the water.
- Can grow up to 18 feet to water surface.
- Easily confused with Hydrilla (another invasive) – Brazilian elodea leaves are smooth to the touch.
Where is it found?
It’s a submerged perennial that can survive free-floating in freshwater depths up to 20 feet. It is found in ponds, lakes and sluggish rivers and streams.
Why is it a concern?
It grows fast and can lead to dense, monospecific mats on the surface of the water. The mats crowd out native aquatic plants, provide poor habitat for fish and impede boat movement and other recreational activities.
Carolina Fanwort
This submerged aquatic plant has been detected in Michigan. Officials believe it was introduced to the state through aquarium and water garden trades or recreational equipment.
How to identify it:
- Submerged aquatic plant, rooted in the mud of stagnant or slow-flowing water.
- Underwater leaves approximately 2 inches across and divided into fine branches.
- Flowering branches grow above the water surface with diamond shaped leaves.
- Flowers small and range from white to pale yellow.
- Stem length usually less than 10 feet but can reach 33 feet.
Where is it found?
It roots itself into the mud of shallow, slow-moving, or stagnant freshwater sources.
Why is it a concern?
Dense mats alter oxygen and pH levels, reduce sunlight penetration and interfere with recreational water use.
Curly-Leaf Pondweed
This submersed, perennial herb has been detected in Michigan.
How to identify it:
- Leaves are dark green with wavy, serrated margins.
- Submersed, perennial herb with thick roots.
- Can reach 2 meters in length.
- Flowering spike grows above water’s surface.
- Starts growing in fall and winter, flowers in late spring, dies in late July.
Where is it found?
It inhabits ponds, lakes and slow-moving streams. It likes brackish, alkaline or eutrophic conditions less than 3 meters in depth. But it can grow in waters up to 12 meters deep.
Why is it a concern?
Curly-leaf pondweed out-competes native aquatic plant species and reduces diversity. Dense colonies can hinder fish movement as well as recreational activity.
Cylindro
This blue-green algae has been detected in Michigan. It produces a toxin that can harm humans, dogs, and wildlife. If you see a bloom, do not let your dogs in the water. Dogs can potentially swallow the toxins or injest them when grooming themselves.
Symptoms of illness can appear in a few minutes to a few hours. Symptoms include vomiting, diarrhea, fatigue, staggered walking, excessive salivation, convulsions, erratic behavior, or physical distress.
If your dog has contact with algae you should thoroughly rinse them or bathe them with fresh water, even if the algae is not of the toxic variety.
How to identify it:
- One of many species of blue-green algae; this species is considered invasive and harmful.
- Blooms are just below the water surface of freshwater habitats and may appear as foggy green/yellow water.
Where is it found?
It has been documented in low densities in many inland lakes and reservoirs surrounding the Great Lakes region.
Why is it a concern?
It’s a species of photosynthetic cyanobacteria that is capable of forming toxins that are harmful to human health when in bloom. They are usually in bloom in late summer, when the water is warm.
Click here to learn more about harmful algae blooms.
Didymo (Rock Snot)
This algae can make recreation unpleasant and is present in some Michigan waters.
How to identify it:
- Didymo is a microscopic algae (diatom) that produces stalks that form thick mats on hard surfaces like rocks in stream beds.
- Looks and feels like white or brown wet wool.
- Ranges from small, cotton ball-sized patches to thick blankets and long, rope-like strings that flow in currents.
- Although often referred to as “rock snot,” didymo is not slimy.
Where is it found?
It thrives in low-nutrient cold water rivers and streams. Didymo cells haves been documented in the Great Lakes Basin and Michigan waters in low abundance. Nuisance blooms have been documented in Michigan in the Upper Manistee and St. Marys rivers.
Why is it a concern?
Didymo can be transported on boats, anchors and fishing gear such as waders, felt-soled boots and nets. It can create thick mats that can cover river and stream bottoms. It alter habitat and food resources for fish and make recreation unpleasant.
Eurasian Watermilfoil
This aquatic plant has been established in Michigan.
How to identify it:
- Emergent, herbaceous aquatic plant.
- Stems are whitish-pink to reddish-brown.
- Leaves are greyish-green with finely divided pairs of leaflets that are ½-2 inches long, giving the plant a feathery appearance.
- Leaves arranged in whorls of 3-6.
- Yellow or reddish flower with 4 parts on a projected spike sitting 2-4 inches above water.
- Usually 3-10 feet tall with a maximum of 33 feet.
Where is it found?
It likes fresh to brackish water and areas that have been disturbed. It’s very resistant and can overwinter in frozen lakes and ponds or survive over-heated areas.
Why is it a concern?
Eurasian watermilfoil forms large mats of floating vegetation that will shade-out native aquatic plants and impede recreational activities. This species is not a valuable food source of waterfowl and may interfere with fish predation. Thick vegetation like this can also clog residential or industrial water intakes.
European Frog-bit
This is established in Michigan and is also prohibited.
How to identify it:
- Free-floating aquatic plant sometimes rooted in shallow water.
