In my previous article about Aeroponic farming, I put forward a concept for indoor-grown tomaotes for sale at local farmers markets.  The project seemed pretty interesting so, as we often do here at TG with these concepts, we decided to set up a proof of concept experiment.  We figured that if it worked out, we’d have tons of tasty tomatoes and would also be able to document the farmers market selling process.  Was it a success?  Well, we harvested lots of tomatoes and there were no great surprises, but we also learned some very important lessons that we were otherwise missing.

Doing the Legwork

Though my previous post was informative, other TG writers didn’t want to pitch in for the experiment without a lot more hard data so we wouldn’t need to re-buy equipment.  We needed to update our knowledge on this area of research as to yield expectations, energy requirements, maintenance, and how to properly measure progress.  So I hit the books, visited greenhouses and farms, read literally hundreds of horticultural papers, and am now even an official member of the International Society for Horticultural Science!  That said, all this prep was more so we would have any clue at all what to expect.

Ultimately though, this was a very rough proof of concept.

Research Findings and Design Considerations

What we discovered in preparation were some key facts, first for tomato growing, and then specifically for aeroponic setups. We decided to plant a heirloom breed of cherry tomatoes, and one heirloom mortgage lifter (very large).  Here are some relevant highlights, with the hardcore science omitted:

• Tomato plants love light.  In fact they need tons of it.  For our situation, we budget 1kW per square meter.
• For fruiting, plants (including tomato) expect warm fall lighting for about 12 hours a day.  The most efficient source of this is HPS (High Pressure Sodium).  Yes, at this point, it is much more cost effective and efficient than narrow band LED lighting approaches.
• Full grown plants can drink a ton of water.  We wanted to leave our reservoir unchanged for a week or more, so we decided on budgeting 20 gallons to out .5 meter grow.
• Ideal nutrient droplet size is around 50 microns.  This takes high pressure and small nozzles that love to clog.
• Plant density can dramatically increase from outdoor soil recommended 1 plant per .5 sq meter.  We saw some experiments as high as 30 plants/m2.  We are ambitious so went for 26 plants/m2.
• Heirloom tomatoes like to sprawl much more per tomato, and are much more unpredictable.  We wanted to see if we could create ideal conditions for them, because that’s what we want to sell (and eat!).
• Best mainstream accepted yield is about 85 lbs per square meter per year.  This works to 3.5lbs per .5 sq m per month.  We think we can do much, much better!
• Tomato plants are insidious, wretched little vines, and need constant pruning.  Big greenhouses require tons of labor.  We decided to regularly prune our plants to a single vine and top them after the first or second fruit cluster.
• The fewer fruit on the vine, the more energy is routed to them so the bigger and better they are–to a point.
• Misting would be ideal for about 1 minute every 5 to create a constant 50um fog atmosphere.

Building The Device

First off let me say this: my design was hacked together from lots of concepts out there.   As you can see below, a large storage container was repurposed for the project.  We put in a 200 mesh filter, some 50-micron mist nozzles with integrated 200 mesh filters, and pressurized the system to about 90 psi using an espresso pump.  Many similar devices use Shur-flo diaphram pumps, sometimes even with all sorts of external pluming and gadgetry.  For 1 minute misting, espresso pumps do a great job on this scale (one pump per square meter, basically).  All of this was mounted on a $12 home depot furniture dolly, as when loaded, 20 gallons of water weighs a good 140+ lbs.  We used the filter’s purge connection to both drain AND fill the reservoir.

Construction was relatively simple–just a lot of PVC cutting and gluing.  The hardest part of course are the thru-hulls or “bulkheads” as plumbers call them.  These are very hard to find, especially in the size/configuration you want.  Also, you will notice that hose is used to plumb the pump itself–this is just to make it easier to move around if needed.

The timing of the pump was done using a simple 555 timer circuit wired to a relay switching the 120v for the pump.  We wanted to just buy a timer but believe it or not, you can’t do this sort of timing for less than $85 for some reason.  Anyway, with the relays: if you carefully read relay spec sheets you will discover that, solid state or not,  these relays do not work forever.  This is an inductive load and (within a year) it will probably melt out the contacts.  For long term, you would need to dramatically over spec this part and still expect to replace it.

How did it Work?

Exactly as predicted!  We got a nice foggy mist every few minutes that was clean and excess recombined and condensed back down into the reservoir.  The clear tubing used for the pump may have led to the algae buildup we later experienced.  Also we initially used plastic barbed nipples to adapt from PVC but with moving, one broke off and created a bit of a flood (and nearly killed the plants while waiting for a replacement)–we replaced those with brass.  The primary issue aside from these was that the plant quickly became very heavy and needed support.  We velcroed on the PVC support structure you see, with metal rods providing support as the vines were attached about 10″ from the base with zip-ties.

The Tomato Results

How much did we yield?  Well, as I mentioned in the beginning this was a very rough experiment, and as such there were many liberties taken.  The plants were sprouted and growing in a space far too small and cold for them, and when transplanted they lost about 75%+ of their roots.  After this, they had to wait for about 10 days with no proper watering device (just a small pump sprayer) while the watering parts were finished.  In addition to THAT, they had to be severely pruned because they had not been correctly maintained in their first few weeks. Lastly, the experiment had to be cut short right when the tomatoes were going nuts.

All of that aside, they did remarkably well!  They were full, healthy plants within just a couple of weeks of planting.  Once they were put under the HPS light and into the aeroponic environment, they had fruit within a week.  In total, with all its mistakes and tomato plant torture, the experiment ran 71 days at which point everything was broken down and the existing fruit weighed.

The bottom line: 1.5 lbs from .5m2.  Considering propogation method (take clones from these adults for new plants), harvest season length, etc, we would expect this to yield about 84lbs per year per m2–right in line with projections.  Could this make it interesting for farmers markets?  Not without a much bigger space and some cheap natural light.  1kW/m2 gets expensive quickly!  If you set up a 12m2 operation, you would have a lot of efficiencies, and it could be done in a small greenhouse in the back yard (about $1200) to save about 70% of the electricity cost.  In this case you would be yielding about 20-30 pounds per week. Go to the market every couple of weeks with your 40-60 lbs and sell them for a premium $5/lb and you would bring in $400-600/mo.   The real interesting part though, is that you could do this 365 days a year.  When the market shuts down in the winter, go to the local restaurants, and you will find very receptive chefs.