How do I prevent blanketweed from growing in my pond?

getting rid of blanket weed

Blanketweed is a form of algae, and is one of the most regular problems we are asked about.  Over the years,The Pond Guys have developed a few options to successfully deal with this challenge.  Blanketweed, along with any nuisance algae, is best tackled with a multi-pronged approach to successfully keep it under control.

Why does blanketweed grow so prolifically?

 For any plant to grow, they require three factors – water, light and food.

In a pond, it goes without saying that it provides an abundance of water!  It also, by nature of normally being outside, will receive a lot of light.  There are of course minor variations between winter and summer, and also different ponds will have varying amounts of shade, but in essence they mostly receive plenty of light.  Food, in the form of nutrients, is the final major element that nuisance algae (or indeed any plant) needs to grow.

Nutrients are the only element we can have any real control over.  The Pond Guys always tackle blanketweed with both good pond husbandry, which controls the incoming level of nutrients, and a water additive to limit the algae’s growth.

What makes up the algae’s food?

Food for the plants is in the form of nutrients.  Many different elements make up these nutrients, but the two we concentrate on are nitrate and phosphate.  These are both measurable by a standard pond test kit set and controlling them always reduces algae growth.

Nitrate is the last stage of the nitrogen cycle, the biological process that happens in any aquatic system (whether it be a pond or aquarium).  Nitrate is considered relatively safe to your pond in lower doses, and in a perfect world, we would carry out small water changes every month with nitrate free water, and keep the nitrate level diluted and under control.   The problem is that nowadays nitrate can often be found in high levels in tap water (one of the results of fertilisers applied to crops in fields which then get into our water system).  Depending on where you live, water changes using tap water are therefore not as effective as we would like.

Phosphate is the other nutrient which is within our control.  It comes into our ponds from four main sources, all of which we can control to some extent ; soil in planting baskets, fish food, tap water and run off.

Planting Basket Soil in your pond is a first source of phosphate, as it contains low amounts of phosphate.  The actual quantity can be quite variable, and garden compost (sometimes recommended in garden books) can have quite high quantities within it.  This phosphate will leach out of the soil over time straight into the pond water.  ACTION – Using a good quality aquatic soil, (or just gravel if the plants you are growing are low in requirements for nutrient) helps to reducing the availability of phosphate from this source.

Fish food.  Phosphorous is put into fish food as part of the diet, being an essential element for fish health.  When you feed your fish, some of the food will inevitably not get eaten and processed by the fish, and therefore it again leaches into the pond water.  ACTION – use a good quality fish food, and do not overfeed.  Feed the fish in small quantities, and make sure all the food is eaten within 3 minutes and not left floating around the pond for a long time afterwards.

Tap Water is also a major cause of phosphate.  It is another additive in fertilisers for crop production and gets into our water supply the same way as nitrate, meaning water changes also add phosphate!  ACTION – This one is a bit trickier to deal with, but with a small pond you can add rainwater to your pond rather than tap water.  Alternatively, add a phosphate absorbing filter media cannister to your hosepipe when topping up your pond, which will dramatically lower the phosphate added to your pond.

Run-off water can also be a cause of phosphate levels accumulating.  Run-off water first washes over your garden, and as it travels the phosphate leaches into the water from the surrounding soil (especially if you add fertilisers to your garden or lawn).    Note, this is only a cause where run off water can enter the pond – a raised pond, or one with a solid barrier around the edge will not suffer from this. ACTION – ensure any water from your garden is directed away from entering the pond.

What else can we do?

As part of a healthy pond, it is always advisable to have a reasonable amount of plants within the pond.  All the different types of plants – marginals, oxygenators, deep-water marginals and water lilies will all also compete for the nutrients in the pond, and will often be successful in using the nutrients over the algae.  Therefore, if you have a reasonable quantity of plants growing in your pond, this will help to keep everything balanced and algae under control.

Good maintenance is also helpful in controlling algae.  The less waste and silt there is at the bottom of the pond, the less there is to break down through the nitrogen cycle and end up as nitrate.  We recommend maintaining your pond at least 10 times a year, ensuring the filters are kept clean, the pond vacuumed, and water changed (obviously refilling with as low nitrate water as possible).  Also ensure the quantity of fish does not get too many for your pond!  Goldfish can breed very well in garden ponds, and as a result if they are very successful you end up with too many fish for the pond, meaning you will add a lot more food resulting in higher levels of phosphate leaching into the water.

It is also becoming more commonplace to use phosphate absorbing media within a filtration system, and then replacing this regularly as it becomes full.  Although this is not a natural process, it can be quite effective as long as the pond is not too large, otherwise it becomes costly!

What about the treatment?

The final line of defence is to dose the pond with an additive to prevent blanket weed.  We here at The Pond Guys only use one treatment – AQUATIC WEED CONTROL PRO – which we have over the years ‘played around’ with, and we now have the dosage and treatment mixed to ensure a good level of success. We have found that by varying the amount and frequency of the dose, depending on each pond’s specific requirements treatment with AQUATIC WEED CONTROL PRO is very successful at combatting the growth of the algae.  We use it regularly in all the ponds that suffer with blanketweed that we maintain, with excellent results.  If you would like to know more, or obtain this treatment, please do give us a ring, with details of your pond, and we will be happy to help tailor a treatment package for you.

If you have any questions about the information above, or would like to discuss your battle with algae, then do give us a ring!  We love to talk ‘fish’ and help people with their ponds and would be more than happy to help you successfully fight your algae problem, or any other pond related challenge you face!!

The Pond Guys are a pond installation and maintenance business based in Surrey, with teams covering London and the South East.

We are specialists in all types of ponds from ‘state of the art’ Koi ponds through to tranquil water gardens and water features.

Do get in contact for any help and advice, or indeed if you need help with any aspect of your pond or feature.