- Leaves are small, 0.5-2.5 inches, round to heart-shaped, with a purple-red underside.
- Leaves form a rosette.
- Single flower with three white petals and yellow center may be visible from June to August.
Where is it found?
It’s found in slow-moving rivers, sheltered inlets, ponds, bayous, and ditches. It prefers waters rich in calcium with no wave action.
Why is it a concern?
European frog-bit can form dense mats on the surface of slow-moving waters like bayous, backwaters and wetlands. Mats of European frog-bit can impede boat traffic and alter food and habitat for ducks and fish. Prolific growth of aquatic invasive plants like European frog-bit can also reduce oxygen and light in the water column.
European Water Clover
This plant resembles a four-leaf clover and has been detected in Michigan.
How to identify it:
- Resembles a four-leaf clover.
- Leaves are smooth and can be floating, submerged or emergent.
- Maximum height: 8 inches.
- Thin green stalks bear a single leaf.
Where is it found?
It anchors into the sediment in shallow, slow-moving waters. It likes sandy and loamy soil environments with semi-shade to full sun.
Why is it a concern?
European water-clover has the ability to form dense, monospecific stands by outcompeting native aquatic species. Its ability to adjust the angle of floating leaflets to optimize sunlight gives it even more of a competitive edge over native species.
Flowering Rush
This has been established in Michigan and is restricted.
How to identify it:
- Resembles a large sedge.
- May grow with upright foliage in shallow water or submerged with flexible leaves suspended in deeper waters.
- Leaves have triangular cross section, are narrow, and twist toward the tip.
- When flowering: flowers grow in round umbrella-like clusters of 20-50 flowers, 6 light pink to rose-colored petals per flower, 9 stamens per flower.
- Difficult to identify when not flowering.
Where is it found?
It grows in shallow sections of slow-moving streams or rivers, lakeshores, irrigation, ditches and wetlands -- although it can survive in very clear water up to 20 feet.
Why is it a concern?
Flowering rush out-competes native species for resources and can hinder recreational activities like boating.
Giant Salvinia
It has not been detected in Michigan and is prohibited.
How to identify it:
- Floating leaves are oblong, ½ to 1 ½ inches long, and vary from green to gold to brown.
- Leaves have arched, white hairs resembling egg beaters.
- Submerged fern fronds are stringy and root-like, but the plant has no real roots.
- Leaves of mature plants grow vertically and curl, creating a chain-like structure.
Where is it found?
It can grow in almost any water system.
Why is it a concern?
Giant salvinia forms chains of leaves that link together into thick mats on the water’s surface. These mats restrict light and oxygen, shading out native plants and organisms and disrupting ecosystems. Giant salvinia reproduces from plant parts and can be transported by waterfowl or boats.
Hydrilla
It has not been detected in Michigan and is prohibited.
How to identify it:
- Submerged aquatic plant with generally green leaves.
- Leaves are whorled in group of 4-8.
- Leaf mid-vein is reddish with a row of spines giving it a rough texture.
- Very slender stems that can grow up to 30 feet long.
- Stems branch out considerably near water surface.
- White 3-petaled flowers (may be reddish or brown on male plants).
- Easily confused with Brazilian elodea (another invasive) – hydrilla leaves are rough and have visible saw-toothed margins.
Where is it found?
It grows in springs, lakes, ditches, marshes or rivers. It can grow in low light conditions and tolerate a variety of nutrient conditions.
Why is it a concern?
Hydrilla is a threat to native aquatic ecosystems. Dense mats shade out native aquatic vegetation and alter the ecology of the water body. Invasion also interferes with recreational activities like boating and fishing.
Parrot Feather
It has been detected in Michigan.
How to identify it:
- Spikes of stiff, feathery leaves grow in whorls of 4-6.
- Bright green upper stem emerges up to 1 foot above water.
- Small, inconspicuous white flowers where leaves attach to the section of stem above water.
- Stems and submerged leaves may be tinted reddish.
- All U.S. plants are female and spread via fragmentation.
Where is it found?
It’s found in slow-moving freshwater habitats such as tributaries, canals, ponds and lakes. It prefers high-nutrient environments with lots of light.
Why is it a concern?
This invasive milfoil plant threatens native aquatic species through competition. It will often form dense mats which provides habitat for mosquito larvae and can impede boats.
Phragmites (Common Reed)
This perennial grass has been established in Michigan.
How to identify it:
- Warm-season perennial grass with a rigid, hollow stem.
- Height ranges from 6-13 feet.
- Leaves are flat, smooth, and green to grayish-green.
- Flowers grow as dense branched clusters on the end of each stem that are open and feathery at maturity.
Where is it found?
It grows in ditches, swales, wetlands and on stream and pond banks.
Why is it a concern?
Phragmites can be difficult to walk through (for humans and wildlife) and often obstructs landowner views because of its ability to grow in tall, dense patches. The exotic strain can reduce native fish and wildlife populations, block out native salt marsh vegetation, and can be a fire danger for nearby residents.
Purple Loosestrife
This perennial herb is established in Michigan.
