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. The plan is to reduce the duckweed to the point at which the ducks and fish can eat all the new growth andkeep it under control.

And here is the cleared pond. The green in the centre is the reflection of a bush in the clear water!

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Surplus pond weeds as a mulch 

Lots of people say blanket and duck weed should be put on the compost heap. Just turned over a year old, well looked after heap, which has been banked up with earth and has plenty of soft material in it, so ideal composting conditions. We discovered none of the pond weeds had rotted. Not such a good idea after all.

However, we’ve increasingly used the pond weeds as mulches to keep the roots of plants damp and to wipe out weeds, and especially the reeds that are trying to conquer the edges of the pond again. So the pond is producing a useful product after all.

Blanket and duck weed – the fight goes on

Last year’s big effort to clear blanket weed by physically removing it has paid off, so far. It has been surfacing again in clumps this spring but not nearly as much, so an hour’s work in waders was enough to clear it. Every bit that comes out removes nutrient from the pond, because – looking on the bright side – you could regard blanketweed growth and removal as a way of sucking up excess nitrates and phosphates and putting the pond in better balance. There is still plenty of it close to the bottom but I suspect that the combined effect of the clouds of daphnaea and the three grass carp – all algae eaters – may be keeping it under control.

Since this is the breeding season for many creatures we pulled the weed out in small clumps and then laid it by the water’s edge so they could escape. We seem to have large numbers of common newts, more than we have seen before, and we made sure they didn’t get trapped. They obviously like it here.

Only very small quantities of duckweed so far.

Dealing with rampaging duckweed

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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.

Harvested duckweed

Harvested duckweed

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.

Sweet smell of success

Great improvement in the pond this year – no smell when the mud is disturbed by walking in it, instead of the unpleasant smell it gave off for the first two years. Discovered the good news when planting a new red water lily, wearing waders.

I wonder why: a better ecological balance now the pond has had time to settle down? Something to do with the fish, now rather large, that escaped from next door? Whatever the explanation, very pleasing to have a sweet smelling pond.

Getting rid of blanket weed

We hate blanket weed. It makes a green blanket (of course) on the surface, and creates slimy bubbles around itself. Light is obscured for the creatures and plants below and given half a chance it takes over a whole pond.

There are all sorts of remedies for it, including ridiculously expensive chemical treatments and the simple, old fashioned use of bales of barley straw, which are sold at extortionate prices in garden centres but can be found cheaply for sale in the countryside, if you look. But barley straw has to be left floating for months to rot, and is unsightly, so we’ve stuck to a simple old-fashioned method, which is to haul it out and put it on the compost heap – or rather, with a pond the size of ours, make a compost heap with it.

We ignore the advice we read that breaking it up makes it spread faster, because that just doesn’t happen. What we do is wait until the surface bloom appear in large quantities in the spring and then remove it all, piling it around the pond for a day or two so creatures can escape and then wheelbarrowing it to the compost heap.

A pile of blanket weed ready to be taken to the compost heap

A pile of blanket weed ready to be taken to the compost heap

We have experimented with several ways of getting it out. The most effective is to get into the pond, grab a chunk of it with your hands, and spin it round slowly, keeping it under water: it will draw in strands from all around until you have a lump the size of a football and you can throw it on the bank. If you don’t like handling it, then use a stick to twirl it. The main thing is to do it gently to avoid breaking the strands. But in an infested pond it is possible to extract a large amount in an hour or two.

If you don’t feel like using waders, a really wide rake can extract a lot of blanket weed. We have a 48 tooth nylon grass rake by Spear and Jackson, which is £23 on the web and advertised as self cleaning. We took off the handle it came with and replaced it with a 12 foot pole cut from a hedgerow, so it reaches right out into the pond. It is also useful for raking out fallen leaves in the autumn.

The blanket weed doesn’t disappear. We find, however, that after removing the first surface bloom in the spring it doesn’t grow back to the surface, and remains through the summer as a bottom-growing algae. Our theory is that once it is trimmed, daphnaea and other creatures that feed on algae take over and keep it under control. Removal is best done as soon as the bloom appears on the surface, while frogs are mating, and before their spawn floats into view.