How to Make a Heat Bench Propagator Using a Soil Warming Cable.

Vegetable propagation equipment

tomato seedlings on a heat benchWe propagate large amounts of vegetable seedlings in the early spring so need some heat to get them going. Clearly a heated propagator or heat mat isn’t really practical as we need such a large area so we use soil warming cables buried in sand to provide the warmth we need for germination. There’s a little bit of work in setting up the bench but once it’s in place it’s simple to use and should last you for years.

What’s the plan?
The idea here is to embed the cable evenly in a box of sand covering the are you want to heat, it’s a bit like an electric blanket but sand instead of a blanket. We need to build a box at a convenient height strong enough to hold a decent quantity damp sand and all your seed trays. Here’s the list of stuff your need:

  • A base for your bench. Old pallets or crates are handy.
  • Sharp builders sand available from your builders merchant.
  • Foam insulation board.
  • Heavy gauge waterproof polythene.
  • 6 inch high planks for the sides
  • Timber board for base
  • Corner posts
  • Screws or nails to hold it all together.

N.B. If you know a friendly builder they will always have odds and ends of insulation board and DPC plastic, it’s worth a go. You could also ask builders supplier if they have any damaged pieces.

Build your support
I’m being a bit vague here because really it all depends on what your have to hand. For a domestic size heat bench an old table will be good (I saw 2 in the dump the other day) or as per the list pallets or crates are handy.
heat bench soil warming cable plansBuilding and insulating your base
Make up a frame using 6 x 1 inch boards and 2 x 2 inch corner posts. It’s square in the illustration but obviously can be any dimensions you like. I find it easier to use the corner posts instead of screwing straight into the planks as it gives a much stronger frame and gives you plenty of wood to screw the base to in the next step.

You will be placing a liner in the heat bench frame so untreated timber will be fine to use as the timber won’t be in contact with the damp sand.


Attaching base to homemade heat bench

Screw or nail the base board to the frame to make a box with a bottom. Cut the insulating board to size and add it to the base of the propagator box.

Here’s the bit everyone gets wrong:
It’s important that the sand remains moist to transmit the heat evenly across the heat bench, if the sand dries out the cable won’t be anywhere near as effective.

Line the box with a layer of waterproof plastic sheet, there’s a dark grey heavy D.P.C. plastic available from builders merchants in bulk rolls but you should be able to get something similar in smaller quantities.

Electric propagator heating cable installationInstalling the heating cable
Cover the insulated base with a 2 inch layer of sand. Arrange the cable in a linear fashion, as we’ve said, a bit like an electric blanket. Cables vary but in general they should be placed 3 inches or 8cm apart. Don’t let the cable cross over itself as this creates a hot spot which can cause damage and failure of the unit.

Once the soil warming cable has been laid cover with another layer of sand. Water the sand, we’re not looking for puddles here but it should be thoroughly moist. At this point you can plug in and start to warm up your bench but I do recommend adding a thermostat to regulate the temperature and make the bench cheaper to run.

Adding a thermostat
To be honest I can’t understand why you wouldn’t put a thermostat on any heated plant propagator. If you don’t it will be on all the time wasting energy and reaching a temperature that may be unnecessarily high to germinate your plants. Like any thermostat the desired temperature can be set meaning the heat cable is only on if the temperature falls below the required level.

Parasene thermostats
We supply soil warming cables with a compatible thermostat that measures the temperature in the sand layer around the cable. To install drill a hole in the side of your heat bench to allow you to slot the probe into the frame.

The soil warming cable is attached to the thermostat by removing the plug and joining the wires to the terminals inside the cover as per the instructions. Do not cut the other end (like some customers, you know who you are) with the blue plastic cover as this will damage it beyond repair.

Soil thermostats
There are other thermostats on the market (BioGreen for example) which use a small probe at the end of a wire that is inserted directly into the compost. These give a reading of the compost in the particular pot you’ve placed it in but will give you a good idea of the temperature of everything else in the propagator.

Using you heat bench propagator
A good average germination temperature for most crops is 18 to 21˚C.  Depending on the air temperature inside your tunnel or greenhouse your soil cable will heat between 18 and 21˚C (isn’t that a coincidence?) at 8cm apart. If the air temp is colder and you need more heat you can place the cables closer together but in normal usage this is unlikely to be necessary.

Seedling plants protected by horticultural fleeceYou can keep your heat bench warmer by making a cover for it but we just use a few layers of horticultural fleece laid over the seedlings to keep conditions nice and toasty. Fleece is good because it’ll keep the heat in but let light through so your seedlings won’t get leggy from lack of light once sprouted. As you know seeds only need heat and moisture to germinate but once they’ve popped up they need light or they will grow tall, weak and spindly.

