What I learned (so you won’t have to)
My goal for this project is to grow tomatoes (and possibly other warm-weather crops) all year round in my greenhouse. I learned several things from my winter experience this year. I thought I’d record them here to benefit anyone else out there trying to grow tomatoes year-round in uncooperative environments.
For the record. I “shut-down” the greenhouse in mid-December. This year (La Nina year), was below average cold for my area. There just wasn’t enough produce to justify the propane heating cost which was turning out to be double what I expected (9 kg of fuel per month). Further, there was no future produce either as flowers were not setting fruit. Reasons listed below.
Light is essential
No duh, right? This winter, I did not have supplemental lighting. As a result, growth was slow, the plans were weak, and fruit didn’t set. A lot of this has to do with improper temperatures, but I believe light was a major factor this year.
Geothermal is unhelpful for winter tomatoes
The concept behind geothermal in the winter is that you can store energy during the day, and recover it at night. In addition, since the geothermal “battery” recharges from the surrounding soil, there’s a temperature floor which helps keep air temperatures at night above freezing.
The problem with the theory is when there is little to no energy input geothermal temperatures are too low for tomatoes. Without the sun, which is typical for winters in my area, there’s no “free energy” to store during the day. To supplement the lack of solar energy, I have a propane water heater as a backup heating source. This energy isn’t “free” light sunlight and I don’t need to store it in the ground. In fact, it’s inefficient to store it in the ground during the day because just like the geothermal battery “recharges” from the surrounding soil, the surrounding soil also “recharges” from the geothermal battery when the battery has a higher temperature. The energy losses to the surrounding soil resulted in lower than acceptable air and soil bed temperatures and wasted propane. The lower temperatures with the light problems led to slow growth and a lack of fruit production.
When I turned the greenhouse back on, I added a bypass manifold that separated the geothermal from the bed and aisle lines. Now I can provide radiant heating directly from the propane water heater without any losses to the earth.
The result was very positive. The water reservoir heated up enough that it was warm to the touch. Soil temperatures went from just 11C to above 16C (March 6th).
Don’t shut down your greenhouse
When I shut down, temperatures were allowed to drop extremely low. It was cold enough to crack some PVC pipes, and burst copper tubes in every single heat exchanger in the greenhouse including the propane water heater. The damage was close to $500… (all in the name of saving $20 worth of propane). Instead of shutting down, I should have just lowered the set-point to around 3C.
Genetics is everything
A number of my tomato plants had poor yields, even in the summer time. Among the poor producers was the San Marzano that I got from The Home Depot. This was especially disappointing as even my grafted San Marzano produced almost nothing. I’m not sure why the San Marzanos didn’t produce well in my greenhouse, but this year I’m trying a greenhouse-specific hybrid: The Pozzano. This is supposed to be the same style as the San Marzano (paste/plum tomato), but really excels in the greenhouse. We’ll see. So far the plants I have look promising.
A new start
It took a while to repair and get things running again. Sometime learning is expensive and I sure learned a lot this year. Early February, I was able to transplant some new Pozzano plants into the greenhouse. I have 7 new plants and 5 of the 7 have flowers. This is going to be year 3 and I’m hoping this will be the year it “just works”. Summer is coming and I have big plans, but I’ll save that for the next post.