Warming Seeds In An Outdoor Garden


Spring is almost here and with that the green thumbs out there are preparing for their summer gardens. It’s usually a good idea to get a jump on all your gardening activities by starting seeds indoors, but with this comes the problem of making sure juvenile plants get enough sunlight. Putting a few seeds on a window sill will keep seeds warm enough to start germinating, but that will drastically reduce the amount of sunlight available for any given day. The best solution is to make sure the 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 around.

[Tim] constructed a simple heater cartridge using a few 5 and 10 Ω resistors. These were sealed inside a piece of copper pipe with heat shrink tubing and silicone. The solution to powering this heater cartridge, though, is an impressive display of thinking outside the box.

The cartridge is powered by a solar lantern – the same kind you’d find illuminating a garden path at night and recharging during the day. After inserting the cartridge in a hill of seeds, the heater provides a little bit of warmth to get the seeds through the night. During the day, the battery in the solar lantern recharges, providing just enough power to cycle through another night.

It works for [Tim] in his native New England, so we’re betting it’s good enough for just about any growing region.

16 thoughts on “Warming Seeds In An Outdoor Garden

  1. I agree it’s a placebo. solar lantern have a circuitry that generate high voltage spikes from an 1.5v battery to light a white led. The led is flashing at a very high rate with a very very low duty cycle. The average power supplied to the led it’s in the low mW or uW scale. I don’t think it can raise the soil temperature for more than few mili or micro degrees.

    I think the “rising” in temperature ( if there is some) is due to heat storage of the metalic part of the lamptern and the wires

  2. if he was using a solar panel MUCH MUCH MUCH bigger, then yes this would work.

    but if he is using a solar panel from one or two solar lights then the amount of electricity is so small it would heat your seed up by MAYBE one degree for MAYBE 1 hour,,, not enough to go all night long. and that is only when there was FULL sunlight ALL day.

    even the solar light itself never has, and never will work as inteded/advertised
    you think your unmodified solar lights get fully charged each day of full sun???
    haha test before assuming!

    example: 16ma in full sun times 6 hours of “DIRECT” sunlight,
    PLUS 8ma in shade times 6 more hours
    plus maybe 4 hours of 4ma (sunrise&sunset)
    equals only 160ma,
    nowhere near the quoted 300, 400, 600 or more ma capacity of the battery

    0.1w of heat is almost nothing, and with 1.2v 160ma/h you get 0.19w/h
    meaning 0.19w for one hour or 0.1w for only 2 hours!
    the original LED was 20ma @ ~3.2v = 0.064w
    BUUUT was running at less then 50% duty cycle! menaing its drawing on average half the power.

    and that explains why solar lights placed on the side of a house run for only 5 or 6 hours, but a solar light placed on the roof runs for 10 – 12 hours (more DIRECT sun)

    1. PS: try the old sunlight heating water trick (PhotoThermal) and store the hot water in an insulated barrel and circulate it through the garden at night, you could use a small (PhotoVoltaic) solarpanel to run the pumpmotor

      1. If garden on a slope you can use an even older/simpler trick. Place barrel at higher level than the thermal solar panel, and beds to be heated on 2 intermediate levels. It should thermo-syphon. Might want to run separate pipes to the panel and add a few (manual,or solenoid?) valves so the heating can be shut off- I’ve seen a thermal panel boil water (in highlands of Scotland). and those kinds of temps wont do plant roots any good.

  3. Man, haters gonna hate!

    OP here. It looks like there is a widespread consensus that this is going to do sweet f-all. My hypothesis (yet to be tested; it’s barely March) is that with the insulating properties of the soil, the gentle heating will be somewhat cumulative. But it remains to be seen. Time may well show that it does nothing and I’ve wasted 10c worth of resistors.

    Only time will tell, but until then, a back-of-the-envelope calculation: The battery inside claims a capacity of 800mAH. Nominal NiCd voltage is 1.2V. Assume the lantern’s voltage booster is 50% efficient (i.e. crap), and that the battery is more-or-less fully depleted during the night (it is). Total heat delivered per night is then (400*1.2/1000) = 0.48 Watt-hours. Multiply by 3600 seconds/hr to get 1728 Joules (watt-seconds). Now assume a 1/2lb (227g) mound of dirt gets its entire mass from water, which has a specific heat capacity (C) of 4.2J/g(degC). With this assumption, we’ve contributed a lousy 1.81degC (3.26degF) to the mound.

    But wait! The dirt mound is mostly well-aerated dirt, not water. Dry dirt alone has a C of only about 0.8. Wikipedia claims a typical field capacity (soil water content 2-3 days after a rain) on the order of 10% to 35%. So let’s split the difference and say the dirt is 23% water. This gives us a composite C of 1.582 for the moist dirt. Assuming the entire mound is heated uniformly, it now rises 4.8degC (a little over 8.6degF). Now, if we take away the unrealistic assumption that the mound heats uniformly, we’re getting somewhere! Most of the heat will be concentrated near the heating element, where the seed is. There will be some heat loss to ambient to consider, but for $0.10 in resistors I think it’s worth a shot so far :-)

    1. “Some” heat loss to ambient? The thermal conductivity of soil like yours (loamy, good for growing things, 15-25% water content) is about 0.76 watts per meter Kelvin. The quarter watt the heater is putting out isn’t even going to blip a thermometer that isn’t physically touching it.

  4. To address some…er…points raised so far:

    “This won’t do anything / solar lanterns suck for heating…”: This is addressed in the blog post (but see theoreticals above). “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.”

    Really, whether it will heat well depends heavily on the design of your solar light. The one I used seems to work reasonably, but I can’t claim to have dissected every one. Nearly all use an inductive booster of some kind, operating 1-2 cells to deep discharge. Some might use a voltage-feedback method where lower resistance = more current. Some might use a primitive current feedback means, or just a fixed or self-resonant oscillator. Some might happily let you pull current until the inductor saturates and becomes the primary heating source ;-)

    “Solar lights only run for a few hours because some idiot always puts them in the shade”: Then don’t put it in the shade. Cloudy days are unavoidable of course (the lights will recharge, but not as fully). Oh yes, and ambient temperatures do vary, freak cold snaps happen, and this is not a life support system, etc…

    “Why don’t you just use direct solar heating”: This is mentioned too. The post recommends covering the seedlings with a coldframe and avoiding shading them, to maximize direct solar heating. Dark earth should really soak up the sun by day, although some black covering can’t hurt. (Also: the solar lanterns were junk left behind by the previous owner, and I otherwise had no use for them, so why not?)

    “Why don’t you just trench the yard and bury pipes attached to a big barrel, solar collector array and pump motor?”: Because I have a wife, and would like to keep her :p Also, that sounds an awful lot like work.

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