Also, with the exception of warm climate crops like tomatoes, courgette, chilies etc… most plants should be taken off the heat bench once the seedlings have emerged. You must protect from freezing night time conditions and a layer or two of fleece will again work perfectly. Fleece is a very light material which will be pushed up by the plants as they grow so won’t damage or hold back tender young plants.

Warm climate crops, once germinated can be left on the bench with the temperature turned down a little, as a rule of thumb about 18˚C is fine.

As we’ve said the bench must be kept moist, check the sand under the pots periodically and water as required.

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26 Responses to How to Make a Heat Bench Propagator Using a Soil Warming Cable.

  1. H mitchell says:

    Thank u information excellent

  2. Sandra Lane says:

    Thank you so much.
    I’ve had a cable warming bench for some time but was never sure if I had set it up right or was
    just using it incorrectly.
    Your article was very clear and the information has shown me exactly where I was going wrong .

    So glad I found it in time for the coming season.
    Thanks again

  3. Andrew says:

    I have just been given a Rodstat warming cable, and was not sure how to use it. I grow cuttings of fuchsias for show plants. Your information was very helpful. Thanks

  4. mr w. l. briggs says:

    very helpfull I nead to root dahlia and chrisanth cutings instead of buying plants each year so I need to know the cost of cable and thermostat .

  5. John Davis says:

    My homemade propagator temperature fluctuates over a great temperature even after fitting a new cable and thermostat from yourselves last year and didn’t have time to experiment with it have you any idea where I’m going wrong after following your building instructions MENY THANKS PS. DESPERATE FOR A SOLUTION!! Getting near that time again

  6. Hazel Gray says:

    I’ve a new propagator and the thermostat is set at 20; it’s been on for 24 hours. Should the sand feel warm (it doesn’t)?

    • admin says:

      Hi Hazel. Have you placed some pots or plant trays on the sand? The sand should feel warm under the tray but often doesn’t if the not covered. If you are concerned that the cable isn’t heating dig a small hole in the sand and feel the cable, it should feel warm to the touch. I hope this helps. Andrew

  7. Dave says:

    Can you tell me roughly how much it costs to run please

    • admin says:

      Hi Dave, It is difficult to say as regards running costs as it depends on outside temperatures, whether you are using a thermostat or not and what you have it set at if you do. What I can say is we have 8 of these benches on the go from late Feb to mid April and they cost is about £150 to run for that period. Obviously that is a lot more than you would need so I don’t expect one bench will make much difference to your electricity bill.

  8. WILLIAM BROWN says:


  9. Susan says:

    Can you cut the warming cable if you have too much?

    • admin says:

      Hi Susan. No, you must not cut the soil warming cable, it will destroy the cable which will not be able to be repaired.

  10. Tim says:

    Do you need to water the plants or is the moisture from the sand penetrating the soil of the trays and providing adequate moisture ?

    • admin says:

      Hi Tim.
      Thank you for getting in touch. Yes, you definitely need to water the plants, the moisture in the sand is really only for the even transfer of heat from the cable. I would soak the seedlings every second or third day depending on conditions.
      I hope this helps

  11. Keith says:

    Hi, I would like to build a heated bench, for spring use, my problem is the space for the bench is my summer growing bed in lean to, used for growing tomatoes. How low would be ok for a bench of this type,Could I grow my tomatoes in pots on the bench if it were ground level

    • admin says:

      Hi Keith. It makes no difference how high the bench is off the ground provided the base in insulated as per instructions. Yes, you could easily have a propagation area at ground level and place pots on it in the summer. You would need to cover the sand with a board or plastic sheet to stop any tomato roots growing into it and becoming entwined in the heat cable. I hope this helps. Andrew

  12. Andrew Tindale says:

    Hi can anyone tell me the correct position for the thermostat . Is it the same height as the warming cable or placed just under finish height of sand

    • admin says:

      Hi Andrew. Are you using a Parsene thermostat? If so put it just under the sand as it is the sand surface temp you want not the cable temp. I hope this helps. Andrew

  13. Bill Nicholson says:

    my cable is to long for my box, what should i do.. ???.. ill have to cut it just to use it and seal the end, what do you advise ??

    • admin says:

      Hi Bill.
      Do not cut the end of the soil warming cable as it will render useless as you will break the circuit. I would use all the cable anyway. If you are using a thermostat you won’t be using any more power as the unit will shut off when it has reached temperature. You will also be able to get your bench a little hotter if required.
      I hope this helps.

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