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Sunday, January 25, 2009

Homemade heat

OK, now for a 'real' propagation project - not just a basement clean/rearrange. ;-)

I've been drooling over those heat mats that go under trays, but they cost a ka-boodle! (Priced at 20" x 48" = $90-$100 Jungs and Parks) . Ouch! And they probably take a lot of wattage, saying they will bring soil temps up to 85-90F. *pant*

So I came up with this idea. Most items I already had on hand, but did need to buy a couple of things. What I had: caulk (silicone/latex), a piece of corrugated plastic (48" x 23"), some cedar molding, and a bag of plastic corks. All I had to buy was 12' length of rope light and a 6-pack of 48" x 14.5" x .75" expanded foam sheets.

My goal was to make an insulated tray filled with rope lights then covered with a sheet of firm plastic. That way the heat from the lights could only move UP, not down or out due to the foam. I decided to go for broke and went for 4' long - that would easily accommodate 4 trays. Luckily the width only needed to be (outside measure) 23" - which was as wide as my piece of plastic. So first thing I caulked 2 sheets of foam together along their length.

Then I caulked my sheet of corrugated plastic so I could 'glue' it to the foam.

Squared up the edges.

And cut off the excess foam - carefully. I needed that piece to make the walls of the tray.

I cut that excess piece of foam and used it to make the sides of the tray.

To arrange the rope lights in the tray, I cut plastic wine corks to the same height as the foam, then hot-glued them to the tray. Here I've used a piece of glass to hold the lights down while I arranged the loops and placed the corks. (A tip, let the lights warm up 3-4 minutes. The rope will be softer and easier to manage.)

Here all the corks are glued down. The 'longer' corks are those I trimmed length-wise and laid them down the center. Both the guide corks and the long corks will support the plastic cover and then the plant trays without sagging. *Note* The rope lights are NOT glued down so that if they need to be replaced or I find that 12' will not give me enough bottom heat, I can easily remove them. (And, should I need to put in an 18' rope, it will not be a hardship to obtain more corks! *sigh* Oh, the sacrifices we do for gardening! ;-) )

Then I cut through the side of the tray to push out the ends of the rope (which are thicker than the rope itself and would NOT have fit within the height of the foam strips).

Then I made a collar out of 1.75" cedar strips. Now all I need is to get a 2' x 4' sheet of plastic, trim it to 23" wide and lay it over the foam tray. (The plastic cannot intrude onto the collar; I have plans to use the collar. More on that later)

Dateline: 4 days later:
After trying out this configuration, I found that there was not as much heat as I'd hoped, so I exchanged the 12' light rope (36w) for an 18' light rope (54w). This time I didn't hot glue the corks, I hot glued the rope -- this rope had a mind of its own! It would not lay flat by itself. When it had been 'tamed' I laid in the Plexiglas.

Within 20 minutes, it got over 85F under the plexi (while covered with foam).

A 2-pack cell was set on the plexi and then covered with an inverted clear plastic tray, a mini greenhouse.

20-25 minutes later, clear results. The soil was heating up (and continued to climb to 74F). Success!

Super results as far as I'm concerned. $10 for the rope light and 1/2 a pack of foam sheets. Total = $13.50. And since it only burns 54w, running this heater won't break the bank. So now, between my 2 existing light-bulb germinators and this tray I will have bottom heat for 6 trays.

And while it's really too early to start seeds, I don't think you can ever plant a little lettuce too early.
"Let's go, Igor.... off to the plant laboratory... woohaha!" :-)

If you find this post informative and you make your own or link to this post, please let me know by leaving a comment.  A little encouragement goes a long way ...  and Igor likes to get comments:-D


  1. great use of your brain and materials, will try this in a smaller scale (6ft light) to fit into my experimental,large tote light box.

  2. Anon - thanks! And good luck with your project. Would love to know how it turns out. Happy growing!

  3. Your whole blog is wonderful and helpful - keep doing it! I have it bookmarked and will visit again - please post on GW again when you add more to the blog - LOVE IT! Also, I emailed you directly, so if you add content, please send me an email. Kay

  4. excellent suggestion !

  5. Thank you for the informative display... I purchased a small shelving unit (3 metal shelves) for my first attempt at starting seeds in the basement... I'm relatively new at all of this but last year's attempt at seedlings near the window didn't give me great results... was wondering about the heating... thanks for the idea... thinking through how to adapt this to my small shelving project... wooohaaaahaaa!!! ;) Liz

  6. Good luck with your shelving/heating project, Liz. Let us know how it goes for you. :-D

  7. Thanks Kris! Amazing post/project!!! We're going to start growing microgreens, and this set up would be perfect to get them started. I really appreciate you sharing your process! =) Deb (under Jeff's sign in)

  8. Can I ask where you got your rope lights?

    1. Jeff & Deb: I found various lengths of rope lights at Lowes and Home Depot. Glad you 2 found the post interesting. I get a lot of use out of the setup. Over time, what with the plexi on top, I found that the rope has melted a channel into the styrofoam. Doesn't seem to be a major issue since I only use the heat for a few weeks in Spring. With your microgreens using it more often, you might consider more venting than I have. Good luck - and keep me posted. :-D

    2. P.S. Are you going to blog about your microgreen project? If so, would appreciate a link if you'd like to share your adventure....

    3. Thanks for the venting tip! Don't have a blog, but will make a note to leave an update comment for you =)


Looking forward to hearing from you. :-D