Garden ProjectsPropagation

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

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.

  1. Sandra Lane

    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

  2. Andrew

    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

  3. mr w. l. briggs

    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 .

  4. John Davis

    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

  5. Hazel Gray

    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)?

    1. admin

      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

    1. admin

      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.

    1. admin

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

  6. Tim

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

    1. admin

      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

  7. Keith

    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

    1. admin

      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

  8. Andrew Tindale

    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

    1. admin

      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

    1. admin

      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.

  9. Rdwsrd

    How much sand should I put on top of the cable? Do I put a particular conpost layer straight on top of it or in pots? How long should I leave the seedling in the heated bed?

    Thank you,

    1. admin

      Hi Edward
      Apologies for the late reply to your question. You can add 2 inches of sand on top as well as below the cable. Place pots or trays on top of the sand. I would not cover with compost or plant roots will become tangled in the cable. It depends what you are growing and what time of year it is but for outdoor crops you are only using the bench to germinate the seedlings, once the plants have come up you can take them off the heat. For warm climate crops like tomatoes, chilis or cucumbers you can leave them on the heat until the weather is warm enough to plant them out. I hope this helps.

  10. George

    Hi Andrew,I have just got a new greenhouse after moving homes and will be putting my home made propagater with soil warming cables and thermostat back into use however I was never sure what type of sand to use and welcome your advice on this. One thing I did when it was last in action was to cover the top layer of sand with kitchen foil this I thought spread the heat evenly and the sand kept moist for longer requiring less watering I will discontinue this practise if do not approve
    Thank you

    1. Andrew

      Hi George
      I have not tried using foil on the sand, our heat benches are so big and have such a high turnaround that it wouldn’t be practical.The only thing I would say is that the moist sand is helpful to keep the compost in the pots or trays moist whereas the foil could get hot on a bright day and dry the compost. As regards sand I use sharp builders sand from the builders merchant. I would avoid soft sand like the stuff you get for children’s sand pits. I hope this helps. Andrew

    1. Andrew

      Hi Nigel. I am not sure vermiculite would work very well as I don’t expect it would be able to spread the heat the way wet sand does. My opinion is the air spaces in the vermiculite will be too large for effective heat transfer to work.
      I hope this helps

  11. andrew

    i am very keen on heat cable for early spring seedling.
    i would like to grow seedling in the beginning of march in my polytunnel unheated.
    can you advice me on the wattage that i should choose ? you know outdoor winter and early spring is still very cold.. too cold for germination and seedlings..

    to choose a small power as bed heater is might well not using any. to choose a too large power,, i fear might cost me more than to purchase seedlings from store..

    what would be the proper power for me according to your level of expertise ?

    how much different is it to have a heat bed made of sand and styrofoam as base, then cement plate with styrofoam as base ?



    1. Andrew

      Hi Andrew

      The length of the heat cable you need will be dictated by the size of propagation heat bench you are making. Our cables are as follows: 6 meters – 50watt, 10 meters – 100 watt, 25 meters 320 watt. I would also use a thermostat so the cable is only powered up when needed which will save power. I do not expect cement to work as a base as it is the water in the damp sand that distributes the heat. The horticultural heat cables we supply are not designed for use in a concrete base.

      I hope this helps


  12. Manni

    Does the damp sand also do the watering , or can I use this with capillary matting?
    How warm is too warm? If I’m starting brassica seedlings to plant outside, when do I move them to the cold frame?
    I have so many questions! Can you recommend a good book on propagation?

    1. Andrew

      Hi Manni
      The sand does not do the watering but it will help prevent the compost plugs from drying out. You will also need to water from above as required. Brassica seedlings will germinate best at approc 18 degrees celcius. It depends on the time of year as regards moving them, in general I would take them off immediately after they germinate and put them in a bright, frost free place.

    1. Andrew

      Hi Manni
      Yes, you can use capillary matting with a soil warming cable heat bench but I would remove periodically to soak the sand below. I have not actually tried this personally but I can’t see why it wuldn’t work for you.

      1. Samantha Brown

        Hi, I have just taken over a greenhouse which I am presently cleaning down and de whiteflying. Should I clear out the old sand in the heated propagation tableand start again or just top up the top layer with fresh sand?

  13. Samantha Brown

    Hi, I have just taken over a greenhouse which I am presently cleaning down and de whiteflying. Should I clear out the old sand in the heated propagation tableand start again or just top up the top layer with fresh sand?

  14. Rach

    Hi, I’m a new propagator and I do have a lot of questions because I wanted to build a rectangular frame instead of square.I wanted to know why the distance between cables have to be 8 cm, can I use plywood instead of timber planks? does it have to be 6 in. deep?

    1. Andrew

      The distance between the cables is determined by the power output of the cable and the ability of the bed to conduct heat.

      Plywood is fine and the depth isn’t too critical.

  15. Dustin Bajer

    Thank you for the article and YouTube videos. I have a quick question about the wattage of the heating cables – I’ve done some searching online and have seen everything from 80W to 300W – have you found that the 80W cables you use are sufficient to keep the temperature where you want it and at what outside temperature do you think that they would start to struggle?

