Tackling duckweed menace – again 

After a year with hardly any duckweed (don’t ask me why) it’s back with a vengeance.

 There are 8 ducks apparently hoovering it up for hours every day, but neither they nor the grass carp, which also feed on duckweed, make much difference when there’s this much on the pond. In the right conditions, according to the books, duckweed can double in 3 days.

We clear it by the very simple method of putting on waders, getting into the pond, and pushing a long 6 inch plank held on edge through the water, so the weed piles up in front of it.

Enough of the plank is submerged to trap the weed in large quantities.

imageWe push it to the bank and then scoop the thick accumulation of duckweed out with a grass rake, onto a large plastic sheet, allowing the water to drain back into the pond along with lots of tiny creatures. Without the sheet, the grass verge turns into a bog.

imageThe duckweed doesn’t rot easily into compost so rather than put it on our heap we use it as a mulch or put it on poor, clayey ground so that in the long term it will work into the soil.

imageIt takes three or so hours to clear the pond completely by repeating the process.

The work is best done on a day with a gentle breeze that moves the duckweed towards you. Working on the downwind side of the pond, a cleared area appears after each sweep of the plank.

imageThis quickly fills again with wind-blown duckweed.image

The other end of the pond clears as the weed moves downwind.

Without wind, the duckweed tends to spread back over the pond again but in a much thinner layer – because there is less of it – which makes sweeping less productive. So if the wind drops, best to take a break till it comes back again. It’s much easier to rest and let the wind bring the weed to you than to chase it round the pond.

Below, the ducks come back to feed on some of what’s left – as a name, duckweed is exactly right.image

This method does of course depend on having a pond you can walk in but many ponds have thick layers of mud that make that very difficult. Ours was like that till we desilted it. So early attempts to clear duckweed used three planks tied loosely together with a rope at the end of each of the two outer planks. With a person each side of the pond holding one of the ropes, the whole apparatus could be pulled  down the pond, sweeping the duckweed along. It worked up to a point, though not nearly as efficiently as getting in with waders.

Either way, the objective is to remove as much as possible of the duckweed so that fish and ducks will control what’s left for the rest of the summer. It is impossible to get rid of all of it. There seems to be a critical area of coverage below which the duckweed grows no faster than than it is consumed by the pond’s inhabitants.

Duckweed survives the winter by changing to a dormant form that sinks to the bottom, rising again the following year. So by removing as much as possible this year, next year’s growth gets off to a slow start.

And below is the pond this week with most of the duckweed cleared. That splash of green near the centre  of the photo is a reflection of a bush in the clear water! Now we just have to keep our fingers crossed that the ducks and the fish eat what’s left around the edges faster than it grows. (PS 20 August: duckweed completely gone).




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