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  • Rafael Saer

Cool Nutes in a Hot Missouri Summer

In late July, E2P was contracted to build a low-cost hydroponic raft system for a commercial customer. This is what we came up with:

A relatively straightforward project, intended to be used with our favorite celestial fireball in a greenhouse setting. These rafts use standard 4x4 flood and drain (ebb and flow) trays, which Ben modified with good ol’ PVC pipe to have an easy drainage system. The trays are sitting on hard foam to insulate the rafts from the greenhouse floor. A venturi pump sucks up the nutrient solution via a PVC pipe that extends diagonally across the raft, providing ample circulation and oxygenation of the nutrients. We used standard rockwool cubes in net cups to grow the plants.

Missouri’s hot summers are a bit of a double-edged sword for growing leafy greens; there’s ample sunlight available, but the heat and humidity can lend themselves to some sad heads of lettuce. One thing that we’re interested in is trialing different types of lettuce varieties in a raft system to see what grows well out here. We’re all about local greens!

You can spend a fortune on climate-controlling a greenhouse, and minimizing energy costs is key to running a successful greenhouse operation. Temperatures in St. Louis can reach upwards of 100F (about 38C), and that makes it very expensive to cool a greenhouse to optimal lettuce-growing temperatures. We wanted to see if we can “cheat” the system per se, by just cooling the nutrient solution instead of the ambient temperature.

Ben converted an old dehumidifier unit into a water chiller for this experiment. The setup was simple: cool one raft system and leave the other one alone. The initial setup looked like this:

Yes, our chiller is pretty ghetto, but it’s got the powah!

Now we just wait…

We observed the plants over the course of a few weeks. Immediately we noticed that the plants in the chilled raft were growing faster than the un-chilled ones. We also noticed this:

The lettuce plants in the un-chilled rafts wilted during the hot mid-day sun. What happened to the plants near the pump, you ask? Well, they got splashed on and died. Splash zones are a big no-no for lettuce.

This video compares the two rafts after approximately three weeks of growth. Some lettuce varieties definitely do better under hot and humid ambient temps (Butter Crunch was better than Boston Bibb), but overall, chilling the nutrient solution provided a large benefit to our floating leafy greens.

Check it out: !

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