Weeds Near Your Pond or Water Feature?
Most weed killers are toxic to aquatic life. Using the wrong product near water can kill fish, frogs, newts and the entire pond ecosystem. Here’s how to manage weeds safely near water.
Why Weed Killers and Water Don’t Mix
Most herbicides are classified as toxic or very toxic to aquatic organisms. When weed killer enters a pond — whether through spray drift, surface runoff, or contaminated soil drainage — the effects can be catastrophic:
- Direct toxicity: Herbicides like glyphosate (and especially its surfactant additives) are toxic to fish, amphibians and aquatic invertebrates even at low concentrations
- Oxygen depletion: If aquatic plants are killed by herbicide contamination, their decomposition consumes dissolved oxygen. This can suffocate fish and other aquatic life
- Algal blooms: Some herbicides disrupt the natural balance of pond life, triggering algae overgrowth that further depletes oxygen
- Amphibian sensitivity: Frogs, newts and toads are exceptionally sensitive to chemical contamination. They absorb substances directly through their permeable skin
In the UK, it is a legal offence under the Environmental Protection Act to allow pesticides to contaminate watercourses. This applies to garden ponds, streams, ditches and land drains as well as rivers and lakes. See our wildlife and water safety guide for the full legal picture.
Buffer Zones: How Far Is Safe?
The critical point: standard garden glyphosate products (Roundup, etc.) are NOT approved for use near water. The surfactants in standard formulations are highly toxic to aquatic organisms even though the glyphosate itself is less harmful. Only specifically labelled “aquatic” glyphosate formulations can be used near water, and these are typically only available to professional users.
What About Garden Ponds?
Garden ponds deserve the same protection as natural watercourses. In many ways, they’re more vulnerable — a small pond has less volume to dilute contaminants, meaning even tiny amounts of herbicide can reach harmful concentrations. Frogs, toads and smooth newts are protected species under the Wildlife and Countryside Act 1981, and great crested newts have additional EU-level protection.
Practical rules for gardens with ponds:
- Never spray within 5 metres of a garden pond with any standard herbicide product
- Never spray on windy days — drift can carry herbicide much further than you expect
- Never spray uphill from a pond — surface water runoff during rain will carry residues downhill into the water
- Check the rain forecast — heavy rain within 48 hours of spraying can wash herbicide residues into nearby water
- Use spot treatment, not broadcast spraying — a paintbrush application of glyphosate directly onto weed leaves eliminates drift risk entirely
Safe Alternatives Near Water
For planted borders near ponds, dense ground cover planting is the best long-term strategy. Plants like creeping jenny, marsh marigold and water mint thrive in moist soil near pond margins and crowd out unwanted weeds while supporting wildlife including bees and pollinators.
Managing Aquatic Weeds Inside the Pond
Weeds growing inside the pond itself — blanketweed, duckweed, Canadian pondweed — require completely different treatment from terrestrial weeds. Never use any land-based herbicide in pond water.
- Blanketweed: Remove by hand using a stick or rake to twirl and lift it out. Leave it on the bank overnight so any trapped creatures can return to the water, then compost
- Duckweed: Scoop off the surface with a fine net or aquarium mesh. Complete removal is difficult — manage rather than eliminate
- Excess submerged weed: Thin by hand, removing no more than a third at a time to avoid oxygen crashes. Autumn is the best time for major clear-outs
- Barley straw: A traditional remedy for blanketweed. As it decomposes, it releases compounds that inhibit algae growth. Available as pads, bales or liquid extract from garden centres
What If Weed Killer Has Entered Your Pond?
If you suspect herbicide contamination:
- Partial water change: Replace 20-30% of the pond water with fresh dechlorinated water to dilute the contamination
- Increase aeration: Run a fountain or air pump to boost dissolved oxygen levels — herbicide-killed plant matter decomposing will consume oxygen
- Activated carbon: Placing activated carbon in the pond filter or hanging a carbon bag in the water can absorb some herbicide residues
- Monitor fish and wildlife: Watch for signs of distress (gasping at surface, unusual lethargy, pale coloration in fish)
- Remove dying plant matter: Dead and dying aquatic plants should be removed before they decompose and further deplete oxygen
Prevention is always easier than cure. If you have a garden pond, adjust your weed management approach in the surrounding area to prioritise non-chemical methods. The pet-safe options discussed in our safety guide also tend to be the safest choices near water.
Frequently Asked Questions
Can I use Roundup near my garden pond?
Standard Roundup (and similar consumer glyphosate products) should not be used within 5 metres of any water body including garden ponds. The surfactants in standard formulations are toxic to aquatic life. There are professional-grade aquatic-approved glyphosate formulations, but these aren’t typically available to home gardeners.
What’s the safest weed killer to use near a pond?
Pelargonic acid is the least harmful chemical option near water because it breaks down rapidly and has no soil activity. But even this should not be sprayed directly into or over water. For the area immediately around a pond, manual weeding is the only truly safe approach.
How long after spraying weed killer is it safe for my pond?
This depends on the product, application rate, distance from water, and weather conditions. In general, allow at least 48 hours of dry weather after spraying before any risk of washoff. If you’ve sprayed well outside the buffer zone (5m+) and there’s no rain, the risk is low. If you’re concerned, wait for a full week before allowing any water movement near the treated area.
Will weed killer on my lawn harm pond fish?
If your lawn drains towards the pond and it rains heavily after application, herbicide residues could potentially wash into the water. Selective lawn herbicides containing MCPA or mecoprop-P are toxic to aquatic organisms. Avoid treating the section of lawn closest to the pond, or treat it on a day with no rain forecast for at least 48 hours.
Are there any weed killers approved for use in water?
In the UK, there are a small number of professional herbicide products approved for use in or near water (typically glyphosate formulations without harmful surfactants). These require professional application and are not available for home garden use. For garden ponds, manual removal is the recommended approach for aquatic weeds.
Need Help With Garden Weeds?
Our weed killers range includes products for safe garden use. Always read the label for buffer zone requirements near water, and use manual methods within 5 metres of ponds and streams.