How to identify it:
- Perennial herb with a woody, square stem covered in downy hair.
- Height varies from 4 to 10 feet.
- Leaves are arranged in pairs or whorls.
- Magenta flower spikes with 5-7 petals per flower are present for most of the summer.
Where is it found?
It thrives along roadsides and in wetlands. While seeds can germinate in water, establishment is much more successful in moist substrate that’s not flooded. It prefers full sun, but can tolerate shade.
Why is it a concern?
Given the right conditions, purple loosestrife can rapidly establish and replace native vegetation. This can lead to a reduction in plant diversity, which reduces habitat value to wildlife.
Starry Stonewort
This plant is established in Michigan and is on the prohibited list.
How to identify it:
- Whorls of 4-6 branchlets/leaves with blunt tips.
- Star-shaped bulbils are produced at the nodes, generally 3-6 mm wide.
- Can reach up to 33 inches in length.
Where is it found?
It invades lakes, ponds, reservoirs, and slow-moving rivers. It inhabits freshwater habitats from 3 feet to 95 feet in depth.
Why is it a concern?
Starry stonewort forms dense mats in lakes and can significantly reduce the diversity of other aquatic plants. Dense mats of vegetation can also impede movement of fish, spawning activity, water flow and recreational activities.
Water Chestnut
This plant has not been detected in Michigan.
How to identify it:
- Green, floating leaves with sharply serrated edges.
- Leaves form a densely crowded rosette.
- Small, white 4-petaled flowers.
- Produce a hard “woody” nut surrounded by sharp barbed spines.
Where is it found?
It prefers shallow, nutrient-rich lakes and rivers. It can grow in mucky substrate.
Why is it a concern?
Water chestnut forms dense mats that shade out native aquatic vegetation, leading to a decrease in biodiversity. Decomposition of vegetation below a dense mat decreases oxygen levels and can cause fish kills. Boating and other recreational activities become almost impossible in an area invaded by water chestnut.
Water Hyacinth
This perennial herb has not been detected in Michigan but is on the watch list. You can purchase and sell it in Michigan.
How to identify it:
- Erect, free-floating perennial herb.
- Short, bulbous leaf petioles.
- Distinctive air bladders that keep leaves afloat.
- Rounded, leathery leaves arranged in whorls of 6-10.
- 14-day flowering cycle produced lavender flowers with central yellow fleck.
Where is it found?
Mainly grows in tropical and sub-tropical freshwater systems. It cannot tolerate below-freezing temperatures for long periods of time or salt water.
Why is it a concern?
This invasive species forms dense colonies in water bodies that block sunlight and crowd out native species. At optimum temperatures, it can double its biomass within a month. Dense colonization can also impede boat traffic, reduce water flow and interfere with hydroelectric power generation.
Water Lettuce
This free-floating plant has been detected in Michigan.
How to identify it:
- Free-floating – forms a rosette of leaves that resembles an open head of lettuce.
- Leaves are thick, ridged, rounded at the end, light green, and have short, white hairs.
- Produces small, white to pale green flowers.
- Many feathery roots dangle under the rosette.
Where is it found?
It grows in ponds, lakes, rivers and streams.
Why is it a concern?
This freshwater perennial creates thick mats that reduce oxygen levels, block sunlight and prevent growth of submerged vegetation. Mats interfere with recreation and can have a negative effect on fish and other aquatic species in an infested area.
Water Soldier
This aquatic invasive species has not been detectedin Michigan, but is on the watch list as prohibited. The sharp edges of this plant can cut swimmers.
How to identify it:
- Submerged aquatic plant, becomes buoyant during summer.
- Leaves are 40 cm long, sword-shaped, sharply serrated edges, bright green.
- Leaves form a large rosette.
- Roots may or may not be attached to mud.
- Looks similar to an aloe plant, spider plant, or top of a pineapple.
- May produce a showy white flower with 3 petals.
Where is it found?
I inhabits freshwater bodies like ponds, lakes and rivers.
Why is it a concern?
Dense mats of vegetation can form to crowd out native species and decrease biodiversity. Water soldier can potentially alter water chemistry and could harm other aquatic organisms. Mats also hinder recreational activities and the sharp edges of this plant can cut swimmers.
Yellow Floating Heart
Yellow Floating Heart has been detected in Michigan and is prohibited.
How to identify it:
- Flowers are bright yellow with 5 petals, located above the surface of the water.
- Leaves are circular or heart shaped.
- Leaves are alternately arranged on the stem but oppositely on the flower stalk.
- Seeds are flat and oval; many seeds per capsule.
Where is it found?
It’s commonly found in slow-moving rivers, ponds and lakes.
Why is it a concern?
Yellow floating heart can create dense mats that shade out native aquatic plants, decrease oxygen levels, increase mosquito breeding habitat and impede boating activity, fishing and swimming. Fragmented pieces of plants can establish new populations and seeds are engineered to disperse by attaching to the feathers of waterfowl.
You can visit Michigan’s invasive species website to learn more about these invasive aquatic plants and other invasive species.