How to grow hydroponic lettuce
Leafy green lettuces may be grown in a lettuce raft, also known as a deep water culture system. In a lettuce raft, plants float on a Styrofoam raft -hence the name- in a container filled with a nutrient-rich solution. The roots of the plants then grow downward into the solution, creating a consistent supply of nutrients for the plants. Hydroponic lettuce is really just lettuce grown in a hydroponic system, and almost any type of lettuce may be grown. Buttercrunch, bibb lettuce and Romaine, along with other greens such as spinach and chard adapt well to this hydroponic system.
Start the seedlings
Start the lettuce seeds of your choice using rockwool cubes. The cubes have a small indentation in the center. Place three seeds in the indentation of each rockwool cube. Seed at least six cubes, as the basic design described here can hold up to six plants. Place the rockwool cubes in a shallow container and mist the cubes to maintain a low level of moisture.
When the seeds have germinated, add a small amount of water, approximately 1/8 inch deep, to the bottom of the shallow container, enough to allow the rockwool cubes to «wick up» the water. Maintain this low level of moisture, keeping the cubes slightly moist, but not wet, until the seedlings have reached approximately 2 inches in height. Add a few drops of liquid nutrient solution to the water, allowing the cubes to soak up the solution.
Prepare the lettuce raft
The lettuce raft consists of a deep plastic container, a section of Styrofoam, tubing, an air stone and a pump. Cut a section of Styrofoam that measure slightly smaller than the width and length of the container. This is the raft and it needs to float on top of the nutrient enriched solution that fills the container.
Use a hole saw to cut six holes in the Styrofoam, making two rows of three holes. A three inch net pot is the best size pot to use for the seedlings, s use a hole saw bit slightly smaller than 3 inches to make the holes so the lips of the net pots sit snugly in the holes.
Fill the container with water and add the nutrients according to the manufacturer’s directions. Place the air stone into the solution-filled container and use the tubing to connect it to the regulator. The regulator prevents the solution from backing up into the pump.
Connect the regulator, via tubing, to the pump. Plug in the pump and test to ensure the water bubbles. The air stone oxygenates the water. With the lettuce raft built, you can now add your lettuce seedlings to the system.
Add the seedlings to the raft
Fill a 3 inch net pot halfway with clay pellets and position one of the seedlings, keeping the seedling in the rockwool cube, in the net pot. Fill in the remaining space with clay pellets to fill in the net pot around the seedling. The clay pellets are an inert material, that is, they do not effect the nutritional value of the solution in the lettuce raft. The pellets also help regulate the moisture content of the rockwool cubes. Place the Stryofoam raft in the container. It should float on top of the water.
Place the net pot with the seedling into one of the holes in the Styrofoam raft. Continue this process until all the seedlings are in the raft. If you have fewer than six seedlings, place net pots with clay pellets in the remaining holes to maintain balance and prevent the solution from bubbling out of the holes.
Maintain the system and harvest regularly
With the seedlings placed in the raft, and the raft positioned in a sunny area, either inside or outdoors, the plants have a steady supply of water, air and nutrients. Monitor the system to ensure the pump and air stone are working properly. Without the oxygenation process, the plants will die.
Renew the nutrient solution every two to three weeks. Lift the raft out of the container and place on two upended bricks. Unplug the pump and take the air stone out of the container. Set aside. Empty the container and fill with prepared nutrient solution. Place the air stone back into the container and plug in the pump. Place the raft back into the container.
Monitor the plants by checking for root growth. Lift the raft up and check that roots are growing from out of the net pots. The roots should be white to creamy white in color, slightly thick and pointing downward.
Harvest the outer leaves of the lettuces regularly as the plants mature. This encourages new growth and provides you with a steady supply of greens.
You can also use a lettuce raft to grow herbs such as basil, oregano, rosemary, parsley and thyme.