How to build a giant heated propagation unit

Now that it’s finished I’ve consolidated 4 previous posts into one, save jumping about.

The first job was to clear out the greenhouse so there was room…

I have begun the changes needed in the greenhouse to achieve peak propagation this year. So far this has involved cutting down the long staging to fit in its new home at the door-end of the greenhouse and trimming down and moving the existing short staging to the other side, next to the existing gravel wet bed, see below. In hindsight I could have achieved the same outcome with less cutting and mucking about if I’d left the short staging where it was and just cut the long staging down to fit a different spot (as per the diagram I drew previously…). Never mind, kept me busy for a couple of hours. All this is really about making space for the large heated propagation bed. I have the soil heating cable, I just need the wood. Next step will actually be to move the potting bench from the greenhouse to the shed to make a bit more room. I have half an idea of making it bigger but that’s not a high priority – one for next winter perhaps, although if my propagating plans go as intended I may regret not scaling it up now while there’s not much else on.

Next I started to build the timber frame…

I paid a visit to Oxford Wood Recycling which is a social enterprise I’ve used before when I’ve needed inexpensive timber. It’s a bit of a trek, 20 odd miles, but well worth it. From their Aladdin’s Cave of wood I bought some long lengths of 4×1 plank which they cut to required size for me on site (£14). These will form the horizontal sections of the frame, about 233cm or 7’6″ long. I ideally would have preferred old scaffold boards or 6×1 planks but the former were too pricey and they didn’t have enough long pieces of the latter. Never mind, I will double up on 4x1s and so have 8in of depth to play with in the frame. I already had pallet rails (£0) which are very solid, which I cut down to 90cm in length to form the legs for the bench. If I didn’t have these I’d have either foraged for more pallets or bought 3x2s from the wood palace.

Legs eleven. Or six.

One of the unforeseen downsides of maximising the size of the bench vs the greenhouse is that there isn’t much wiggle room to construct in there so I first built the sides in the shed. The 4×1 planks were screwed square to the legs with 2in screws, flush with the tops, two planks on each side.

I added an extra leg on each side for the sake of robustness and so I can brace the base a bit more. It will eventually be holding a heavy load of sand, water and compost so it needs to be strong. I don’t want it all falling through the middle and wreaking propagation havoc.

It would have looked neater to put the middle legs on the inside of the box frame but I wanted to have an uninterrupted run inside the frame to make it easier to spread out the liner when the time comes.

I should’ve bought another couple of 4×1 planks for the bottom shelf but miscounted so will buy a couple in B&Q when I get the other bits I need. I moved the shorter side pieces into the greenhouse and after a bit of staging shuffling, screwed in the side pieces (93cm or 3′) which completed the basic frame for the bench.

After completing the basic frame, the local DIY emporium (B&Q) was my next port of call.

MDF base secured to box frame

I’ve used a sheet of 18mm MDF board (£19 for 8’x4′) for the base which was cut to size for me in the shop. I love that cutting service, I’ve always wanted to have a go at the big saw rig. Sigh. Anyway, I drilled and screwed the base onto the underside of the side rails, using plenty of screws. Wet sand is pretty heavy so I added some extra bracing underneath the base, running diagonally from the corner legs to the central legs and square across the centre. Hopefully that’s enough…!

Extra bracing for the base.

I made the bottom shelf mostly from bits of battening from the dismantled staging  (£0) but I needed a few more (£10).

Plenty of storage space.

Since I’ll be heating the bench I want to make sure the heat goes where it is needed, into the sand and more importantly the plants, not down into the base. I had a brainwave and asked my local builder if he had any offcuts of Celetex, excellent insulation and just the job. Not only did he he have some knocking about but he also dropped them off to the house (£0!), saving me a few quid. Thanks Mick!  I’ve cut it to size and jammed it in, nice and snug.

Snugly insulated.

As the bed will get a lot of watering, it needs to be lined to prevent the wood from going soggy and quickly rotting in the humidity. I’ve cut to size a sheet of damp proof membrane (£20 for 4mx3m in B&Q), a pond liner would work too. I have used a double layer of DPM to be on the safe side, this should be nice and watertight.

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.

For the bed itself, 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.

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.

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.

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.

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.

Finished!

So that’s it finished! 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.

It’s already filling up with germinating seeds, cuttings, seedlings and so on.

 

UPDATE 22-Feb-2018

The bench has been in action now for over a year. I did add a thermostat which allows me to more closely control the temperature as well as making it cheaper to run.  The bench has been in constant use, for seedlings, for cuttings, for overwintering delicate plants. I have found that the sand needs watering once a week or so – two or three watering cans does the trick. This is because wet sand conducts heat better than dry sand.  The structure of the bench has held up very well. With the cover closed, condensation does build up on the inside of the cover which in itself is not a huge issue, but I have found that this drips off when I open the cover. Some of these drips find their way down the back of the cover and underneath the bench and the MDF base is looking a little puffy in a couple of places. In hindsight I might have used marine ply for the base instead, or just treated the MDF with wood preserver to protect it.  I still haven’t got the electricity hooked up properly, I must get around to that…