Hydroponic lettuce

Easy Low-Cost Hydroponic Lettuce 

Hydroponics is gaining popularity as a way to grow food plants indoors, especially in winter. Traditional hydroponic methods rely on a pump to circulate air, water and nutrients to the plant roots. It needs daily maintenance and electricity to operate the system. While traditional hydroponic systems work very well, we wondered if there was an easier and cheaper way to grow lettuce and other leafy greens and there is! This method is adapted from the research by B.A. Kratky of the University of Hawaii.

All it takes to grow one lettuce is 1 g of fertilizer, 1 L water, 1 seed and 1 Jiffy-7 peat pellet, at a cost of as little as 10 cents per plant. Recycled containers hold the water and fertilizer solution. The other supplies needed are very light weight and inexpensive to buy and to ship – a big advantage if you live in a remote community. It’s water-wise too, since there is very little water lost to evaporation because it is a closed system. It is rarely necessary to add water so you can go away for days or even weeks without worrying about your plants. A circulating pump is not needed: the roots access oxygen because they are exposed to humid air inside the water container. It only takes 6 – 8 weeks to harvest leafy greens.

What can you grow using this method?

Leafy greens are best grown this way. Choose an open leaf lettuce variety instead of the type that produces heads. Some good lettuce cultivars include: oak leaf, Red Sails, Prize Head and Grand Rapids. You can also try water cress, pak choi, arugula or spinach. Other good prospects are mizuna, a Japanese herb and shiso, also known as perilla. Basil works well too.

Can I grow other vegetables using this method too?

This indoor method works best for smaller leafy greens which have low light needs compared to vegetables like tomatoes, peppers or cucumbers. These need larger containers, more water, more nutrients and a lot more light. Information about growing larger vegetables can be found in the link at the end of this document.

Do I need a grow light to grow lettuce?

Leafy greens have fairly low light requirements compared with other vegetables. Hydroponic lettuce can be grown easily on a sunny windowsill in most parts of Canada even in winter. However, if you live in artic and subarctic regions where there is virtually no light in winter, you may need artificial grow lights for the lettuce to turn green (photosynthesis).

If you don’t have a grow light, a regular shop light fixture also works well for this. Use one T-12 cool light and one T-12 warm light fluorescent bulb in the shop light. Hang the shop light about 30 cm (12“) above the plants. One shop light will provide enough light for 45 cm (18”) by 120 cm (48”) area, or about 6 – 8 hydroponic lettuce plants. One 120 cm (48”) light fixture will provide enough light for about 24 plants (3 rows of 8 plants) based on the containers recommended in this method.

Materials:

• Plastic container which holds about 1 litre of water such as a 1 litre plastic milk container or plastic container with lid
• Water soluble 20-20-20 fertilizer
• Jiffy 7 peat pots
• Clear plastic dome or bag
• Shallow tray
• Seeds

Method:

1. Start the lettuce seedlings:
   1. Soak one or more Jiffy-7 peat pellets in water for about 30 minutes until the peat pellet has fully expanded.
   2. Press one or two lettuce seeds in the top of the peat pellet. Place the peat pellets in a shallow tray. Cover with a dome or clear plastic. It takes about a week for the seed to germinate. Place the tray in light as lettuce needs light to germinate.
   3. After about 3 weeks, the seedling will have several leaves and be about 3 cm (1”) tall. It is now ready to grow in the hydroponic container.

2. Prepare the container:
   1. If you are using a plastic tub with a lid, cut a round hole in the top of the lid that is slightly smaller than the width of the expanded peat pellet. If you do make a hole slightly larger it is ok, a couple of tooth picks will hold the plant from falling in. then use an old bag or plastic wrap to tuck in around the hole to keep the humid air inside the container.
   2. Another option is to use a 2 litre plastic milk jug. The peat pellets fit well in the opening in the top. Just squeeze gently to fit it in.

*If the container is clear plastic, it’s a good idea to spray paint the container to prevent light from causing algae to grow in the nutrient solution. Before you paint, place a strip of masking tape vertically on one side of the container. Peel off the masking tape after painting. The unpainted strip allows you check the level of nutrient solution.

