Duckweed grows on benign neglect in the wild. However, to cultivate it in a garden setting, duckweed does have a few definite needs if it is to thrive consistently.
1. Like any vascular plant, duckweed needs a minimum of nitrogen, phosphorus, potash and micro nutrients. Sources can be as simple as a little humus and/or soil or compost tea. To achieve high protein levels of 40% or greater, duckweed needs added nitrogen, preferably in the form of ammonia from animal waste. This is why it thrives so well in fish aquariums.
Nitrogen sources can include fish wastewater, chicken coop drainage, some types of grey water, vermiculture liquor, or aged manure. If a solid, place in a gunny sack and lower into the water column. This releases a steady amount of nitrogen and trace elements for a couple of weeks. Replace if you see duckweed roots grow to an inch or longer. Some people will occasionally spray the duckweed with an organic-based foliar spray as another source of nutrients.
2. Duckweed thrives at a pH of 6.0 to 7.5 If algae is present in large quantities, it can raise the pH to dangerous levels by virtue of CO2 production at night. The trick is to monitor pH, especially if algae is present. Treat pH extremes as described in my prior post. Encourage an adequate surface covering of duckweed at all times to suppress algae production.
3. Harvest duckweed as needed but leave about 1 1/2 to 2 pounds of wet duckweed per square meter on the surface of a pond daily. This insures enough of a cover to slow down algae growth or suppress it altogether. As an added benefit, this much covering also helps with mosquito control, water evaporation and temperature issues. More than 2 1/2 pounds of wet duckweed per square meter will result in its demise, as it self mulches at that concentration.
4. Duckweed prefers water temperatures of 50 to 90 degrees. Above or below that range, duckweed just sort of sits there. Much above 90 degrees, your duckweed crop will crash and it won’t be pretty. Ways to circumvent this is light shading from surrounding trees, plants or shade cloth hung over the growing area.
5. Keep water movement to a minimum. As duckweed floats on the surface, any strong wind will push it to the edge of the pond where it will begin to pile up in layers, effectively self-mulching the layers beneath. Grow taller food crops around the perimeter to shield it from the wind.
Duckweed is an amazing crop that gives much more than it gets, but still needs a few basic “gotta haves” in order to reach its full potential in a garden setting.