Aquatic weeds, including true plants and various forms of algae, quickly disrupt a pond ecosystem. Uncontrolled growth leads to aesthetic issues and impacts recreational activities like fishing and swimming. A dense mat of vegetation can cause oxygen depletion when the weeds die and decompose, consuming dissolved oxygen and potentially leading to fish kills. Managing this overgrowth requires a systematic approach: identifying the specific growth, implementing physical or non-chemical removal, and, when necessary, applying chemical treatments safely.
Identifying the Specific Aquatic Growth
Effective control of nuisance aquatic growth begins with accurate identification, as misidentification often causes treatment failure. Aquatic weeds generally fall into four categories based on their growth habit:
- Submersed plants, such as coontail or hydrilla, are rooted in the pond bottom and grow entirely underwater.
- Emergent plants, like cattails or bulrushes, are rooted in shallow sediments, but their stems and leaves stand above the water line.
- Floating plants, like duckweed or water hyacinth, either float freely or are rooted with leaves floating on the surface.
- Algae are non-rooted organisms, ranging from microscopic planktonic types to large, stringy, filamentous mats.
For accurate species identification, especially with invasive varieties, consultation with a local university extension office or a professional pond management service is highly recommended.
Non-Chemical and Physical Removal Methods
Mechanical Removal
Mechanical removal involves physically extracting the weeds using specialized aquatic rakes, cutters, or mechanized harvesters. This method immediately removes the biomass and the nutrients contained within the plant material. However, it is labor-intensive and must be done carefully to avoid breaking plants into fragments that can regrow.
Environmental Manipulation
Environmental manipulation techniques reduce the sunlight available to the plants. Applying non-toxic, aquatic-safe dyes to the water column is a common method that filters out the light spectrum necessary for submersed plant photosynthesis. This shading is most effective in deeper water and must be maintained as the dye degrades. Another physical method involves covering sections of the pond bottom with an opaque material, like a specialized benthic barrier, to completely block sunlight and kill rooted plants.
Biological Control
Biological control involves introducing organisms that naturally consume the nuisance vegetation. The triploid grass carp, a sterile fish species, is a common biological agent used to graze on submersed aquatic weeds. The use of these fish is highly regulated, and permits are often required to ensure only the sterile variety is introduced. This method offers a long-term, low-maintenance control option, but requires selecting the correct stocking rate based on the pond size and the specific weed species present.
Choosing and Safely Applying Aquatic Herbicides
Application Safety and Dosage
When non-chemical methods are insufficient, aquatic herbicides offer an effective solution, but require careful planning. The first step involves accurately calculating the pond’s volume to determine the precise dosage of the chosen chemical. Applying too little may be ineffective, while over-application can harm non-target organisms. All herbicides must be registered for aquatic use and applied strictly according to the product label instructions, including wearing appropriate personal protective equipment (PPE).
Types of Herbicides
Aquatic herbicides are generally categorized as either contact or systemic. Contact herbicides act quickly to kill the plant tissue they touch, making them suitable for widespread algae and some floating or submersed weeds. Systemic herbicides are absorbed and translocated throughout the plant structure, offering a slower but more thorough kill, often targeting the roots of perennial plants. Copper-based compounds are frequently used as algaecides to control filamentous and planktonic algae, but application rates must be carefully monitored, especially in soft water, to prevent toxicity to fish.
Mitigating Oxygen Crash
A significant danger following large-scale chemical treatment is the risk of an “oxygen crash.” When a large mass of weeds or algae dies rapidly, decomposition consumes a massive amount of dissolved oxygen. This can lead to a fish kill within days. To mitigate this risk, treat only small sections of the pond—no more than one-third of the total infestation—at a time. This allows for gradual die-off and oxygen recovery before treating the next section. Pond owners should also check with local and state authorities for any required permits, as aquatic herbicide use is frequently regulated.
Long-Term Management and Nutrient Control
Sustained pond health requires managing the underlying cause of weed growth: an excess of nutrients, specifically phosphorus and nitrogen. Killing existing weeds only addresses the symptom; preventing new growth requires limiting the nutrient load in the water.
Aeration and Circulation
Aeration systems, such as diffused air systems or decorative fountains, are effective preventative tools. These devices circulate the water and increase the dissolved oxygen concentration. This helps break down organic material and limits the release of phosphorus from the bottom sediments.
Reducing External Inputs
Reducing external nutrient inputs is a crucial long-term strategy. This involves installing vegetated buffer strips around the pond’s edge to filter runoff from lawns or fields, intercepting sediment and nutrients before they enter the water. Within the pond, beneficial bacteria products can be introduced to aid in the breakdown of organic muck—the accumulation of dead plants and leaves. These specialized bacteria consume the organic sludge, locking up nutrients and making them less available to fuel aquatic plant growth.