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

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

We 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 plans

Building 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 installation

Installing 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 fleece

You 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.

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