Floating hydroponic gardens are ideal for growing leafy green plants such as lettuce, kale and mustard greens, and herbs such as basil, mint and watercress. Other crops can also be grown, but larger plants need a thicker foam layer. The floating hydroponic system, called the raft system, consists of a tank or container of nutrient solution with the plants floating on the surface. The root ball suspends above the solution to allow access to air. Some of the roots grow down into the nutrient solution. It is critical that the roots have exposure to both air and water.
Obtain a large shallow container to use as the nutrient basin. A plastic wading pool, plastic tub, or plastic-lined frame works well. Build a rectangular frame from 2-by-6 inch lumber and line with with 6-mil or thicker plastic sheeting.
Cut a piece of 1 1/2 inch thick foam insulation to fit the container. The foam should fit the container while allowing enough room for it to float freely on the nutrient solution. Larger containers may require several sheets of foam fitted together.
Fill the nutrient basin with water, measuring the volume as you fill. Check for leaks and be sure the foam floats freely on the surface.
Cut holes in the foam by using a hole saw or sharp knife. For 3-inch net pots, use a 2 1/2-inch hole saw. If you are making your own net cups, cut holes that will suspend the bottom of the cup level with the bottom of the foam sheet. Space the holes approximately 6 inches from the edge of the foam and approximately 8 to 12 inches apart. Float the cut foam on the nutrient solution.
Add nutrients to the water according to the directions on your nutrient solution, or add fertilizer at a rate of 2 tsp. of fertilizer per gallon of water. Use a balanced water-soluble fertilizer, such as 20-20-20, that contains micronutrients. Also, add 1 tsp. of magnesium sulfate per gallon of water when using fertilizer. Mix the nutrients well.
Suspend commercial or homemade net pots in the foam so that the bottom of the cup touches the water, but is not submerged. Make your own net pots from foam or plastic cups. Cut small slits or holes in the bottom of the cup to allow water in and the roots to grow out.
Germinate seeds in a soil-less media such as compressed peat pellets. Place the fully rooted transplant and its growing media into the net pot. The root ball sits loosely in the cup, so do not pack it in with additional growing media. The roots need to be exposed to air.
Replenish the nutrient solution, as needed, to keep at least 5 inches of nutrient solution in the tank. Completely change the nutrient solution between crops.