3. Prepare the fertilizer solution:
   1. Measure 1 gram (1/4 teaspoon) of fertilizer and mix it into 1 litre (4 ¼ cups) of water.

4. Insert the Jiffy-7 seedling into the container:
   1. Gently squeeze the peat pellet and insert into the hole in the lid or the opening in the milk container.
   2. Make sure that the bottom of the peat pellet is just barely touching the top of the nutrient solution. The roots from the lettuce will eventually grow out of the peat pellet and into the water.
5. Place the container near a sunny window. If you live in Northern Canada where there is very little light in winter, use a grow light.
6. Ensure the roots can access oxygen
   1. Once the roots grow out of the peat pellet, ensure there is a 1-2 cm (about 1/2”) air space between the top of the fertilizer solution and the bottom of the jiffy pot. Roots need oxygen as well as water to be healthy. This is why you do not need a pump to circulate the water.

Harvesting

There are two ways to harvest: either wait 6 -8 weeks until lettuce is full and mature and cut off the entire head; or harvest the outer leaves continually as the lettuce grows.

Succession planting

If you start all your plants at one time, they will mature at the same time. To have a constant supply of lettuce, start fewer plants more often (perhaps every other week), depending on how much you need for your family. 

Can I re-use the peat pellets and the nutrient solution when I start new plants?

It’s best to use a fresh peat pellet when starting new plants. Same for the fertilizer solution – it’s better to mix up a fresh batch for each new plant. Old fertilizer solution can be used to water houseplants. 

 

Sources:

https://www.ctahr.hawaii.edu/hawaii/downloads/A_Suspended_Pot_Non-circulating_Hydroponic_method.pdf

Tanino, K.K., Gray, Gord; Benic, Elena; Nair, N.P.M. (2019) Understory Greenhouse Production and Home Windowsill Edible Low Light Tolerant Vegetable Plants. University of Saskatchewan (unpublished)

 

Lettuce has excellent potential as a year-round indoor crop.  Lettuce grows rapidly, even in situations of relatively low light and temperatures.  Buttercrunch (aka Boston bibb or semi-head lettuce) type lettuce is particularly well-suited to indoor production as it produces compact high-quality plants that can be harvested one leaf at a time or as an entire mature head.

Recirculating hydroponic systems allow the very effective use of growing space and nutrients and are well suited to the indoor production of high-value crops such as lettuce. 

Production in hydroponic systems started with 4-week old lettuce seedlings grown in rockwool cubes (2.5 cm * 2.5 cm * 3.0 cm deep).  The seedlings were grown at room temperature at moderate light levels and were fertilized weekly with 20-20-20 plus micronutrients at 100 ppm N.  After 4 weeks the lettuce seedlings were about 10 cm tall and had 6 true leaves (Fig. 1) – at which point they were large enough to be transplanting into the hydroponic production system.

The University of Saskatchewan has evaluated the performance of 26 different cultivars of buttercrunch type lettuce in hydroponic production.   So far, the best cultivars have been ...

• Red Sails (Johnny’s Seeds) - produced excellent yields of large, red/green open-type heads.  Red Sails received the highest flavour rating of all cultivars tested.    
• Optima (Osborne Seeds) - is a great looking and good tasting semi-head type lettuce

Lettuce grows well in either Floating Culture or Nutrient Film Hydroponic systems

Floating culture - Is the simplest production system.   Plants are grown on a 1 cm thick sheet Styrofoam that floats on top of a nutrient solution held in a grow tank (aquarium, plastic tub etc)(see Fig. 2).  The 4 week old lettuce seedlings are inserted into holes cut through the Styrofoam block so that the roots are fully immersed in the nutrient solution, but the growing point remains above the nutrient solution.  The nutrient solution is kept aerated using a standard aquarium aeration system.   Spacing plants about 20 cm apart allows enough space for the lettuce to reach full size without wasting space. 

