Google “how to with deal duckweed” and the search produces answers from around the world, as pond owners from Florida to Australia and from Cornwall to Cumberland struggle to contain the green monster. It is just like doing a search on blanket weed, which can take over a pond in the late spring with an unsightly mass of bubbling green sludge floating on the surface.
Last year we had virtually no duckweed, but this year’s warm weather has encouraged it to grow at a ferocious rate so that after a 10 day absence we returned to find a bright green pond with no water visible beneath the smooth carpet of weed. The long term solution is to try for a better balance of nutrients in the water but the short term treatment is simple – rake it out.
We have two ways of doing that. The simplest is to use a 3ft wide Ransome’s self-cleaning grass rake with an extra long (10 foot) pole cut from a sapling in the hedge row. This only works after a good breeze has piled up the duckweed at one end of the pond, and when that happens the rake can retrieve very large quantities,with the wind pushing more and more weed into the gaps produced by raking. The rake is plastic and very light and can be held easily at the level of the water surface. The duckweed is piled on the bank and left overnight for creatures to escape. However, the grass underneath suffered so now we throw the compost onto an old tarpaulin and then move it to a compost heap or use it for a mulch.
The other method we use when the pond is completely covered: put on waders and use a large plank (a scaffolding board works well) as a boom to push the duckweed to the edge of the pond, where it piles up in a deep mass and can be spooned out with a smaller plastic grass rake. The plank is floated on edge, so it goes deep, and the weed can’t escape from behind.
It took about two hours to largely clear our 100 foot by 50 foot pond. The solid green layer with which we started is actually very thin so the heap of raked-out weed was quite small for the size of pond.
This isn’t a long term solution: the weed can double in a few days so it will be back. So far this year we have had two big raking sessions from the bank and one with a plank and waders in the water. But we’d rather do the work and have an attractive pond to look at. More light also gets into the water and other plants have a better chance without the weed.
The other reason the duckweed will be back is that it has a very successful survival mechanism. Each tiny plant winters by sinking to the bottom, lying dormant till the following year when it rises to the surface and starts reproducing again.
Being interventionists, the pond gets a good raking in the late autumn to remove at least some of the rotting weeds and sticks on the bottom. A byproduct of this is that it also pulls out a lot of old blanket weed from the bottom and perhaps that will also reduce the amount of dormant duckweed waiting for next year’s warmth to revive it.
Mechanical removal also works with blanket weed – see earlier posts – but is a similarly short term solution. However, longer term methods for are even more unsatisfactory. A bacterial control advertised for duckweed sounds risky. Proprietary chemicals for blanket weed are ridiculously expensive for a large pond and we suspect them on principle. The old solution for blanket weed of floating bales of barley straw is unsightly once they blacken and rot, and a pond our size would need several. Anyway, cleaning weed from a pond can be a pleasant and relaxing exercise and it produces very quick results, even if they are not permanent.
Postscript: Four days later, the remaining duckweed had almost disappeared. I had been expecting it to start multiplying again in the warm weather, as it had after this summer’s previous rakings. The likeliest explanation is that something is at last eating it faster than it grows, and that the most recent clearance efforts got it below a critical mass. The grass carp have been noticeably active and the ducks, six of them, have returned after an absence of quite a few weeks; both eat duckweed. See this useful Royal Horticultural Society link.