Lettuce in the midst of winter

hydroponic lettuce under a grow light
Hydroponic lettuce under a grow light.

Homegrown lettuce in the dead of winter or the heat of late summer? It’s possible with hydroponics. And you don’t need a fancy setup with electric pumps and a water circulation system. The Kratky method lets you do it with a grow light and an empty coffee bin. 

Developed by horticulturist Bernard A. Kratky of the University of Hawaii at Manoa, the Kratky method is ridiculously simple. Plant roots need access to oxygen. When grown outdoors, a plant’s roots find this oxygen in air pockets within the ground. In a commercial hydroponic system, pumps circulate air to the plants’ roots. In the Kratky method, an air pocket is formed as the roots take up water, lowering the water level. This air pocket provides all the oxygen a plant needs at the root level.   

What You Will Need

component parts for a hydroponic system
Component parts. A simple hydroponic system is ready to be assembled.

To get started you need a few things:  a container, a grow light, water, water-soluble fertilizer, a net cup, growing medium to hold the base of the plant, and some seeds. 

Container:  A plastic coffee bin works well for one lettuce plant. A plastic shoebox will hold two. To discourage algae growth the container should be opaque. If you use a clear plastic shoebox, spray paint the outside or wrap it in aluminum foil or paper. Using a doorknob drill bit, drill a two-inch hole in the container’s lid, this will be just wide enough to hold the net cup.

Grow Light:  A bright window might work for starting seedlings, but to grow a full head of lettuce you will need more light. Either a fluorescent or LED fixture will work. During the summer and winter months, my seed-starting lights do double duty as part of my Kratky setup. Twelve to 14 hours of light will produce a nice head of lettuce.

Water and Fertilizer:  Tap water mixed with a fertilizer designed for hydroponics supplies the trace minerals found in garden soil. I have not tried an all-purpose fertilizer, but it might do just as well.

Net Cup and Growing Medium:  A net cup is a small plastic cup with slits cut into its sides and bottom.  They can be purchased online. They are reusable, so a bag of fifty net cups will supply a neighborhood full of gardeners. Or, make your own net cup by slicing slits in the side of a 3 ounce Dixie cup and poking a few holes in the bottom. Something needs to go into the net cup to hold the seed or seedling. Professional growers use rockwool, but a Jiffy-7 pellet works great. And, recently, I have just formed a cup from a scrap of newspaper, added some soilless growing mixture and fit that into the net cup.

Lettuce seeds:  Most types of lettuce should work well in a hydroponic system, but I particularly love hydroponically grown butterhead lettuce. The Kratky system creates light, delicate leaves with crunchy, moist ribs. Seed companies offer special cultivars designed for hydroponics, such as Rex, but any type of lettuce seed you have should work. Compared to lettuce grown outdoors, you can expect slightly smaller heads, but the plants will mature faster. 

a shoebox with two lettuce seedlngs
A shoebox with two lettuce seedlings.  

Putting the System Together

The bottom of the net cup should just touch the water. The water level will drop as the plant grows, but you won’t need to refill it. Once the system is set up under a grow light, nothing needs to be done until harvest time. You can plant your seeds directly into the growing medium, although I have had better luck starting seedlings in a seedling tray (I use a blueberry container from the store) and then transplanting the seedlings into my net cups. The seedling tray facilitates the application of bottom heat, which increases germination. 

I have had two problems growing lettuce using the Kratky method. One year aphids hitchhiked into the house on a Meyer lemon tree brought in for over-wintering. The aphids loved my lettuce as much as I did. A more persistent problem is inner leaf tip burn. The tips of the inner leaves turn brown and die when the plant fails to transport sufficient calcium to the young growing leaves. A small fan blowing air across the plant increases transpiration and can prevent tip burn. Start the fan as soon as the basal leaves form. A related problem of outer leaf tip burn arises when the water solution has too high a level of dissolved salts. Reducing the amount of fertilizer solves this problem.

girl holding up lettuce grown using the Kratky hydroponics method
Hazel Cook shows off the root system of lettuce grown using the Kratky method.

The Kratky method offers an easy introduction to hydroponics and produces wonderful heads of lettuce in both the height of summer and the depths of winter. For more specific information and lots of YouTube videos, just search “Kratky method.”

By Robert Cook, University of Maryland Extension Master Gardener, Baltimore City

2 Comments on “Lettuce in the midst of winter

  1. Hey I am inspired to try this! I have a question though. Would using a non-food container be hazardous? Would my plants absorb BPA from a storage container utilized, or would the aluminum of a baking pan get absorbed in the plant?

    • Jan: Sorry for the late reply. Unfortunately that question is above my pay grade. I have read a lot about whether plants absorb lead from the soil (it doesn’t seem that they do from what I have read), but I have seen nothing about whether plants absorb non-metallic substances from the soil or containers. Given the fact that most of us (and the industry) grows seedlings in plastic pots, I’m surprised there has not been more research. If it’s a concern, skip the shoe box idea and stick with coffee bins, which I like the best anyway.

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