Introduction: Heated Propagator

Picture of Heated Propagator

In this instructable, I'll take you through the construction of a heated, temperature controlled propagation bed which is useful if you'd like to germinate seeds in cold climates to get a head start on the growing season. I grow chillies which require a long growing season in order to produce mature fruit. Chillies also require higher than average (for my climate) temperatures to germinate reliably and being able to control the temperature precisely will usually improve yield.

I used a commercial soil heating cable which is the best bet if you are at all unsure about mains electricity, but there are other options for the more adventurous.


  • 1x Commercial soil heating cable, mine was a 75W one
  • 1x polythene sheet, available from building supplies merchant
  • 1x underfloor heating controller with external sensor
  • some lengths of 6" - 9" x 1 - 1 1/2" softwood timber
  • 1x piece of ply 18mm/1 1/2", dimensions as per later discussion
  • 1x piece of PIR insulation board, preferably 50mm/2"+ thick, same dimensions as above
  • horticultural grade sand - I needed about 60kg but that will depend on your exact box dimensions

Step 1: Build a Wooden Box

Picture of Build a Wooden Box

I built my wooden box out of some lengths of 6" x 1" softwood timber with a piece of 18mm or 3/4" ply for the base. I joined the corners by mitring and screwing into 6" lengths of softwood square section, about 1" - 1 1/2" or 25-40mm square.

The dimensions aren't critical but remember to check the recommendations in the heating cable instructions.

I didn't document that part as I'm sure there are good box building tutorials out there already :-)

Step 2: Cut PIR Board to Fit Box

Picture of Cut PIR Board to Fit Box

Cut the PIR board to fit snugly inside the box, so it can rest on the ply bottom panel. I used a box cutter and scored each side of the board along a straight edge then gently tapped the board to crack it.

Step 3: Cut the Polythene Sheet to Size and Fit

Picture of Cut the Polythene Sheet to Size and Fit

Cut the heavy duty polythene sheet to size. I opted for a double layer but that was probably unnecessary.

Step 4: Add the First Layer of Sand

Picture of Add the First Layer of Sand

Fill the box with a layer of sand about 50mm/2" deep.

Step 5: Lay the Soil Warming Cable

Picture of Lay the Soil Warming Cable

As you work your way around, place some damp sand on top of the cable to keep it in place while you're working. Also, my cable recommended being switched on for 5 minutes or so before use, in order for it to be more pliable.

Lay the controller's external sensor cable, placing the sensor in the centre of the bed.

Step 6: Final Layer of Sand

Place another 50mm/2" layer of sand over the cable being careful not to disturb it.

Step 7: Wire Up the Controller

Picture of Wire Up the Controller
  • attach a mains socket back box to the wooden box
  • remove the mains plug from the soil warming cable and prepare the conductors
  • following the guidance from the controller manufacturer, attach the warming cable to where the controller has 'load' marked
  • attach mains cable to terminals on controller, again following the manufacturer's guidance, mine was marked L N E
  • attach the external sensor to the correct terminals
  • pulling the wires gently through the grommet, push the controller face in to the socket back and screw in place.

Step 8: Just Add Water, Power and Plants!

Picture of Just Add Water, Power and Plants!

Remember to keep the sand bed moist for best performance and least risk of burning the cable out.

Apply power and check the bed is warming as expected. It will take a long time to heat up and become stable.

Here you can see some happy chilli seeds enjoying perfect conditions.


sagelike (author)2015-05-05

nice setup and great 'ible, but I was wondering, with the cost and amount of items required, would it not have been cheaper and easier just to buy an actual propagation heating mat which are also temp controlled? I might be missing something as I am fairly new to starting seeds indoors, but I'm from a cold climate (Alberta, Canada) as well, so I understand requiring the additional heat.

jsr38 (author)sagelike2015-05-11

Hi there fellow gardener from even colder climes!

You're right - the costs are comparable but I think still slightly cheaper if you're able to find good sources.

My costs were as follows:

digital temperature controller about 10 GBP from ebay
timber from wood recycling centre 12GBP
sand - I ended up paying rather a lot which was about 15 GBP
heating cable 25GBP from ebay
total: 62 GBP

I just looked up the price for a temperature controlled purpose built mat which was 80 GBP.

Also, the thermal mass of 60kg of damp sand is rather a lot so it might be more temperature stable.

I must admit though, it's not easy to move around ;-)

Tecwyn Twmffat (author)2015-04-05

I've been propagating hot chilli peppers here: and somebody said to try using a digital water bed heater, which sounds like a good idea as there's no wiring to be done?

jsr38 (author)Tecwyn Twmffat2015-04-05

Hmmm. Sorry. Can't help you. I don't even know what a water bed heater is! Good work though for not doing mains wiring if you're not comfortable with it.

Tecwyn Twmffat (author)jsr382015-04-05

I can do high-voltage electrics, but it really is down to pure laziness at the end!

tomatoskins (author)2015-03-15

Here in Utah I've never used a heater to germinate my seeds. Usually just a grow light in the basement has worked for me. It must be much colder where you are.

jsr38 (author)tomatoskins2015-03-15

I imagine that's probably right. I am in the UK which is a fair way further north than you are. My yields are close to 100% with this setup.

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