Here are some tips and tricks for using duckweed in the classroom.
Duckweed Nutrient Solution
There are a number of different recipes for duckweed nutrient solution. Here is a very easy one for classroom use.
Use Miracle Gro (24-8-16). The important thing to check is the N-P-K ratio on the side of the box. From my experience, the one labeled 24-8-16 is the one that DOES NOT say “Nursery Select”.
The target concentration is 0.058g/L. You can achieve this by adding ¼ tsp of Miracle Gro to 5 gallons of water. For smaller amounts, you can dissolve ¼ tsp of Miracle Gro in 1 cup water, and then add 3 Tsp of that solution to 1 gallon of water. Either use distilled water or add chlorine binding drops as directed on the bottle. Some municipalities (including Lincoln, NE) use chloramine instead of chlorine gas to treat their water, so you can’t just let the water sit out, you need to use chlorine drops. You can get them at any fish supply store.
As the nutrient solution sits out, water will evaporate and the volume of solution in the cup will reduce. Just add water to refill the cups the original volume. The water is evaporating, not the nutrients. Therefore, replace it with water, not nutrients.
However, the duckweed will reduce the nutrients with time. How quickly this happens will depend on the volume nutrient solution and the quantity of duckweed. One convenient rule of thumb is to replace the nutrient solution once a week (or move the duckweed to a container with fresh nutrient solution).
You can collect duckweed from many permanent, standing, small water bodies. Urban ponds can be good place to find duckweed because of the increased nutrients found in these locations. Even in the winter, you can collect duckweed. Simply dig up some pond muck in an area where you've previously found duckweed. Let it sit inside for a few weeks, and duckweed will likely emerge from the muck.
Algae will often find itself into your duckweed cultures, especially when collecting duckweed from the wild. Maintaining stock cultures at high duckweed densities will help reduce the amount of light availlable to the algae.
Duckweed will do fine under banks of fluorescent lights set 6-12 inches from the duckweed. An easy set-up is a shop fixture supported by a PVC stand with standard cool white fluorescent bulbs. I like to get the 40W bulbs with the highest lumen output.