Dealing with weed in a carp lake

By Andrew, the owner of Molyneux lake;

Weed is not particularly desirable in large amounts in a carp lake for 2 reasons – it’s difficult for anglers to deal with when fishing and it can have a negative effect on the eco-system.

Phytoplankton and weed are in direct competition with each other in a lake, they both photosynthesize and require the same nutrients to grow. Weed is a trophic dead end as far as carp are concerned, the can’t eat it and although they can eat the animals that feed on the weed, these are limited in numbers and difficult for the carp to find when compared to the animals that eat phytoplankton.

Phytoplankton will add more oxygen to the lake than weed will, so all things considered it’s more beneficial to reduce the amount of weed in a lake and increase the amount of phytoplankton.

The weed in my lake came out of nowhere! I had no weed growth for 2 years and then suddenly one summer it appeared. In the last two years I’ve completely stopped the weed growth in my lake and the weights of the carp have rocketed upwards. It’s been hard work but certainly worthwhile.

So if you want to reduce the amount of weed in your lake what options do you have?

Mechanical removal

A very hard and time consuming job but does reap instant results… it doesn’t seem to have a lasting effect though. I bought a very expensive electric cutter, it looks like a hedge clipper mounted on the end of a pole that clips onto the back of a boat and is very effective if you can get afloat. Its best done on a windy day as the cut weed floats and will drift into the side where it needs to be collected as you don’t want it to rot in the lake.

A large rake does a very good job and is a lot cheaper. I made mine out of a couple of wrought iron gate posts with a number of long bolts driven though it. It needs to be taken out with a boat as it weighs around 20kg and it needs 2 people to pull it back due to the weight of weed that collects on the rake. The good thing with a rake is that it removes the weed from the lake as it cuts. The hardest method but incredibly effective!

I’ve personally not tried this method put was told if you attach a length of chain to the bank and then row the other end out into the lake and winch it in in an arc that it will cut the weed just above the roots and the weed will float free of the chain.


Most of the effective weed killers have been banned in France, although they can still be found elsewhere but if you get caught using them the fines can be very harsh. Using chemicals is not something I would recommend as you can cause serious oxygen problems in the lake… it’s certainly something for the experts.


There are a lot of studies that have conflicting results of how effective Grass carp are in controlling weed growth, I’ve found them to be incredibly effective! Some people worry that Grass carp will compete with the carp for food and therefore slow the carp’s growth rates down, however I’ve read studies that show the opposite. The Grass carp eat the weed that for the carp is a tropic dead end, the Grass carp will then defecate into the lake, this is almost perfect fertilizer for the phytoplankton. As the amount of phytoplankton increases so does the zooplankton and benthic organisms, these are then fed on directly by carp.

If anglers want to avoid catching Grass carp then by fishing tight to features rather than open water and not feeding particles they can be almost avoided.


3 thoughts on “Dealing with weed in a carp lake

  1. John says:

    Whilst excess weed is not desirable, the “perfect fertilizer for the phytoplankton” you mention is also the ideal food for algea, which if a lake has to excess can be far more detrimental than weed, and far more difficult to control.
    Carp lake life is not easy!

  2. Andrew says:

    Hi John

    The phyotoplankton should easily be able to out compeat the algae, particularly as with grass carp the nutrients aren’t created in one big hit but in slowly increasing levels over a few months untill the amount of weed starts to decrease due to being eaten and ‘shaded out’ by the phyotoplankton. Thats why Koi keepers can struggle with algae, as they don’t want phyotoplankton but crystal clear water insteed. It’s usually the death of phyotoplankton that allow an algae bloom as there is suddenly nothing to compeat with the algae and a large excess of nutrients that allows algae growth. I know that some people suggest stocking silver carp if you stock a large amount of grass carp, as they will eat algae, but it’s probably best not to stock too higher level of grass carp in the first place and give them two seasons to do the job. I should have added to the article that combining two methods with give better results than using just one.

    Controlling algae is hard, silver carp can help as can barley straw but it’s best not too have the algae in the first place. You need to be very careful when altering an eco-system as if you push too hard it can cause problems, it’s best to do it in carefully thought though little stages.

  3. Mehdi says:

    Very good article!
    I had weed growing more and more every year. I decided to introduce small grass carp in a winter. The result is amazing. The weed quantity visibly reduces the first spring and then the second season , most of the weed was gone ! 2 things are important : the size of the grass carp, small are better because they eat mostly weed but then over 10kilos they start to eat other food like carp. So about 1kg is perfect. Second point, grass carp doesn’t eat all the weed species. It works well on myriophil and some potamots. They eat a few of elodee but not all.
    The last good point on grass carp is you catch them ! chemical or mechanical is not so effective in long term , much more expensive and you can’t fish for them ! lol

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