Having run out of daylight again last weekend, work continues on the final elements of the heated propagation bench.

The last thing I did was to line the frame with damp proof membrane. At this stage it doesn’t need to be too carefully pushed into the corners as the weight of the sand will do that. I did consider whether to insulate the sides as well as the base as I have left over Celetex, but I would lose about 3 sq feet of propagating space as it’s quite thick. I figure that space is more important to me than some lost heat.

Building a heated propagation bench
Basically a pond in the greenhouse

I am using sharp sand (£20 for 13 large bags in B&Q), partly because it is cheap but also because it has an open structure that should aid capillary action into the pots (as a source of water). A 2 inch layer of sand first, leveled off.

Heated propagation bed, greenhouse.
First layer of sand in

The soil warming cable (£34, Amazon) goes next, carefully laid out on the sand. The closer you coil it the hotter it will heat the surrounding area but obviously the less space it will cover. I have a 12m cable so I have aimed for 12-15cm gaps which should be just about enough to warm the whole bed.

Soil warming cable for heated propagating bed
Laying out the soil heating cable

Per the instructions I plugged the cable in for 10 minutes to make it more pliable but it was still a job to get it to lie where I wanted it. As you can see it ended up quite wiggly. I found that the cable isn’t quite long enough for the area of bed I have but the 24m cable would have been too long. In the end I compromised and opted to have a cooler end of the bed with no cable.

Soil heating cable, propagating bed
The cooler end…

I’ve drilled a hole for the cable to feed into the bench at the right height. This is also where the the thermostat would be wired in, if/when I get one. Once in place I covered the cable with a further 2 inches of sand, leveled off.

Make a heated propagation bed for your greenhouse
Burying the cable in sand

Although the bench is in the greenhouse, I will be able to more closely control the humidity on the bench if I provide a closed area. This is particularly important for cuttings while they are in that vulnerable not-yet-rooted stage. It will help keep the heat in and knock back any strong sunlight a bit. I’m doing this with slightly opaque 250 micron polythene sheet (£11.99 for 2mx4m roll on Amazon) which is typically used for polytunnels so it’s nice and strong and UV stable. This is stretched over hoops made from 25mm plastic piping which I have had lying around for years (£0). I need to be able to open it easily for access and ventilation so I’ve attached the sheeting firmly on the rear side and just left it loose on the front so it can be furled and unfurled over the hoops. To make this a little easier I’ve stapled the front end to a length of battening to give it some weight. I’ve also covered the outside of the end hoops, using the same polythene, so that there is a closed environment. I’ve just cut to shape and stapled it to the hoops.

Completed heated bench project

So that’s it finished, finally! My out of pocket expenses were as follows:

  • Soil heating cable – £33
  • Damp proof membrane – £20
  • Timber – £26
  • MDF board – £19
  • Screws – £2.50 ish
  • Polythene sheet – £12
  • Sand – £20

The total cost of building this bench, 21.3 sq ft of propagating joy, was £132, give or take. It’s a non-trivial amount when added up like that, but still, that equates to £6.20 per square foot, even better value for money than I estimated here. This is way, way less per sq ft than the cost of buying a heated prop unit. And, it’s much much bigger. I could add a thermostat for another £25ish, which would allow closer control of temperature and make it cheaper to run. Might do that, will see.

If I wasn’t so impatient I could have scavenged for more of the wood as well as the insulation offcuts which would have reduced the cost even further. Overall, bargain!!

I’ve still got to get the electricity connected properly but I would have had that cost if I had just bought a heated prop unit. For now I’m just running it via the extension cable from the shed which importantly has an RCD plug on it.

Most of all I’m looking forward immensely to filling it up with germinating seeds, rooting cuttings, divisions, seedlings and so on. I’m expecting great things! Plenty of posts and photos of that to come.

I’ll be back!