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I have an almost empty pond covered with, I think, duckweed. Should I fill with water or wait for rain?

Sent in by Eliane Zimmer, Hampshire

What is duckweed?

Duckweed is the common name for any of the floating plant species of the family 'Lemnaceae'. Lemnaminor being the most common variety. The individual plants consist of just a leaf or two floating on the water surface with one small root that hangs freely below. It can be found anywhere on earth in still or slow moving bodies of water except for arctic and tundra climates and can cause problems for pond management.

The problem it causes

They are adapted to grow very rapidly and when fuelled with plenty of nutrients, duckweed will dominate still water by creating a carpet that shades out any other plant competition and consumes the pond's nutrients – which could lead to ill effects on other pondlife.

In temperate regions such as the UK when temperatures consistently drop below 6 to 7 °C, it develops small, dense, starch-filled organs called 'turions', which become dormant and sink for winter. The following spring, stimulated by increased temperature, they restart growth and float back up to the surface.

A small amount is good

The main reason that duckweed is commercially available is that it provides rapid pool cover and uses up excessive nitrates, therefore inhibiting the growth of algae. This allows higher plants in the water get chance to establish and do a similar job by removing nitrates and offering cover for the newts, keeping the pond ecology in a good balance.

How to control duckweed

Perhaps the simplest and most obvious way to remove duckweed from a pond is using a skimmer net to scoop it out of the water. This is ok if you have a relatively small pond, but other methods will have to be applied to keep it in check - especially if it is a larger pond. Also, try to remove old decaying vegetation or leaf litter from the pond - this could add fertiliser to the water, adding to the duckweeds spread - by raking it up around the shore.

If it is a small pond, it's worth adding an electronic aerator to the pond as this adds oxygen and reduces the levels of nitrogen and phosphorous.

Although many garden centres may recommend ponds with fish to keep the spread in check it is best avoided as they consume amphibian eggs and spawn. 

When and how to fill your pond

You could collect rain water and use a water butt during autumn to fill the ponds and rake the excess duckweed.

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