Duckweeds are nuisance plants or useful plants, depending on your point of view.
In places where they’re not welcome, duckweeds live up to the ‘weed’ part of their names. They can be invasive and almost impossible to eradicate.
But we don’t have to hate these tenacious little plants. My fellow blogger Tamra Fakhoorian at Duckweed Gardening urges us to support the International Lemna Association. The Food and Agriculture Organization says that duckweeds have ‘enormous potential for agriculture and environment’. Open Source Ecology says that duckweeds have ‘tremendous potential for cleaning up pollution, combating global warming and feeding the world’. Dr Landesman the Duckweed Crusader says that ‘duckweed may be the most promising plant for the twenty-first century’.
Here in Britain, the common duckweed (Lemna minor) floats on the surface of quiet, nutrient-rich water. It’s often to be found in ponds, ditches and canals. It can cover a water surface within weeks and it’s here to stay. The Biological Records Centre (BRC) says that L. minor is native, that it’s found throughout most of the British Isles and that its distribution hasn’t changed since BRC records began in the early 1960s. So there’s at least one plant whose range isn’t known to have changed in response to our changing climate.
In fact, particularly in the South and East, we don’t find only L. minor. My fellow blogger Jeremy Biggs at The Garden Pond says that we can find five native duckweeds and two introduced duckweeds.
These are our native species:
* the common duckweed (L. minor)
* the thick, fat or gibbous duckweed (L. gibba)
* the star or ivy-leaved duckweed (L. trisulca)
* the greater duckweed (Spirodela polyrhiza)
* the rootless duckweed (Wollfia arrhiza)
The Royal Horticultural Society says that British ponds may contain wild or naturalised invasive duckweeds: L. minor, L. gibba and L. minuta. The Centre for Ecology and Hydrology describes those three species and L. trisulca. L. trisulca grows submerged for most of its life cycle and it’s rarely invasive.
The Wildlife Trusts say that L. minor belongs in British wetlands. It provides food for waterfowl, and shelter for other species including spawning amphibians. But sometimes, L. minor and other duckweeds can become a nuisance.
If you have a duckweed problem you can remove the plants with a net, rake or floating boom but they’ll grow back quickly. You can shade the water with trees, shrubs or waterlilies (Nymphaea spp.) You can encourage or introduce waterfowl, or fish such as the grass carp (Ctenopharygodon idella). They’ll eat duckweed but their muck will enrich the water, leading to more duckweed growth.
You can use Ecopond’s Duckweed Control which contains ‘micro-organisms specifically selected for their ability to remove nutrients that support the growth of duckweed in pond water.’ Which microorganisms? I suppose that information would be a trade secret. You can bring in a contractor to use a herbicide such as Monsanto’s Roundup Pro Biactive. That’s a preparation of glyphosate, the world’s most popular weedkiller.
But above all, you can make the duckweeds go hungry. These plants like nutrient-rich water. So the best way to slow their growth is to keep excess plant nutrients out of the water. Don’t overfeed your fish or your ducks, don’t over-fertilise your fields or your garden. It’s a good idea to use greywater on your land but don’t put it directly into your pond.
That shows us a reason to value duckweeds: they tell us about the water. Just as a lush stand of the common or stinging nettle (Urtica dioica) indicates nutrient-rich soil, a lush bloom of duckweed indicates nutrient-rich water. The Water Habitats Trust calls this ‘nutrient pollution’ (scroll down to the bottom of page 1). When water becomes very nutrient-rich, that’s eutrophication. Eutrophication can cause duckweed bloom.
That leads to another reason to value duckweeds. They like nutrient-rich water so much that they pull out the excess nutrients. In other words, they can be used to clean up. Dr Landesman the Duckweed Crusader tells us about Lemna wastewater treatment.
Duckweeds are productive, too. Dennis and Daniele McClung at The Garden Pool use duckweeds as part of their small farm. Duckweeds can be grown on a larger scale as animal feed. Brandon Keim at Wired tells us how duckweed farming is being developed in the States.
I’ll finish by restating my gratitude to Tamra Fakhoorian at Duckweed Gardening. Tamra got me thinking about duckweeds as useful plants. I wanted to show you how that’s being put into practice here in Britain, but so far I’ve seen nothing about duckweed farming over here. Why not? Do let me know if you see anything.