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The Homesteader School » Growing Food » How to Make a DIY Heat Mat For Seedlings

How to Make a DIY Heat Mat For Seedlings

When you are learning to grow from seeds, often you might see reference to a heat mat for seedlings.  Heat mats can really help speed up germination for many seeds, especially tomatoes and peppers.  You can buy them, but here I will show you how to MAKE one.

Here’s what I used:

  1. 2 old humidity domes for seed flats (the flats had been broken)
  2. 1 string of indoor/outdoor Christmas lights (the orange lights were not working)
  3. All-purpose sand

That’s it!  It’s actually quite simple.  I thought of it when I was watching my dad install the tubing for his radiant floor heating.  You can use pretty much anything for the tray.  Even old drawers!  I actually have an idea bouncing around in my head for a grow rack made from an old dresser… anyway, back to making a heat mat for seedlings!

The basic concept is this – set out your trays, whatever you want to use to hold it.  They need to be at least the same size or bigger than the seed flats or boxes you will be placing on to them.  Arrange the lights in such a way that they are on the bottom of the tray and evenly spread out.  Kinda like this!

Making a DIY Heat Mat for Seedlings

Then I poured sand over the top to almost completely cover the lights.  It went a lot better if I physically held the lights down while I poured the sand, less lights poking out the top.  It ended up looking like THIS:

Making a DIY Heat Mat for Seedlings

After that, I just set the seeded flats right on top of it.  You can use pretty much anything for this, anything that has sides that are at least a couple inches high.

Heat Mat for Seedlings

OK, the photo above has unseeded flats on it just for demonstration.  So here’s a photo of the flats that I actually DID seed and use with this.  This is a day after sticking them on there.  Note the condensation on the humidity domes – the sand does a phenomenal job of holding and distributing the heat from the Christmas lights.  Consider, also, that this is in an unheated garage, it’s probably in the 30s in there normally, maybe in the 40s.  Inside the grow rack, it is just the right temperature for my seedlings.  You can feel the warmth when you stick your fingers in the sand – not hot, but a nice gentle warmth.

DIY Heat Mat for Seedlings

The best part?  The whole thing cost me a big fat whopping NOTHING!  Much better than that!  Having said that, if you do decide to opt with buying a heat mat for seedlings (and let’s face it, $30 isn’t really THAT much), feel free to check them out at, which is where you generally will find the best prices.  Anything purchased through the below link will help go to support this site (at no extra cost to you!) so we certainly wouldn’t mind if you bought one from them ;)

——>> Find Heat Mats for Seedlings on Amazon

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26 Responses to "How to Make a DIY Heat Mat For Seedlings"

  1. Wretha says:

    Very ingenious! Thanks, I wouldn’t have thought of something so simple. :)


    1. Bethany says:

      Thank you!

  2. Charlotte says:

    This is the best new idea I’ve come across in a while! I want to expand my indoor garden to a larger project in the basement, but the cost of heat mats or otherwise heating up a space held me back last winter. Now I’ll have to look around for free sand since I don’t have any… Thank you!

    1. Bethany says:

      Well thanks! I can’t really claim original because I’m pretty sure I read it somewhere ages ago, but it sure worked perfectly :)

      1. dorie says:

        Hi. I was wondering if you could use just the regular white christmas lights cause i dont have any colored ones? Thanks!

        1. Bethany says:

          Hi Dorie!

          Yes you can definitely use the white ones – icicle ones, pretty much whatever will work.



  3. [...] to see if I could do better than that, so I puttered around on the internet for a while and found this link.  So I scrounged around my house and garage and this is what I came up [...]

  4. Kat says:

    What a wonderfully simple idea! Many thanks for sharing this. I’ve got sand and extra Xmas lights, so I’m good to go.

    1. Bethany says:

      Thanks and you’re welcome. Let me know how it works out!

  5. Matt says:

    VERY NICE! So simple, so cheap, and looks to be very efficient way germinating seeds. Thank you for the post! Almost everyone has extra xmas lights (or something similar). Gonna use to start some chili seeds, thanks again!

    1. Bethany says:

      Thanks Matt! Let me know how it works for you :) I’m just about to set this system back up to start my own seeds for this year. I can sat with confidence that it really helps the chili and tomato seeds with germination, even in a cooler temperature like a garage or shed.

  6. Tammy says:

    I love this mat! How many hour do you leave it on for and how many days?

    1. Bethany says:

      Hi Tammy! It really depends on what you want to do. Generally most people leave it on constantly until the seeds have sprouted. You can turn it off at night, which can help some seeds, but it depends on what you are growing. I just leave mine on all the time and then unplug it when most of the seeds have sprouted.

  7. Jodie says:

    Thank you this is awesome idea
    I’ve been struggling for years to find the right way to heat my seed most of them dying

  8. Tomeco says:

    Can you use a baking foil pan?

    1. Bethany says:

      Hi Tomeco,

      Yep I think a baking foil pan would work just fine!


  9. Doug Bishop says:

    I used an old water bed heater. It even come with it’s own thermostat!

  10. Steve says:

    FYI: Your link searches for “heating mat” on Amazon and returned only three items. Changing the search to “heat mat” returned hundreds.

    1. Marie says:

      Steve, thanks for the tip–we’ve changed our search term to “heat mat.” You’re right–lots more options that way!

  11. John Campbell says:

    Can you use a water bed heater as a heat mat?

    1. Marie says:

      John, I’m sorry we just now saw your question! Apparently our notification system didn’t work. We’ve never tried using a waterbed heater, and I’m thinking it might actually heat up too high for the plant trays. It might work under sand though, just to heat the sand, which would heat the soil tray above it. Anyone out there have an idea?

  12. LexingtonNC says:

    I’ve seen rope lighting used as well. It has the advantage of being water proof.

  13. [...] you can be prepared to raise your own seedlings with an assortment of seed starting containers, heat mats, lights, grow racks, and perhaps even a cold frame or [...]

  14. […] have combined the two together in my own version and have built something I think will work well.…for-seedlings/…-seed-starting I liked the idea of the sand as it will […]

  15. […] Even if they are deep they will still come up. I think they are just being slow because they don't have the bottom heat, still it's only been a few days so without bottom heat it will just take longer. Just continue keeping them in a warm place and eventually they should break the surface Or you could just make your own bottom heat~I would love to see someone make this one and see how well it works for them!…for-seedlings/ […]