    I ask because I plan on using my bed to start dormant hardwood cuttings when it’s still cold (below freezing) outside. The theory – which I’ve seen others do – is that the cutting stays dormant above ground while setting roots in the warm soil. If you were doing something similar, would you add a more powerful wire or would you consider spacing the wires closer together?

    Thank you,
    Dustin Bajer, Forest City Plants in Edmonton, Alberta, Canada

    1. Andrew

      Hi. Our soil warming cables have different power ratings according to their length – 6m (50watt), 10m(100watt) and 25m(320w), which will also determine their load. They tend to cope well with subzero temperatures although we do cover the plants with fleece overnight. Adjusting the spacing will increase the dissipation area but ultimately has no relation to the power rating of the cable.

  16. Michael Doyle

    Rather than make a wooden box I have a large plastic heavy duty oil drip tray, 150cm deep x 1000 x 550cm, could I use this for my parasene warming cable and thermostat?

  17. Joseph

    If I was to set up a heated bench like this in a concrete garage complete with grow lights, am I likely to have any luck growing chilli plants during the winter? Would it be beneficial to build a perspex ‘tank’ over the top of the bench to help keep the heat around the plants too, or is it just the roots and soil that needs to be heated?

    1. Andrew

      Hi Joseph. I would suggest this would be a lot of work and expense to grow chilies. The air temperature would also need to be warm enough for the chilies to product fruit and given the lack of air circulation in a garage I suspect your will be fighting against fungal diseases. Technically it would be possible with a lot of heartache, I would be inclined to give it a miss.
      I hope this helps.

    1. Andrew

      Hi Patrick. I wouldn’t worry about the sensor, it just needs top be moist and can never be too wet. If you can feel warmth on the sand surface when you lift a pot or seed tray it is OK. If it’s too dry it won’t transmit heat. I would water mine every 2 weeks or so. I hope this helps. Andrew

    1. Andrew

      Hi Tony. You will need 20m of cable to cover this area with the cable placed at the normal spacing of 8cm apart. The cables are only available in 6, 10 and 25m lengths so you will need the 25m version. You can fit it by placing the cable loops 6cm instead of 8cm apart. I hope this helps.

  18. scott berridge

    Can i use builders sand for bottom layer and then lay cable and use small stone chippings because i can’t get hold of any sand at the moment?
    best regards Scott

    1. Andrew

      Hi Lynda. I would not use pea shingle as it will not hold enough moisture for the cable to work. You need to use sand as it is the moisture trapped in the sand that transfers the heat, pea shingle is much larger and will not work.

  19. John Brown

    Andrew I have built a bed on a formica top 50mm sharp sand soil warming cable 50 watt 6mtr and another 50mm sharp sand and pvc cover on top plus thermo2 stat with monitor in between s bends it it right and do have keep sand moistened John

    1. Andrew

      Hi John. Yes, you need to keep the sand moist as it is the water molecules that distributed the heat, if it’s dry rot won’t work. This is why I recommend lining the bench with a plastic sheet to keep as much moisture in as possible.

  20. Steven mcculloch

    Superb information on your site regarding sowing and germinating. I’ve been growing veg for years, but never quite understood the link between heat and light. Thank you

  21. Dave D

    Hi, I’ve set my system up as described, my boxes are approx 90cm x 60cm. 5cm sand below, 6mtr cables laid no more than 3 inches apart , another 5cm of sharp sand, using bio green thermo2 set to 18 degrees but it just will not get to anything above 14/15c.
    I have watered the bed. The greenhouse ambient temp is very low, around 1 or 2c as it’s soo cold outside. Could this be a reason ??. My other concern is working out how wet to make the sand , at one point the sand was really gloopy so i sponged some water.

    1. Andrew

      Hi Dave. The ambient temperature is the issue, if it is very cold in the tunnel/greenhouse the cable will not be able to overcome it. The cable can only warm the soil by +12˚C so if it is 1 or 2˚C outside the best you will achieve is 14˚C. Once temperatures rise a little you will get to 18. I hope this helps.

  22. Patricia Mumford

    Is it essential to have 2 ” of sand above and below cable. I want to use a plastic tray but will only be able to get 1″ above and below

    1. Andrew

      Hi Patricia
      it is important to have enough sand at it is the water in the sand (it must be damp) that holds and transfers the heat. I guess you could try with a smaller tray but make sure you keep the sand moist.

  23. Sandra

    Hi Andrew
    Ive got my hot bed finished and working but is there a way to keep the sand moist without having to remove plants and seed trays .thanks for your help

    1. Andrew

      Hi Sandra
      If the bench is well soaked and covered with pots and trays it will stay moist for a significant amount of time. If you used a waterproof liner as per the article you will also be able to remove one or two trays and add a significant amount of water which will dissipate over the full bench (provided it is level).
      I hope this helps

  24. Bridie Drain

    I used sharp builders sand in my heated propagater and the sand below the cables dried out and caked in lumps . I believe this is because of the salt in it. It’s difficult to reset or keep wet. I should have used washed sharp sand or horticultural sharp sand

    1. Andrew

      Hi Bridie
      That is a good point. I have had builders sand cake when it dries completely but if you completely soak it for a few days it comes back around. I take your point that washed sand may be better.

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