Invasive aquatic plants are generally defined as non-native species from other regions or countries that cause ecological or economic harm to natural or managed ecosystems.
Invasive plants disrupt ecosystems by inhibiting native plant growth and interfering with human activities like boating and fishing. They form thick surface canopies that block sunlight, alter water flow, and degrade fish habitats. These dense plant beds shelter small fish, reducing the success of natural predators and leading to imbalanced fish populations. Additionally, invasive plants impact water quality by increasing sedimentation and nutrient loading, speeding up the natural aging process of lakes. The dense growth of these plants restricts oxygen circulation, sometimes leading to fish kills. Invasive species also reduce the diversity of native plant communities, which negatively affects the variety of fish, aquatic insects, and other organisms, ultimately harming the overall biodiversity and function of aquatic ecosystems.
There are several invasive aquatic weeds that have caused significant economic and ecological impacts in the United States. Some of the most notable and their management approach includes
Useful information for treating the some of the most invasive aquatic weeds in the US:
Hydrilla (Hydrilla verticillata)
Description:
A submerged plant that forms dense mats, clogging waterways and outcompeting native plants. Hydrilla severely disrupts native plant communities and decreases oxygen levels in water, harming fish and other aquatic species.
Herbicide Treatment:
- Contact herbicides – fluridone, endothall, diquat, or copper-based herbicides.
- Systemic Herbicides – Fluridone, Penoxsulam, Imazox, topramezone or Florpyrauxifen-benzyl
- Fluridone for whole-lake treatments, diquat for spot treatments. Apply on calm water days for best results.
Best Time to Spray:
- Late spring to early summer (May to July), when water temperatures are above 60°F and the plant is actively growing.
- Late fall to early spring when using endothall
Eurasian Watermilfoil (Myriophyllum spicatum)
Description:
A submerged aquatic plant that grows in dense underwater stands and spreads through fragmentation. This species outcompetes native aquatic plants, reducing biodiversity and altering fish habitats by creating dense, impenetrable stands. It obstructs watercraft movement, disrupts recreational activities, and requires expensive management strategies to maintain waterway access and prevent further spread.
Best Time to Spray:
- Late spring to early summer (May to July) when water temperatures reach 60°F.
Herbicide Treatment:
- Use 2,4-D, Fluridone, Centrazone, triclopyr, penoxsulamor diquat.
- Systemic herbicides like 2,4-D or triclopyr for root control; diquat for fast top-kill.
Water Hyacinth (Eichhornia crassipes)
Description:
A free-floating plant that forms thick mats on the water surface, blocking sunlight and oxygen, and negatively impacting fish and other aquatic life. It also promotes sediment buildup and water stagnation. This species clogs waterways, impedes commercial navigation, reduces water quality, and incurs high costs for removal and maintenance of water flow in lakes, rivers, and irrigation canals.
Best Time to Spray:
- Late spring to early summer (May to June) when actively growing when water temperatures are above 50°F (10°C).
Herbicide Treatment:
- Use glyphosate or 2,4-D with a surfactant to penetrate the plant’s thick leaves.
- Apply on calm, still days. Repeat treatment in late summer if necessary.
- Other herbicides to consider: Diquat, Imazamox, Penoxsulam, Triclopyr or Florpyrauxifen-benzyl.
Giant Salvinia (Salvinia molesta)
Description:
A floating fern that forms dense mats, reducing oxygen levels and shading out aquatic plants, and destroys habitats for fish and other aquatic organisms. It damages fisheries, obstructs water traffic, and leads to significant management costs for clearing waterways and restoring ecosystems.
Best Time to Spray:
- Late spring to summer (May to July) when water temperatures exceed 65°F.
Herbicide Treatment:
- Use glyphosate or diquat with a surfactant.
- Re-treat as necessary to prevent regrowth, especially in warm, stagnant waters.
Curlyleaf Pondweed (Potamogeton crispus)
Description:
A submerged plant with wavy leaves that grows in early spring and forms dense mats. This plant spreads quickly in lakes and rivers, outcompeting native species and altering nutrient cycling, which can lead to algal blooms and oxygen depletion. Curlyleaf pondweed clogs irrigation systems and interferes with recreational water use, often requiring costly control and maintenance efforts.
Best Time to Spray:
- Early spring (April to May), before it reaches the water surface.
- Late summer/early fall (August-September) for systemic herbicides function effectively at peak growth.
Herbicide Treatment:
- Use endothall, fluridone, or diquat.
- Apply before the plant flowers for best control. Multiple treatments may be needed.
Brazilian Waterweed (Egeria densa)
Description:
A submerged plant that forms thick mats and interferes with water movement and recreation. These thick mats can reduce oxygen levels in the water, and result in fish kills and diminished aquatic health. They also clogs irrigation canals, rivers, and lakes, impacting water flow and navigation.
Best Time to Spray:
- Spring to early summer (April to July), when the water is 65°F to 75°F.
Herbicide Treatment:
- Use fluridone or penoxsulam for long-term control, diquat for spot treatments.
- Ensure minimal water movement for effective herbicide application.
Common Reed (Phragmites australis)
Description: A tall, emergent grass that invades wetlands and shorelines, crowding out native species. They alter water flow and sedimentation patterns, and are highly flammable, increasing the risk of fires in the wetland areas and along waterways. Common reed can obstruct drainage systems and irrigation channels, increasing maintenance costs for land managers, farmers, and municipalities. It can also invade and reduce the functionality of natural flood control areas, leading to more frequent and severe flooding.
Description:
Best Time to Spray:
- Late spring to early fall (August to October), before seed production.
- The ideal range for herbicide efficacy, between 50°F and 90°F. Avoid extreme temperatures, as they can stress the plants and reduce herbicide effectiveness.
Herbicide Treatment:
- Use glyphosate or imazapyr.
- Systemic herbicides should be applied when the plant is moving nutrients to its roots. Use a surfactant for better absorption.
General Guidelines for Aquatic Herbicide Application:
Weather Conditions: Apply herbicides on calm, wind-free days to reduce drift and ensure even coverage. Avoid rainy days to prevent herbicide wash-off.
Water Temperature: Most herbicides work best when water temperatures are between 60°F and 80°F.
Adjuvants/Surfactants: For floating or emergent plants, use a surfactant to improve herbicide penetration, especially on waxy leaves.
Multiple Treatments: Aquatic weeds often require repeated herbicide applications for full control. Regular monitoring is essential to prevent regrowth.
Aquatic-Approved Herbicides: Always use herbicides approved for aquatic use to protect non-target species and meet regulatory standards.
