Heated Germination Box

It's not pretty, but it works!

As discussed on the Conclusions for 2000 page, some household changes caused "[t]he ambient temperature profile during the germination process [to change] from the 1999 season to the 2000 season, and this impacted germination". My solution was to build a shallow, heated open-top enclosure to hold seed cups during germination and the early vegetative growth stage with a view to improving germination percentages and initial growth rates. I built the enclosure and used it during the 2001 season, and overall germination and early growth rates were dramatically better than the 2000 results.

The box that I built was made from inexpensive lumber, and was (outside dimensions) 40.5" long by 17.5" wide by 7.75" tall including the legs. The enclosed space (inside dimensions) was 39" long by 16" wide by 5.5" tall. I use 5 ounce disposable plastic Solo cups to start my seeds; given a spacing of 1/2" between cups and between the outside cups and the walls of the enclosure, I can fit 60 cups inside the enclosure in a 12 x 5 pattern. Since the cups are 2.75" tall, there is 2.75" of airspace above the rims of the cups, giving the seedlings a good bit of room to grow before they begin to protrude from the warmth of the enclosure.

My original plan was to heat the box by attaching an under-floor heating element to the underside of the box. These elements are plastic sheets that contain super-thin, low-output metallic heating grids and are installed under tile floors so that you can keep your bathroom floor (for example) toasty warm. I got this idea from some friends in the construction trade that keep reptiles; they used these elements under the floors of the glass aquariums that housed their snakes to provide a bit of extra warmth to the interior of the aquariums. When I finally tracked down a dealer that carried what I wanted, though, the cost was prohibitive, as he would not sell me anything less than a 10 foot long roll, at $148.75, plus $150 for a thermostat to control the heat output. (The trade name of the heating element is SunTouch, by the way).

I ended up going to an exotic pet store and buying a similar product that is sold specifically for heating empty aquariums used to house reptiles. It cost me $50 to buy two of the largest elements. Highway robbery, I think, but worth it to keep my peppers happy and healthy.

Click for larger image

The box itself was simple to construct, requiring a 40.5" x 17.5" piece of 1/4" thick plywood for the bottom of the box, two 40.5" long 'one by sixes' (actually 0.75" x 5.5") and two 17.5" long 'one by sixes' for the sides of the box, and four 2" long pieces of 'one by one' (actually 1.25" by 1.25") for the legs. A circular saw was used for all cuts, and the box was assembled using common wire nails. The one by sixes were cut to a 45 degree bevel at each end to provide a more secure joint; all other surfaces were joined at 90 degree angles.

Assembly, as depicted below, was trivial: the legs were attached to the plywood base, then the sides were attached to each other; finally the box was completed by attaching the sides to the base. The heating elements, which had an adhesive backing on one side, were attached to the bottom of the box as shown above.

Oblique view- created with Bryce 4.0
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Overhead view- created with Bryce 4.0
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When loaded with germination cups under fluorescent lighting, the temperature in the grow enclosure, measured at the center of the inside height of the enclosure, was consistently 7 to 8^oF warmer than the ambient temperature outside the enclosure.

The biggest improvement was seen in germination percentages; changes in initial growth were mixed. I made two comparisons: an overall comparison between 2000 and 2001 that considered all seed types, and a narrower comparison that considered only Cubanelle, NuMex Big Jim, Scotch Bonnet, and Yellow Corno di Toro, which were grown both seasons from the same seed stock (thus the seeds used for these varieties in 2001 were 1 year older than the seeds used for the same varieties in 2000, and would thus be expected to show, if anything, decreased performance).

	Germination %		Days to Show		Days to Stand
	2000	2001	2000	2001	2000	2001
Overall	50.8	64.6	10.9	8.9	12.5	12.1
Narrowed Group	60.0	67.0	10.2	10.5	12.6	13.8

The overall comparison, which is not a very good one since the varieties used differ greatly between the two years, shows big improvements across the board. The 'narrowed group' comparison is better, as the same seed stocks are used, but is still not a perfect comparison because it was not performed side-by-side. The 'narrowed group' data also showed a significant (but smaller) improvement in germination percentage, but shows a decrease in early growth rates. Given that the seeds were a year older in 2001, I would expect to see (in anything) decreased performance in 2001, so the decline in early growth rates is not too surprising.

Since I didn't do a side-by-side test in 2001, and instead used data from two different seasons for comparison purposes, I can't say for certain that using the heated enclosure provided either a benefit or was detrimental in terms of early growth rates. However, even considering the imperfect nature of this 'experiment', I feel pretty good about claiming that using the heated enclosure improved my germination percentages.


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