How to Solve Your Blanketweed or String Algae Problem in Your Pond?

IMPORTANT NOTE ...

This article was written before I discovered John McClauchlan's Viresco solution to blanketweed control. If you know you've got this dreaded pond curse and can't be bothered to read the article go straight to the solution here ... Viresco Aqua will eliminate your problem quickly too. The product can be sent straight from the manufacturer in the UK to any place in the world and the price is the same worldwide including delivery.

This is the only solution I feel confident in recommending although Bradshaws help desk in York seem to have other modern powerful blanketweed solutions too.

What's it all About?

I would sill nevertheless suggest that you read this article to get a good understanding of the prolem that for many ponds has become a serious issue.

The information below was sent to me by John Kinross of the School Of Life Sciences, Napier University, Edinburgh and John has given me permission to publish it. John is an authority on pond algae as you will see by visiting his site. It was in response to a question I posed asking for the latest information on possible blanket weed or blanket weed solutions for ponds. John is without doubt an authority on all matters to do with algae as you will quickly gather by visiting his site. This is John Kinross's reply ..

Blanket weed is a real evil and nuisance for many pond keepers

I don't have problems in my own pond, but that's partly a question of perception: I'm quite happy to have algae to look at, which is probably not a feeling shared by most! Also, I don't have fish, and I think that may be where much of the problem lies. (Tony comments ... no doubt about this since nutrient levels favouring formation of blanket weed are created by feeding pond fish)

Two of my colleagues here have had problems with blanket weed in their fish ponds this last summer (which has been exceptionally hot and dry for us), and we've also had a lot of algal growth accumulating in rivers, which is quite unusual in our climate. I think in both cases the cause can be put down to an excess (whatever that may be) of nutrients, either excreted by the fish, or in the case of the rivers, from agricultural sources and sewage treatment plants.

I was interested to read about your use of the UV 'filter' for clearing the algae in ponds: My sister has recently had one of these installed in a newly refurbished pond, and it seems to be doing the trick. However my feeling is that this may not stop the build-up of blanket weed, since you are not removing nutrients from the system. In addition blanket weed does not pass through the UV so cannot be impacted upon by the UV radiation. John is perfectly corect here.

The question is, how can this be achieved?

The major nutrients favouring blanket weed or string algae growth are nitrogen, as ammonium or nitrate, and phosphorus, as orthophosphate, and plants require these in the molar ratio of 16:1 N:P.

Typically, the cause of eutrophication in fresh waters is put down to an excess of phosphorus due to human activities (sewage discharge, use of detergents, etc.), since there is usually an ample supply of nitrogen. (This may not always be the case in shallow waters, however). The treatment of eutrophication in lakes is seen therefore as controlling the inputs of phosphorus, and this may also involve trying to control the release of phosphate from the sediments, eg by dredging, or by chemical treatment to precipitate it.

In a pond, dredging out sediments which have accumulated over several years may be possible, but unless inputs are controlled also it is unlikely to do a lot of good. If you are feeding fish in the pond, there will be a constant input of nutrients. The only chemical process I can think of which would not harm fish or other aquatic life is to pass the water over some kind of material which will adsorb it:

A student here did a trial of the use of maerl ('calcified seaweed') in sewage treatment and found it very successful, but any form of mineral, especially if it contains limestone, which offers up a large surface area would do the job. Shells might be a good source.

My own preference would be to try to make use of the plants themselves to remove both nitrogen and phosphorus, by continuously harvesting them. (this is an approach favoured by many water gardeners and makes sense in the fight to reduce blanket weed infestation)

This approach is being tried by a number of people around the world for polishing of treated sewage effluents, eg in the US and Australia, and is usually referred to as 'Algal scrubbing'. I've no direct experience of this myself, other than some experiments I did using artificial streams some years ago, which showed me that a good lawn of blanket weed could strip the nutrients (especially nitrogen) from a growth medium in a few days.

The relevance to a real pond situation would depend on the size of the pond, nutrient levels, and the area colonised by the algae, but I could envisage a series of small troughs with a good flow of water through them, and some rigid plastic netting suspended in the flow. A good turbulent flow, with air injected in at the head of each trough would give the best results. The nets could be removed in rotation and scraped down to remove the growth. Such a system could be put downstream of your UV filter, if there was enough flow.

It might be necessary then to have the nets colonised in the pond first for a week or so before placing them in the UV sterilised water. Of course, a shell filter could be used simultaneously.

These are my suggestions; if we have more hot summers here I may get a chance to try them out if I can talk my colleagues around to it, but if you think it worth trying, I'd love to hear what success you have.

Best wishes,

John

PS ... comments by Tony Roocroft

The 2 suggestions by John seem well worth trying to me since they make very good technical sense and should therefore improve if not resolve most blanket weed or blanket weed problems. Should anyone try either of the two (or both) please let me know of your results so that others can benefit. An UVC has NO effect upon string algae or blanketweed

Here's a view from Peter J May

Blanketweed in ponds (string algae) infestation

Dear Peter J. May: I have been searching for a cure for my pond problem. We have this horrible green, slimy, stringy algae on our pond. At first other pond experts around the area told us it was from the heat or sun. Then over the winter it continued.

I clean it out every day, and every day it comes back. It seems to choke out my plants after a while. We have plenty of plants that is supposed to help with that. We also have clean clear water except for the green stringy stuff. I hate it. It looks so awful and it is annoying to have to clean it out every day. We have also tried barley pads, balls, you name it. Do you have any suggestions that will help with this problem? I am getting desperate!

Thank you ... Mary Young

Hello,

The person that invents a cheap cure for blanket weed infestation will become a millionaire over night. There is always some algae in natural water and some algae prefer certain conditions over and above others. You will notice that now you have blanket weed, at least you haven't got any other algae problems! I heard of a man in Birmingham back in the 60's that sold it to keep ponds clear!

Anyway that doesn't help you does it? The conditions favourable to blanket weed:

It loves limey water, high pH. Run off from concrete slabs, cement, soil, the lime in tap water even. If you have to top up the pool a lot, it just encourages it.

Topping up with tap water also feeds it the nitrates and the phosphates it seems to relish. Phosphates also come from out of the soil.

Blanket weed can grow at much lower temperatures than most of the competing plants in the pool and so it gets a head start on all of them.

Control: First and foremost, hook out as much as you can with a rake before any other treatment. Persistently doing this, you will find that the rest of the plant life in the pond restricts its abundance. If not ensure that you have plenty of oxygenators in the pool and lilies for cover.

The pool needs to have two thirds cover by the height of the season otherwise the algae get a look in. Otherwise you need a biological filter. If you get one of these, then using this in conjunction with a u/v and a an ecological algicide that breaks up the threads, it MAY keep the problem in order.

Dont over feed the fish and make sure that as few tree leaves as possible get in the pool keep the pool well oxygenated to encourage the right bacteria.

There are some phosphate treatments available on the market that are meant to be very good, but in general they are not effective unless the water is below pH 8. They are not cheap at 15 minimum for a single treatment. Although Laguna do a sachet of volcanic rock that you float in the pool for a bit less Beware of standard algicides as these can make the problem worse, since the decaying matter that is dead blanket weed makes the perfect fertiliser for new growth.

Hope this helps you get on top of it.

Pete

PS Snails love it too.

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