To weed or not to weed. That is the question…not least because what some people consider a weed, others consider a boon.
I recently helped a friend move into a cottage on 200 acres overlooking the Obi Valley in Australia's Sunshine Coast hinterland.
The dwelling sits near a spring-fed dam that sources a creek which runs into the valley below through assorted waterfalls and swimming holes.
It's an idyllic location rich in wildlife and majestic vistas. There is, however, a problem (the solution to which might also provide some unexpected benefits).
The little dam is covered in duckweed. An aquatic plant that sits on (or just below) the water's surface, duckweed is renowned as the smallest flowering plant on Earth (each flower barely 0.3mm in size). Despite this claim to fame, duckweed has the potential to be highly invasive and a threat to aquatic health. In the case of my friend's dam, the plant had spread a veil of greeny-brown growth across the entire surface of the water.
But duckweed isn't always a blight. Like many aquatic weeds, duckweed proliferates when nutrients (think animal manure) contact the water and exceed healthy levels. With the right temperatures and environment, duckweed moves in quickly to help purify the water.
"Duckweed is an extremely efficient absorber of ammonia, nitrate, phosphorous, potassium, magnesium, calcium, sodium, chlorine, boron and iron. Duckweed can remove as much as 99% of the nutrients and dissolved solids in waste-water".
In short, duckweed can (under certain conditions) help to keep water healthy.
Duckweed is also understood to offer other benefits to the natural environment. High in protein (some say as much as 35-40% protein), it offers an important food source for waterfowl (hence the name "duck" weed). Apparently, in some parts of South-East Asia, humans eat it as an alternative to soy beans. This might explain why it's sometimes referred to as the "water lentil".
Besides its role as a food source, duckweed also provides habitat for certain amphibians and insects, and in some circumstances its light-reducing capabilities may help to reduce the growth of noxious algae blooms. During times of low rainfall, duckweed can also prevent evaporation and water loss from a dam or pond, for example.
So why, if duckweed is so good, did I advise my friend to remove the weed from her dam? Why tout duckweed as a blessing one minute, and a pest the next?
Well, here in Australia, this simple aquatic plant can prove detrimental to aquatic health if left unfettered. It spreads quickly, suffocates native aquatic plants and prevents sunlight and oxygen from entering the water. It creates an environment that is unsuitable for the macro-invertebrates which so readily inform the monitoring of aquatic health (those little microscopic fellas are testimony to water quality).
Duck weed can also become home to other pests as well. This includes, for example, a parasitic mite that is rumoured to have a fetish for burrowing beneath animal (including human) skin where it feeds on subcutaneous cells. Whilst this is hearsay, the rumour cautions care and diligence in the weed's removal.
Besides restoring health to the dam and creek by my friend's home, removing the weed from the water offers other benefts. It should, for example, help to minimise the spread of the weed into the rivers and creeks in the National Park that borders her property. This in mind, we're also removing the scotch thistle and grass weeds along the riparian zone (ie. the area beside the dam and alongside the creek) to help avoid weed spread into the valley below.
Amongst all this weeding, the good news is that this humble plant, once removed and dried, can provide a host of benefits. It makes terrific mulch for the garden and is also promoted as a wonderful food for chickens (particularly if you mix it together with other scraps). It makes great fish-food as well. Indeed, once duckweed is controlled, consider stocking your dam with native fish to help manage future spread of the weed.
So how do you remove duckweed? Depending on the size of the water-body in question, there are three main options: chemical, biological and mechanical. Chemical weed control may be appropriate for large water bodies and massive infestations (whilst it has potential environmental challenges, chemical treatment is often the most cost-effective and beneficial for long-term weed management in large water-bodies).
Biological intervention relates to (amongst other things) stopping the source of the problem. So be sure to keep livestock away from dams and other still water-bodies as all too frequently their poo is the reason for the weed in the first place. As mentioned above, once the weed is under control, consider stocking your dam with fish to biologically control the spread of duckweed.
For my friend's dam, we're using the mechanical method – physically removing the weed from the water. Given the size of the water-body, it's a relatively simple task: either from the water's edge or from within a dinghy on the water, we simply rake the weed out of the dam and put it into large buckets. We then spread the weed onto an area where it can dry in the sun.
I always wear gloves when working outdoors and aquatic weed removal is no exception – especially if there might be nasty parasites in the mix! Long-sleeve shirt, gum-boots, long cotton drill pants, long (elbow length) gloves, a long-handled steel rake, a big bucket, and a couple of hours in the dinghy and – voila! – the job is done.
The only question my friend needs to answer now is whether she wants to grow and harvest duckweed – in a sustainable and integrated way – to generate yet more mulch for the garden, fuel for the compost, food for the chooks or freshwater fish. Doubtless, there are other uses for duckweed too. The trick, however, is to remember that it is a weed. And that means managing the plant, rather than letting it run wild.