Bottom heat can give your seed starting a real helping hand – particularly for some crops such as peppers. For a home scale operation that usually means using one of the flexible rubber mats with the heating loop imbedded into it that slips under your seed starting trays. I’ve been using this method for over a decade and it really does make a major difference for some heat loving plants – BUT, I’ve found that these tend to wear out and I end up having to replace them every few years.
Given these are fairly expensive when my last one gave up the ghost last year I decided to come up with something better and longer lasting than simply to order another and continue the cycle.
Here’s what I came up with – a STC1000 digital temperature controller – 110volt model (under $10 on ebay from china ) and a 250W 110V cartridge heater ($5) form the basis of the system.
Both the cartridge heater and the thermocouple for the temperature controller fit into holes drilled in a 3/4″ thick block of scrap aluminum that is then lag bolted through a 3/16″ thick piece of aluminum plate to a 2X3 piece of lumber. The aluminum plate is long enough to fit two grow trays while allowing a gap for the heater at the center.
The temperature controller is housed in a 3D printed box that is mounted on the top of the 2×3 with the cords all held nicely in place by a strap at the back of the wood piece.
The whole assembly is supported by some scrap 1″ thick pieces of pine left from building beehives over the usual trays on my growlight assembly.
I used a couple of additional sheets of aluminum to spread out the heat more evenly under the grow trays.
In operation I set the temperature of the aluminum block and by trial and error move it up until the temperature of water placed in my grow trays hits the desired temperature for the seedlings I am producing. If you were using a much thicker block of aluminum you could set the temperature much more in line with the desired setting – but with this setup the temperature of the block ends up being considerably higher than you would normally want in order to see the necessary amount of heat generated to be distributed through the aluminum plates.
It’s not a perfect system, but for about $20 and some scrap I have a solid bottom heat source for seed starting that should last decades and allow me to produce loads of heat loving seed starts at home – paying back the investment in less than a year compared to buying transplants at the garden center.