Nutrient film technique (NFT) - in NFT production, plants are grown in troughs and a thin film of nutrient solution is kept circulating down the troughs and past the root system.  The production troughs can be as simple as sections of circular or rectangular eaves trough (Fig. 3).  Trough lengths are kept relatively short (<5m) to minimize nutrient and oxygen gradients.  A hole saw is used to cut holes (5cm diameter) into the upper surface of each trough.  The holes are large enough to allow the 4 week old lettuce transplants in their rockwool plugs to be dropped easily to the bottom of the trough – with the leaves arranged so that they protruded through the hole in the top of the trough.  The holes were spaced 20cm apart and the troughs were also spaced 20 cm apart – this spacing provided each plant with enough room to reach full size, without wasting space.  The troughs were kept on a 2% slope so that the nutrient solution ran down the trough and over the root systems.  At the end of each trough, the nutrient solution was gathered back into a capture tank and then pumped back to the top of the channels using a small circulating pump.  The nutrient solution was pumped through the production troughs at the rate of 1L/min.    Insufficient flow volumes result in depletion of nutrients and oxygen as the nutrient solution runs down each channel.  However, excessive high flow rates waste pumping capacity and may interfere with the uptake of certain nutrients.  The 1L/min  flow rate was achieved by installing pressure compensated button-type emitters in the spaghetti tubes that supplied each grow channel.  These emitters are available locally from suppliers of irrigation equipment.  However, these emitters are easily plugged by plant debris or algae in the nutrient solution.  Installing a 100 mesh in-line filter just upstream of the emitters solved this plugging problem.  Four troughs operating at 1L/min could be run off a single circulating pump.  The nutrient solution in the capture tank was kept aerated using a small aquarium pump.

Crop and Nutrient Management

Successful hydroponic production depends on supplying the plants with adequate, well-balanced nutrient supply.  Nutrient recommendations for hydroponic lettuce are presented in Table 1.   While it is possible to purchase pre-mixed complete nutrient formulations suited for general hydroponic production, creating your own nutrient solution will allow you to more closely meet the needs of your selected crop and in the long run will also be more cost-effective. 

Hydrobuddy is a powerful, yet easy to use program designed to assist growers in preparing nutrient solutions for hydroponic crop production.   Hydrobuddy is available as a free download and can be located by entering “Hydrobuddy” in your favourite search engine.  

Table 1:  Recommended fertility regime for hydroponic lettuce (from Hydrobuddy). 

 

Hydroponic lettuce grows best at a pH of 6.0, whereas the water in Saskatoon has a pH of around 8.0.  Adding household white vinegar to the nutrient solution at the rate of 2-3 mL/L gave the required shift in pH.  The amount of nutrient solution to prepare depends on the size of your nutrient holding tank and the number and size of plants being grown.  About 1L of solution per plant is more than enough to meet the nutrient and water requirements of lettuce from transplanting through to maturity.

Crop Growth

Lettuce was ready to harvest 4-6 weeks after being transplanted into the hydroponic system – with the rate of growth and size of plants harvested being highly dependent on cultivar grown and the amount of light provided to the crop.  No problems with disease or insect pests have been observed.  Algae tend to build up in the systems as each crop progressed.  A thorough clean up of the entire production system at the end of each crop cycle was important for controlling algae and also prevented disease problems.   Clean up involves flushing out the system to remove any root material, scrubbing off any algae, then rinsing with a dilute (5%) solution of bleach, followed by a final rinse with clean water.       

Supply List

Seedling production 

• Seed of a suitable variety
• Rockwool plugs or Jiffy pots to grow transplants

Floating Culture system

• A sheet of 2 cm thick Styrofoam
• Nutrient Holding Tank – should be at least 30 cm deep and hold 1 L of nutrient solution for each lettuce plant being grown
• Aquarium aeration pump + air stone

NFT System

• Grow troughs – standard rectangular or circular eaves troughs work well although the materials used have not been made to Food Grade Standards
• Submersible water pump (10 L/min capacity is plenty)
• 2 cm diam PVC tube to carry nutrient solution from the reservoir to the high end of the NFT system
• 100 mesh in-line filter – removes plant debris from the circulating nutrient solution
• 1 cm diam spaghetti tubing - to take the nutrient solution from the PVC supply hose to each production trough
• 0.5 to 1L/min pressure-compensated emitters – regulate the rate of flow of nutrient solution into each trough
• Nutrient holding tank – see description in the floating culture system
• Aquarium aeration pump and air stone