Lettuce plants are grown on polystyrene sheets that are floating in the hydroponic tanks. "Dow® Board," also known as "blue board", is used. The sheets are 2 lb. density, closed cell, extruded and unscored. The sheets are 4' x 8' x 1.5". Seventy-two (72) sheets are needed to fill the hydroponic tank growing area. The sheets are painted on one side with white, non-toxic, roof paint. Paint keeps the sheets from deteriorating from UV light. White paint is coolest and best for plants. Non-toxic roof paint is available here in the USVI because we catch water from our roofs and hold in cisterns for washing and bath use. A lead-free latex paint may be used but other toxic compounds from the paint may leach into the system water and contaminate or kill the fish.
The polystyrene sheets float on the water in the hydroponic tank. The 4' width of the sheet fits loosely in the tank. Twelve sheets fit into the tank length (12 times 8' equals 96').
The polystyrene sheets have a pattern of holes drilled through them. A drill and hole saw are used to make 1.75" holes for the lettuce seedling transplants. Hole spacing is determined by the variety of lettuce grown. Summer bibb - 88 plants per sheet, Crisphead - 60 per sheet, Romaine - 48 per sheet. Net pots, available from hydroponic suppliers, are seated into each hole. Each net pot holds one lettuce transplant.
Lettuce is harvested weekly. Lettuce seedlings are transplanted into one-quarter of the system each week (18 polystyrene sheets). These plants grow to maturity in four weeks and are harvested. New seedlings are transplanted into the harvested area. In our system each hydroponic tank holds 12 sheets. One-and-a-half tanks are harvested each week. Sheets in the un-harvested half of the tank are pushed to the end of the tank where harvest will take place the following week. In systems with wider tanks holding 3 sheets across only 6 rows would be harvested each week. The remaining 6 x 3 sheet rows would be moved to the harvest end for harvest the next week.
Insect control