Easy Lettuce Growing with a Raft Aquaponic System

by Dr. Chris Kline

Easy Lettuce Growing With a Raft Aquaponic Garden

If you love salads and have always wanted an easy way to grow them at home this article is for you. If you have been put off starting your own system of hydroponics at home because you think it’s too complicated, this article will offer you an easy method for getting started with simple hydroponics while producing some great salad greens. A homemade hydroponics system can be very simple and perhaps the simplest system is what is called a water culture raft system. This is made even simpler by incorporating aquaponics in the raft system because expensive hydroponic nutrient solutions and monitoring are not needed. Aquaponics is the combination of re-circulating aquaculture (fish farming) and hydroponics (growing plants without soil). The fish waste provides the nutrients the plants need and the plants, in taking up these nutrients, help to purify the water in which the fish live. This is an organic process with the only input to the system being fish food which creates a sustainable ecosystem where both plants and fish can thrive.

This aquaponics raft system is particularly successful for growing leafy crops like lettuce, herbs, chives, spinach, arugula etc. because of the high nitrogen content in the water. The raft system is ideal for these small plants that do not need a great deal of physical support. These leafy crops also do well with the lower phosphorous and potassium of the aquaponics system. The majority of hydroponic lettuces marketed commercially are bibb or European butter crunch types. This is often marketed as “Hydro-Bibb” lettuce. It is a good idea to start with these types and move on to other gourmet greens later. Generally you can start harvesting from 40-50 days after planting so it is best to plant each week to stagger your harvests. One of the best things about growing your lettuce inside with this method is that the average home temperatures below 78F are ideal for growing most lettuce year round.

What about the Fish?

Tilapia are generally the fish of choice for larger aquaponic systems because they are fast growing, can tolerate a wide variety of water quality conditions and have firm white meat. For small hobby systems, koi or gold fish work well. It is a good idea to also include a plecostomus cat fish to help keep the tank clean. Start with approximately one inch of fish for each gallon of water in the tank.

Materials for the System

Tank

Virtually any clean container that can hold ten or more gallons of water will work, but for esthetics and function, fish aquariums work best for most small systems. Fortunately, these are readily available used by looking in the Bird and Fish classified ad section of any major newspaper.

Under Gravel Filter and Pump(s)

These can be built, but it is easier to buy them and generally they come with used tanks. The filter needs a means of pulling water through the gravel to promote bio-filtration. This can either be done with an air pump and air stones, or with devices known as “Power Heads” that have an active water pump that draws water up the riser tubes of the filter. The best setup is to have the power heads for the filter and then supplement the dissolved oxygen in the tank with an air pump and air stones. Air hose tubing will be needed as well as a two way gang valve.

The Raft

For the raft we will use a sheet of Styrofoam about one inch thick that can be purchased at any hardware store and easily cut with a razor to fit the size needed.

Aquarium Gravel

Don’t skimp on this one. By buying aquarium gravel rather than using some other type of gravel you will be sure that it will not leach into your water and impact the pH. Buy enough to cover the under gravel filter with two inches of gravel.

Growing Medium

For this system, we only need a small amount of seed starting medium to get the plants started. This can be Rockwool, Oasis Cubes or Sure to Grow media. Some lettuce seed needs a cold period to help them germinate. After soaking your medium, add one or two seeds to each cube and place in the refrigerator for 48 hours.

pH Test Kit

This system is very easy to maintain, but starting with proper pH and maintaining it is the key to success. pH test kits are available for as low as five dollars and can be purchased at hardware and pool stores.

Siphon Cleaner

This inexpensive tool is available at all stores that sell aquariums and is indispensable for monthly tank maintenance.

Light Source

If the tank is placed near a sunny window external light may not be needed, but lights often come with used or new tanks and they can be used by changing the bulb to one appropriate for plants. This may not be adequate if the system is relying entirely on artificial light. Twenty to fifty watts per square foot is a good rule of thumb for grow lights if used exclusively. A small stand can easily be made of PCV pipe from which to hang the light fixture. Artificial light is generally provided to lettuce for 16 hours each day.

Setting up the System!

Tank Setup

The tank will be set up just like any aquarium. Place the under-gravel filter in the tank with the riser tubes installed, add gravel and then water. Connect air stones to tubing from the air pump and insert them in to the riser tubes from the filter or insert and plug in Power Heads. Once water has been added to the tank, the pH will need to be checked and adjusted as needed. If the system was just for raising fish it would be best to try to maintain the water at a pH of about 7.4. In combining fish culture with growing plants (aquaponics), the needs of both the plants and fish must be slightly compromised by maintaining a pH as of about 6.8. More often than not the pH of tap water needs to be lowered. Products for raising and lowering pH that are safe for both plants and fish are available at both hydroponics and fish stores and online retailers. If fish are to be added immediately, the water must be de-chlorinated, otherwise wait 48 hours before adding the fish. To start your system, add approximately one inch of fish for each gallon of water.

Raft Setup and Planting

The raft can be as large as you like as long as it can freely float on top of the water and allow room for feeding fish, filter tubes etc. If the tank is large it is better to cut several smaller sections of raft so they can easily be moved for tank maintenance. Starting 3 inches from the side, holes can be cut every 3 inches for planting tubes. Cut holes slightly smaller than ½ inch so that you will have a snug fit for the planting tubes then cut ½ inch PCV pipe sections of 4 inches each for each planting tube. Fill each planting tube by packing it snuggly with the chosen growing medium. The growing medium will serve to support the roots and to wick water up to the germinating seeds. Once seeds have been cooled for 48 hours they can be placed along with their medium cube, into the top of each tube. Next insert each tube into the raft with 2 inches of the tube below the raft protruding into the water. Depending on the type of fish in the system, they will trim any roots that grow out of the tube. Having a 4 inch tube allows space for sufficient roots to grow unmolested by the fish.

Maintenance

The good news is that this system is very easy to maintain. Each week, check the pH. If the pH is between 6.6 and 7.0 things are going well. If the pH is outside of this range it should be adjusted by using either pH Up or Down as needed. A pH of 6.8 is ideal. Fill the tank to top off the level with de-chlorinated water. Each month, use a siphon cleaner to vacuum the excess waste out of the gravel. Take the tank water level down ¼ with this process. Don’t throw this water down the drain; it will do wonders for indoor and outdoor plants. Fill the tank back up to the top with de-chlorinated water

This small system can not only be an attractive conversation piece, but it can provide enough salad greens and herbs for the whole family.

Dr. Christopher J. Kline is a master gardener and writer living in Paradise Valley Arizona. Chris can be reached at CKline72@Cox.netThis e-mail address is being protected from spam bots, you need JavaScript enabled to view it .