Solar seed warmer to get a jump on spring

Spring is coming… here is a tiny little hacklet from the bench of Tim.

I live in New England. I don’t mind it here, but the growing season is a bit short. So here is a scheme to give outdoor direct-seeding a little head start.

Seeds for many food plants, such as melons and peppers, will not germinate until temperatures rise above a certain point consistently. They could in theory be started indoors, but I’ve never had good luck with this in a prison-windowed New England house: even if I remember to water the seedlings consistently (hint: I don’t), regardless of how well I position them in my best south-facing window, they still end up weak and spindly for lack of sunlight. If they don’t outright die when hardened off and transplanted, they seem to go into some kind of shock and stop growing for several critical weeks. I’ve found direct-seeding outdoors is a lot more reliable overall. The tradeoff is that by the time it gets warm enough long enough to trigger germination, they will not set fruit until the tail end of the growing season. What a pain!

The previous homeowner was nice enough to leave behind some things, including some solar garden lights. A quick tweak to them makes them into seed warmers.

Solar garden light converted into a nighttime seed warmer

Solar garden light converted into a nighttime seed warmer

String of low-valued resistors insulated with black heatshrink

String of low-valued resistors insulated with black heatshrink

Assembled view of the seed warmer element

Assembled view of the seed warmer element

Ingredients:
1 old solar garden light
Heat shrink tubing
A few low-value resistors (5-10 ohms)
Small bit of copper tubing

Process:
Sould be pretty self-explanatory from the pictures.
solder one to a few of the resistors in series to achieve the desired length. Connect to each end of the resistor string with some thin insulated wire. Heat shrink this assembly so that the wires exit on the same side (the heat shrink will prevent the bodies of the resistors from wearing through against the tubing and shorting out). Cut a piece of copper tubing to a sufficient length that this can be stuffed inside. For best results, the assembly should be a snug fit inside the tubing to ensure good thermal contact. Finally, seal the ends of the tubing with RTv or similar watertight material, and optionally coat the copper with something to prevent corrosion.

Remove the LED from the solar light, and wire the heating tube in its place. DONE!

Now, when you plant a hill of outdoor seeds, drive the heating tube into the center of the hill, and place the solar light off to the side a bit (so it is not shading the hill). The sun will help keep them warm during the day, and the heater will take over during the cold nights.

Notes:
How much heating will you get? Short answer is “it depends”. In theory, you can measure the voltage output by the light unit and use Ohm’s Law to calculate the power dissipated over your chosen resistors (power in Watts = I*V = V^2 / R). Depending on the design of your solar lights, the output voltage may not be remotely constant or easily characterized, and the circuit inside may have its own current-sourcing limit, reducing your total output. The actual amount of heating you get may be pretty modest. You will not (and should not) find the tube uncomfortably warm to the touch during operation. Fortunately, dirt is a pretty good insulator.

Some plants just plain don’t like to grow in the cold, even if you can trick them into germinating early. For better results, combine with a coldframe to keep the aboveground bit a little warmer too.

6 Responses to “Solar seed warmer to get a jump on spring”

  1. [...] seeds are kept warm outside, but for wont of a properly placed clothes dryer vent [Tim] decided to make a solar soil heater using junk he had lying [...]

  2. [...] seeds are kept warm outside, but for wont of a properly placed clothes dryer vent [Tim] decided to make a solar soil heater using junk he had lying [...]

  3. [...] seeds are kept warm outside, but for wont of a properly placed clothes dryer vent [Tim] decided to make a solar soil heater using junk he had lying [...]

  4. freeflyer says:

    I’m not sure it’s the best way to do it..
    The efficiency of a solar panel is like 5 or 10%

    What about a short copper or aluminium wire connected to a black heat sink surface exposed to the sun..

  5. [...] are stored cozy exterior, however for will not of a appropriately placed dryer port [Tim] chose to make a solar soil heater   using crap he’d already [...]

  6. [...] seeds are kept warm outside, but for wont of a properly placed clothes dryer vent [Tim] decided to make a solar soil heater using junk he had lying [...]

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