Aquatic plants are vital to the health of New Zealand’s ponds, lakes, and dams, but invasive aquatic weeds can threaten water quality, biodiversity, and aesthetics. Understanding what plants you have, how weeds spread, and the best control methods will help you maintain a balanced, vibrant water ecosystem.
Types of Aquatic Plants & Weeds
Aquatic plants are grouped broadly by their growth habits and where they live in a waterbody. Some are benign or even beneficial, others are invasive and can cause serious problems.
| Category | Description | Examples |
|---|---|---|
| Emergent plants | Grow rooted in shallow areas; part of the stem & foliage rise above the waterline | Species that spread over wet ground or shallow margins |
| Free-floating plants | Float on the surface without anchoring in sediment | Duckweed, watermeal and similar small species |
| Submerged plants | Grow entirely underwater, rooted or not | Hornwort, Egeria, Lagarosiphon etc. |
| Native submerged or low-impact species | Less aggressive, often part of the natural ecology | Native milfoils, native pondweed species |
Why Aquatic Weeds Become a Problem
Aquatic weeds flourish when certain conditions are met. Some of the key contributing factors include:
- High levels of nutrients (especially nitrogen and phosphorus), often from runoff or decaying organic matter
- Stagnant or slow-moving water, which lets weeds settle and spread
- Accumulated sediment or sludge that acts as both substrate and nutrient source
- Lack of regular maintenance or oversight
When weeds like dense floating mats of free-floating species or thick submerged growth dominate, problems can follow: reduced oxygen, blocked sunlight, fish stress or kill, murky water, unpleasant smells, and degraded pond health overall.
Control Methods: How to Manage Aquatic Weeds
Here are effective strategies to manage weeds and maintain aquatic plant balance:
- Selective Herbicides / Chemical Treatments
- Use registered herbicides that target specific weeds. Make sure to apply correctly and safely.
- Combine with agents that help improve penetration or speed decay of dead plant matter.
- Beneficial Bacteria / Microbial Treatments
Introduce or boost populations of beneficial bacteria in your pond. These microbes digest decaying plant matter, fish waste, and organic sludge, reducing the nutrients that aquatic weeds (and algae) feed on. Products like Natural Blast® from Healthy Ponds are designed for this. Healthy Ponds
Also, Healthy Ponds suggests using beneficial bacteria regularly to maintain water clarity, reduce sludge, and reduce floating organic matter. - Physical & Mechanical Intervention
- Remove sediment or sludge to reduce the weed’s substrate and nutrient source.
- Use mechanical removal (rakes, cutting) for emergent/weedy edges.
- Improved Water Movement & Aeration
- Install aerators or fountains to improve circulation and oxygen levels.
- Circulation helps prevent stagnant zones where free-floating weeds thrive.
- Preventative Maintenance & Best Practices
- Limit nutrient inputs (runoff, excess fertilizer).
- Remove dead plant material regularly.
- Monitor weed growth early, before it becomes dense.
- Favor native or low-impact species to occupy space and out-compete weeds.
- Integrated Approach
- For best results, use a combination: herbicide or targeted removal to reset heavily weeded ponds, then follow-up with aeration, sediment reduction, and ongoing maintenance.
Long-Term Best Practices
- Always identify the species you’re dealing with before treatment
- Apply treatments when weeds are young and active for better results
- After treatment, restore balance with oxygenation & substrate/nutrient control
- Regular monitoring (seasonal) to catch new outbreaks early
Aquatic plants can support healthy, beautiful ponds but unchecked aquatic weeds degrade water quality and ecosystem health. Using a thoughtful mix of treatments, mechanical, and biological control methods can restore balance. By identifying your problem species, reducing nutrient inputs, improving water movement, and maintaining regularly, pond owners can keep aquatic weeds under control and enjoy clear, vibrant water all year.
