Water is great in a garden and whether you have a large pond, a formal pool, a miniature pond in a half barrel or a self-contained fountain or other water feature, the chances are that from time to time you will have been frustrated by the fact that the water turns green: the so-called ‘pea-soup’ effect! Generally, this will be due to one of many different types of algae infestation, and this problem tends to get much worse during warmer weather. Sometimes it completely ruins the appearance of the water feature and on other occasions it is just a passing phase, but what should you do about it?
Blanket weed is so called because the long filaments of algae tangle up together, making a mass that resembles a dense green blanket. The easiest way to remove this once it has taken hold is to simply pull or rake it out. Over large areas a rake is worthwhile, providing you can avoid bringing the pond plants with it. Alternatively, use a stick or bamboo cane that you twirl into the mass of algae (rather as you would load spaghetti onto a fork) and then compost it down in your compost bin.
Surface floating pond weeds such as duckweed, which looks like lots of very tiny oval leaves floating on the water’s surface, can be a real nightmare too. Sometimes it is impossible to work out how it got there, but I think it is often brought in on new plants or simply on the feet of birds or other creatures as they come to the pond to drink. It builds up very rapidly, soon forming a bright greenlayer on the surface and the best control for this is to regularly scrape the weed off using a good-sized kitchen sieve. Again it can be composted, but do make sure that you remove every scrap of it from your pond or it will soon be back.
Where possible, try to avoid the build-up of algae by creating your pond carefully and taking precautions with what you grow and keep in it. Avoid making a new pond in a very sunny position, as lots of sunlight increases the algal growth and makes the problem worse. Bear in mind, however, that a pond situated too close to a large tree will soon become clogged by leaves over the autumn months! You should also make sure that the pond is as deep as possible: shallow water is much more inclined to develop algal problems than a pond which has deeper water. If you want fish that’s fine, but avoid putting too many fish in a pond as their excreta raises the nitrogen level in the water and this encourages algae. It really helps if you grow plenty of surface floating plants – water lilies are a wonderful solution, as they help to shade the water’s surface and so are particularly useful if the pond is in a fairly sunny site.
I’d also suggest that with a new or existing pond, you go down to your local garden centre or pond plant supplier and ask for a selection of oxygenating plants. Some of these float in bundles in the water, others can be grown in containers, and all of them help to raise the level of oxygen in the pond and so decrease the problem with algae.
Installing something which moves the water around, such as a spout or fountain, will help to reduce the amount of algae because it also oxygenates the water, but do bear in mind that water lilies do not like water that is in motion.It is important not to give up on a pond too quickly – if you keep changing the water in an attempt to keep it clear, the pond never gets the chances to create its own natural balance and the situation will not get better. Try to be patient and usually, provided you try all the above, the pond will eventually sort itself out and you can enjoy seeing clear water again.
You can consider using various proprietary physical and chemical controls for algae, but always make sure that the one you have chosen is suitable for your size and type of pond and that it can be used safely where wildlife or your pond fish are living.
The much-loved method of using barley straw to clear algae in ponds does work, but it is essential that you have straw taken from barley and no other cereal crop. If you can get hold of this, cram some of it into something like an old pair of tights and submerge in the water by weighing them down with several bricks. Alternatively, you can buy pads of barley straw, in some instances mixed with lavender stalks, and these will help to do the job for you.
by Pippa greenwood
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