Notes from Windward: #67


Harvesting Duckweed

getting started with a key crop

     A key part of the research work we're doing involves finding ways to adapt crops used in warmer climates to our elevation and latitude--a key example is the remarkable aquatic plant known as "duckweed." Under the right conditions, this fairly primitive plant as the remarkable ability to double in volume in about three days--conditions which we only enjoy for part of the year. So part of what we're working on involves over-wintering our duckweed so that it's ready to take off and produce when the weather warms up in the spring.


     This summer we scoured the local waterways looking for a native strand of duckweed, but came up empty. We found all sorts of aquatic plants, and the ducks certainly were happy to consume them, but no duckweed. A few weeks back we were able to find a strain of duckweed that is used in tropical fish tanks to purify the water and feed fish, and were able to get a colony of it started in an aquarium localed in one of the kitchen's garden windows.


     What we're working with is a miniture form of duckweed since the larger strains tend to contain oxalic acid which lessens their utility as an animal feed, but this tiny strain is growing fine. Now that it's formed a floating cover all across the tank, we seined about half of it and transferred that to the barrelponics tank for the goldfish to enjoy. Now we'll watch to see how long it takes for the remain duckweed to recover the top of the tank.


     Duckweed isn't a perfect food, but studies have shown that access to fresh duckweed can cut the cost of feeding ducks in half. In order to grow aggressively, duckweed needs nutrient rich water such as that given off by concentrations of food fish, which ties in with aquaponics in that it allows for a rapid expansion of total system biomass when that's needed to balance the bio-loading created as fish grow to market weight.

     Our aquaponic goal is to produce lettuces and other greens for the kitchen, but those plants take time to germinate and grow to a size where they will have a notable nutrient uptake--with duckweed you can get a rapid increase in total uptake by just allowing the duckweed to keep growing, or conversely after harvesting a significant amount of fish, you can reduce the bio-load by harvesting as much duckweed as needed in order to maintain nutrient levels.

     It's this rapid response capacity that makes duckweed a key component in managing a sustainable aquatic system, but there's more in that certain strains of duckweed can be grown for the kitchen as described in United States Patent 5,269,819.

Notes From Windward - Index - Vol. 67