Lettuce Give Thanks

Some time ago, while researching greenhouses, I came across a website run by a fellow in southern Virginia. Bobby Smith is the “mhpgardener” whose U-Tube videos are a treasure trove for do-it-yourselfers. While his adventures in building greenhouses were enlightening, it was his experimentation with hydroponic culture that got me interested in trying it myself.

Specifically, Bobby introduced his viewers to the work done by Dr. Bernard A. Kratky (ret.) of the University of Hawaii. Prof. Kratky published a short paper in 2002 outlining how to build a small hydroponic system from a bottle; his hydroponic method was patented and is free to use, but if used commercially outside the State of Hawaii, must be licensed. Thus, hobbyists and home gardeners can use it freely. Further papers outlined larger scale development of the system.

The essence of the system is that plants develop two types of roots; water-absorbing roots and air-absorbing roots. If a plant is allowed to grow air roots, then its water roots can be permanently suspended in solution without drowning the plant.

In the Spring of 2019 I determined to try out the Kratky system. Following Bobby Smith’s instructions, I built the first 24” x 24” tank from recycled basement stair treads (2x10s ripped to 5-1/4”), scrap 3/4” plywood subfloor, and some clear 6 mil poly.

The cover was a 24” x 24” x 1” extruded polystyrene project board from Lowe’s ($6).

From Indoor Gardens on Romig Road in Akron I started out with some perlite ($13 for a 4 cu ft bag and nine 2” net pots (18 cents each). A 2” hole saw was used to cut the holes for the net pots. From Copley Feed I bought a 1/8 oz packet of Buttercrunch lettuce seed for $2.09 - likely enough seed to last me into the 22nd Century. I had lots of Miracle-Gro, so to save money (and against Bobby’s recommendation) I mixed up a fertilizer solution and set to work. The lettuce was seeded into some potting soil;

when it was about an inch high, I transplanted it into the net pots in perlite.

The plants grew for about a week, then died. I tried it again, with a higher concentration of Miracle-Gro, convinced that the solution must have been too weak. Same result.

After the garden was planted, in late June, with the greenhouse cleared out, I tried it again. This time I used the Miracle-Gro Tomato formula, and I also built a second tank with a 7” depth.

The second tank was filled with a solution made with MaxiGro hydroponic fertilizer (1 kg bag at Indoor Gardens for $13). Indoor gardens did not carry the Master Blend fertilizer recommended by Bobby Smith or the Chem-Gro recommended by Dr,. Kratky.

Seeds were started in glass-wool blocks (package of 200 for just under $10).

The nitrogen in Miracle-Gro is almost entirely urea based; without soil bacteria in the tank to convert the urea, that fertilizer is better suited to in-ground culture. The MaxiGro uses ammonium nitrate and calcium nitrate as the nitrogen source; the nitrates are directly absorbable.

Unfortunately, summer in Ohio is not a good time to grow lettuce in a greenhouse that is poorly vented. I am pretty certain that the plants cooked in the 100+ degree daytime temperatures. I gave up for a while.

I was determined to try again. On October 23 I seeded 9 glass-wool blocks and on October 27 I set them in the 7” tank in the greenhouse using 1 teaspoon of MaxiGro per gallon plus a teaspoon of Epsom salts. That day (the 27th) I seeded another 9 blocks, and 3 days later set up the 5-1/4” tank under lights in the basement and installed the plants.

The first week of November was cold. The plants inside were doing very well; the plants in the greenhouse were not making progress and had hardly grown at all. I drained the solution into 5 gallon buckets, moved the tank into the basement onto the shelf below the one that was already there, refilled the tank, and the plants began to grow.

On November 25 we started using lettuce from the 5-1/4” tank, and have had a salad almost every day since without having to buy lettuce from the store. The 5-1/4” tank was replanted on December 8 and that planting has almost been harvested.

The plants filled the space between the tank tops and the lights; there was only about 6” of growing room over the 7” tank. I also noticed that only about half the solution was used during the growing period, so I disassembled the 7” tank, cut it down to 4-1/4” inches, replaced the 2x2 bottom support rails with 1/2” plywood, and replanted it on December 23. Matt and Angie expressed interest in the system and gave me several Folgers coffee cans; the 3 lb size holds about 1 gallon, and I planted the December 1 crop in those.

We finished harvesting that batch on January 17. From that batch I learned that on average, over the 6-7 weeks from seed to end of harvest, the lettuce plants used roughly 1/2 gallon of solution.

Tanks #3 & #4 were built 4-1/2” deep; they have about 11” clearance from the tank top to the lights, which allows Romaine to grow nicely.

When the 5-1/4” tank is done in a few days (bottom right in the picture, it too will be disassembled and cut down.

If I assume I can buy a head of Romaine lettuce at the store for $1, I can look at recovery costs. At this point, with all 4 tanks in production, I have calculated that the average cost per head of lettuce is about 7 cents in fertilizer, 5 cents in the glass-wool cube, and 46 cents in electricity for the lights, for a total of 58 cents in consumables. At about 1 teaspoon of perlite per pot, that is negligible and could be recycled if I felt like it.

The tanks and lights are fixed costs, but the tanks were built from scrap lumber with the polystyrene tops being the expensive part. The six 2-tube fixtures I had already, but they run about $15 each at Ace Hardware if I had to buy them again, and I replaced the flourescent tubes with 17 watt 5000K direct-wire LED tubes purchased on-line for about $8 each. The net pots at 18 cents each are reusable indefinitely. So I figure it cost about $200 in setup costs to produce a rotation of 36 heads of lettuce in one full cycle, or $5.56 per head.

It takes about 4-6 weeks to cycle through the system, so the hard costs would be reasonably recovered in 10 cycles (about 1 year), at which point we can expect to harvest bug-free, contaminant-free lettuce year-round for far less cost than in the store. Having observed two notable nationwide lettuce recalls due to E. Coli contamination in the past 12 months, in my opinion, the effort is worthwhile.