Stapeley Water Gardens

Although a UVC based filter system is the best control option, there are others, plus there is a type of algae not controlled by UVCs - blanket weed!

What is Blanket Weed and why might it become a problem in my Pond?

Blanket weed is a generic name for any and all of the many types of filimentious algae. Sometimes it has the appearance of fine wispy hair growing off the pool sides, other times it forms a thick, choking, candy floss type mass that can choke plants or float at the water surface.
As with green water causing algae, blanket weed thrives in full sun and nutrient rich, preferably alkaline water.
Blanket weed isn't harmful in itself, in fact it will actually improve the water quality for the fish in the same way as other plants. It is, however, unsightly and in some cases can choke plants. Green water can be more of a problem as the billions of single-celled algae can alter the water's chemistry, firstly by reducing the water's dissolved oxygen content during the hours of darkness (as can blanket weed and so-called oxygenating plants when growing in excess). Secondly, this fluctuating input and output of oxygen and carbon dioxide between day and night can, in some cases, also result in a daily fluctuation of the water's pH value, with sometimes dangerously high pH readings occurring in the afternoon.

How to prevent blanket weed and Green Water

1. we said it before, and we will say it again - plants, plants and more plants. 50 - 70% surface coverage from floating plants and lily leaves can make a lot of difference, and don't forget to establish the recommended quantities of submerged plants, which will compete with the algae for nutrients.

2. excess fish stocks will encourage the build-up of waste nutrients such as ammonium, nitrate and phosphate, so don't over stock especially in the first few seasons whilst plants are establishing.

3. ensure that lawn or plant feed isn't washing into the pool when it rains.

Prevention is better than cure, but algae treatments still have their place. Here is a round up of the types of product available.

1. Aquatic Herbicides - added directly to the pond water, these chemicals directly poison all types of algae. They are amongst the most effective products due to their strength, but this is also their draw back, for if they are overdosed, or certain plants (essentially floating plants, water lilies, and submerged aquatics) are struggling then these could also be affected.

Strongly recommended in unplanted ponds or features, where they are good value and may only need to be used once or twice a season. In stubborn cases they can even be used at double strength and will have no direct effect on fish or wildlife.

2. Algicides - in simple terms these are similar to herbicides - just weaker. Most algicides are totally safe with pond plants or fish as long as they are used as directed. There are many formulations on the market, ranging from the good to the indifferent, but few tend to work as well as herbicides and they usually need re-dosing more frequently. That said, you may feel happier using them in a planted pond.

3. Settling Agents - some of these products are sold appropriately as products to settle ponds that are muddied or hazy with suspended debris or clay particles. Others are sold as green water controllers but all they do is settle the algae out - unlike algicides they don't continue to prevent algae.

4. Dyes - some products are available which work by dyeing the water blue on the basis that this will reduce sunlight penetration. Safe, cheap and to some people, attractive, but it is poor as a treatment and is really only recommended as a green water preventative rather than a cure.

5. Water Improvers - these are products that don't kill the algae directly but aim to improve the water conditions, and it is this reduction in nutrient levels which will often prevent or control green water or blanket weed. To get the best out of these products they need to be used on a regular basis, and tend to work better where there is also good surface coverage. The big appeal for most people though is that most of these products are seen as a natural control, as they simply boost the pond's natural level of beneficial bacteria and enzymes. Not only do these products improve the water conditions for the fish but some of them also reduce sludge build-up.

6. Barley Straw - not the wonder product some people hint it is as it usually only works about 50% of the time, but when it does, its big advantages are its long term effectiveness (about 4 months) and its great value. One warning however, it is possible to do more harm than good if you use a very excessive amount at any one time, where as the usage on prepacked pond barley straw is totally safe when used as directed.

7. Daphnia - these 'water fleas', which are sold as live food for aquarium or pond fish, when added to a fish free pond will multiply and are then excellent at creating and maintaining clear water.

Blanket Weed Only - Controlling Products

1. Algae magnets - available in two forms, these products aim to alter how certain salts present in water interact with the blanket weed. They also reduce the build-up of calcium carbonate (limescale) on UV quartz sleeves, pipe work, and pump components, reducing maintenance. The most basic type uses permanent magnets in a tough housing which simply connects, via stepped hose tails, to your flexible pipe work. Success is variable but they are well priced and do reduce limescale build-up. The better system needs a mains supply and is connected to existing flexible pipe work by wrapping two special cables tightly around the pipe. This system is more expensive but has a very good success rate, and as the price of the units has fallen dramatically in recent years they are set become as useful as the UVCs.

2. Grass Carp - these Chinese fish can be the way to control blanket weed but it really does depend on the pond. Due to their size, 45 - 90cm (18 - 36 inches) in time, they are only for larger pools, especially as they are very apt to jump. They can consume good quantities of blanket weed, but they will also eat ordinary fish food and possibly certain other soft-leaved plants, which all reduces their uptake of blanket weed! They won't solve the problem overnight, especially as they are usually only available to buy at smaller sizes 5 - I5 cm (2 - 6 inches), plus you will need to experiment to find the right quantity for your pool.

Other Hints and Tips

1. most books will say that a lot of new pools will turn green but not to worry as once the plants are established the water will clear. Whilst this can occur, what can also happen is that the plants don't establish as a result of a lack of light due to the green water and choking blanket weed. Work on the basis that it is easier to keep clear water clear than get green water clear. Consider not adding fish for a season and using daphnia, or, add a plant-friendly algicide at the first hint of hazy water.
2. whilst just about all algicides etc. are safe with fish and wildlife when used correctly occasional problems could occur due to the dead and dying algae, especially in terms of oxygen depletion. all that you need to do to reduce this potential problem is to remove as much blanket weed as possible before treatment, and consider carrying out a 30 - 40% water change every day for a couple of days before using a green water treatment (just don't forget to use a de-chlorinating product to condition the new water). Not only will this reduce the problem of dead algae but most treatments will also work more successfully at preventing the growth of algae.