Aquatic Weed Control

Aquatic Weed Control



'Aquatic use' means spraying in, on or near water. Watercourses can be defined as 'open' (large lakes, coastal areas) or 'closed' (rivers, streams, ponds, etc.) - check the product label before spraying.


Due to the way these herbicides work, the best time to spray most aquatic weeds is in late August-September when the plants have reached their peak and are beginning to store food in their roots/ rhizomes. Please see The Environment Agency literature: SC120008 - Aquatic and Riparian Plant Management, for a good guide on when to treat each particular weed.


Since the loss of approval of dichlobenil, aquatic weed control has become a major challenge. The availability of products for this type of spraying is limited to those containing the active ingredient glyphosate. Glyphosate products approved for aquatic situations (such as Roundup ProActiveRoundup ProVantage Gallup Biograde) will only work on pond weeds if the weed leaf is dry or above water. 

Different weeds in the aquatic environment represent different threats & challenges. Some, like Australian Swamp Stonecrop (Crassula helmsii), Curlyleaf pondweed (Potamogeton crispus) & Canadian Pondweed (Elodea canadensis) are invasive thugs, taking over and choking waterways. They raise water levels and overpower much of the native flora & fauna in the aquatic ecosystem. Other common weeds, such as Bulrushes or Common Reed (Phragmites Australis) grow tall, in large clumps that narrow waterways and affect boating access or angling sports.

However, if emerged leaf is evident, or indeed, the water level could be reduced to expose more of the plant, then Roundup ProActive / Roundup ProVantage  sprays can be extremely effective to tackle the invasive growth. Glyphosate works by entry into the plant leaf - it then translocates to the growing points and kill the plant shoots & roots.


The Department of Environment, Food and Rural Affairs (DEFRA) enforces strict rules on the use of herbicides in aquatic areas. Using any herbicide product in/ near a watercourse requires Environment Agency approval; contact your local EA office before work begins. A form is available online to fill in to seek approval for spraying pesticides in watercourses: AQHerb01.


Roundup Aquatic Weed Killer

Roundup ProActive / Roundup ProVantage are professional strength glyphosate products, known to many, which provide excellent aquatic weed control. They control both emergent and floating weeds, such as those detailed above and on the product label:

Emergent weeds – bulrush, common reed, creeping bent, reed canary grass, reed sweet grass, sedges, soft rush, watercress and whorl grass floating weeds – white water lily, yellow water lily.

When Roundup ProActive / Roundup ProVantage is used as directed, there is little risk to spray operators, wildlife or the environment. It may be used in waters stocked with fish (if used in accordance with the recommendations shown on the product label). Use a knapsack sprayer or weed wiper to apply.


Floating or emergent pond weeds will require an adjuvant with glyphosate to help it ‘stick’. TopFilm, (a natural biosponge called Biocar made from maize) may improve control of species where herbicide wash-off is a problem, e.g.: parrots feather (myriophyllum aqauticum), fringed water-lily (potamogeton natans), floating pennywort (hydrocotyle ranunculoides), water primroses, Canadian waterweed (elodea canadensis), Australian swamp stonecrop, fringed water lily (nymphoides peltata).

TopFilm is an approved adjuvant for use in water.


When are aquatic weeds a problem?

They are usually an issue during the warmer months of the year when the water temperature can reach above 6°C. The warmer temperatures allow the weeds to grow more rapidly.

Why are aquatic weeds a problem?

There are multiple reasons the weeds can be an issue, from covering the water surface and cutting the light to other plants, to smothering the more desirable water plants with their sheer size

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