A different kind of duck: all about duckweed

Ducks tend to be very rare in gardens. I realise that I’ll now get a flood of letters from people telling me that they DO get ducks, but those gardens are the exception, not the rule. It generally takes a big garden containing a very big pond (or a lake just over the fence) for ducks to be a regular feature; as an example, my 15-metre long pond has yet to attract a single duck in four years.

What many of us do get in gardens, however, is duckweed – those floating plants that individually look like a tiny water-lily leaves but en masse can quickly carpet the surface of a pond.

Before we explore how to tackle an infestation of duckweed, let’s take a closer look at the plant itself.

The first thing to say is that there are actually six different species of duckweed in the UK, and what makes them so unusual is that each plant has just one leaf, technically called a ‘frond’. One species, the Rootless Duckweed, is rather rare but is notable in being our smallest flowering plant, with its frond only about 1mm across.

There is then the Common Duckweed, the invasive non-native Least Duckweed which is spreading quickly, the Fat Duckweed, and the largest of them all, the Greater Duckweed, which can have fronds up to 1cm across. The final member of the family is the rather different Ivy-leaved Duckweed which has fronds which cohere into groups, and which usually lives under the water surface rather than on it.

So how do duckweeds replicate so fast? It seems as if they must divide again and again like amoeba, so rapidly do they proliferate.

Actually, that analogy isn’t so far from the truth. Each duckweed frond has an area called the meristem which produces new little cloned plants which break off from the parent, without any need for sexual reproduction. It is called vegetative budding, and means that one plant can become two, four, eight… very quickly over a matter of days. No wonder it spreads with such speed.

Our native duckweeds are a natural part of pond habitats, so it may be that you choose to keep it. However, they can produce such a thick, dense mat of fronds that they exclude almost all sunlight from the pond, and that can lead to pondweed beneath it dying off, and a reduction in the pond creatures that like to see the light.

So how can you tackle duckweed? Well, there are two useful techniques. One is to use a fine-mesh net. I find the most effective are those which are square-ended rather than rounded, because there is a larger edge to skim across the surface.

And here's how the pond looks after just one minute of netting.

The other technique, which can be used instead or in combination with a net, is to take a long baton of wood with a straight edge and draw it across the water surface. This skimming pulls masses of duckweed towards you, where you can then grab handfuls (or net it out). (Obviously, net or skim in a safe position – I don’t want my postbag also filling up with stories of those who have toppled in while duckweeding!).

See how just one skim across the surface (above) clears huge areas of much of the duckweed.

Put any duckweed you remove on the edge of the pond for a day or so for there are bound to be pond mini-creatures caught up in it; they can then crawl back into the water. I find that you have to be especially careful in spring and summer not to skim out tadpoles, so go carefully. The duckweed can then go onto the compost heap.

Just a couple of minutes’ skimming each week will keep duckweed levels acceptable. Or you can really go for it and put in concerted effort over a month or so to net out every piece you see - it requires great determination and diligence but can be done, especially in smaller ponds. Your choice.

The added benefit of spending this time 'duckweeding' is that you'll probably get even more intimate with your pond and all the wonderful wildlife treasures within it.

PS Since posting this, my good friends John and Shena Maskell have been in touch to remind me of the 'ice sheet' technique, which is indeed another great way to remove duckweed. After the first frosts, you can lift panes of ice out with whole slabs of duckweed fronds stuck within. Finger numbing but rewarding!