DIY Propagator

Need some advice, please. I'm building a 40" x 20" heated propagator with an alu body / polycarbonate top supplied by Two Wests & Elliott. Have lined the base with sheet polystyrene and would like your thoughts on my plans to finish it.......
Hoping to be able to use perlite instead of sand to bury the cable, to keep it light and portable. From a safety point-of-view, would this work?
Ditto heavy-duty sheet plastic.
Would this seem a logical 'sandwich'? (From the bottom up)
Polystyrene, sheet plastic, 1" layer of perlite, heating cable with long staples to hold it down, layer of geotextile to stop cuttings' roots tangling round cable, 3" layer of perlite.
As the potting compost here is awful stuff, and seems to come packaged with millions of sciarid flies, I'm going to try rooting cuttings directly into perlite. Compost for seed trays and pots of seeds will be treated with Sybol powder.
What do you think?

 

Sounds fine to me - but I'm no expert.

Just needs to be waterproof I expect? With thinner material there is greater risk of puncturing, or it perishing sooner, but what will that mean? that water leaks and drips on the floor? then you will know the time has arrived to fix/replace it! (Assuming I am correctly understanding the purpose of the plastic sheet)

The Thicker the Better Or use Celotex / Kingspan (I think Kingspan has to remain dry, and Celotex can get wet, but I may have it wrong-way-round, and there is also possibility, if memory serves me, that those materials are not available in France??). You might get offcuts as give-aways from any local building work that is going on.

Is that as per instructions? I would expect staples to cause local hot-spots of the heating cable and increase the likelihood / shorten the time before the cables burns out.

I suppose Sand would be more likely to keep the cable in place, without having to anchor them, than something lighter, like Perlite??

Not sure about that. Important that the substrate around the heating cable is damp - heat won't transmit through a dry substrate. Will the geotextile prevent / reduce the percolation of water to the bottom substrate?

That was how it was done on a propagation nursery I worked on. Benches of Perlite ... we laid perforated metal steam pipes in them, heaped the perlite over, covered with tarpaulin and steam sterilised - then removed tarpaulin and pipes, levelled out, firmed and stuck billions!! of cuttings in the benches. They were all under mist too - are you going to add mist to your propagation bench? Best thing I ever bought (winced at the price though - but I can tell you where I got it - cheapest I could find). I don't bother with heating - but I only use it for propagating during the warm months.

 

Aren't Perlite and (expanded) Polystyrene both insulators

1) There used to be a problem that resulted in degradation of cable insulation when polystyrene foam backed insulation board came out and cables were buried in the insulation.

2) Even domestic electric cables (which don't emit heat) have to be increased in size when buried in insulation.

Personally, I'd check with the manufacturer before burying heating cables in insulation material (rather than laying on top and then covering with something that would transmit the heat to above).

 

Thanks both! @Kristen, We have plenty of offcuts of Knauf insulation, so will see how much space that leaves me for the sand-substitute layer. The 'geotextile' is a weed-suppressing, fairly open-weave fabric, so shouldn't be a problem with letting water through. Will be aiming for 'damp' rather than 'wet' though, as don't want standing water in the aluminium base! My 'staples' are plastic-covered paper-clips cut in half...improvisation rules OK Likewise misting...much as I'd love a pro unit, it's going to have to be surveillance and a hand-held sprayer.
The cable has good insulation, seems well-made, so fingers crossed that this unit lasts longer than the pathetic objects that I used in the UK!

@Scrungee I'm sure that the traditional sand would be more effective, but the thing needs to be portable and I hate to think how heavy it would be using sand! Can you think of any other medium that would work better
Manufacturer's recommendations, hmmm...was given a heating cable which prompted the whole idea. It's German-made, with a mouse-nibbled instruction booklet (in German). Reckon I'm on my own there

Will keep you posted on how it goes

 

I always understood the damp sand was to transmit the heat, I think perlite might, cause the cable to overheat, being a good insulator.
But I've never tried it, so just guessing.

 

I'm not sure that wet Perlite is a good insulator? But its only gut feeling, not something I actually know.

 

Well, given the consensus, I'll go for a thinnish layer of sand around the cable. Don't want to have to dismantle it all if it burns out! Will top this off with the originally-planned depth of perlite which will be kept damp. This is the easy bit.....getting OH to construct a stand could take years

 

IF that is insulating (I'm not sure that wet Perlite will be) then it will prevent the heat getting to the base of the cuttings, so you are back to square one I reckon. It won't burn out (thermostat will switch off warming cable when sand gets hot enough), but it won't get to the plants either.

But I'm just thinking out loud, so no idea if my assumptions are right.

 

Perlite is an important commodity in the horticulture industry where it is mixed with soil. The addition of perlite to soil increases the amount of air (i.e., oxygen) held in the soil, as well as the amount of water retained by the soil. This obviously improves the growing conditions for plants.
An excerpt from the Minerals Education Coalition site. Don't really see how a material can be both insulating and water-retentive but if that's the case, pity the site doesn't mention heat conductivity as well! Suppose I could hedge my bets and mix sand with perlite for the deep, top layer to lessen the weight a bit, but I've never had much luck striking cuttings into sand.

 

Ah, sorry, Perlite is ONLY for the top layer for rooting - and not part of the bottom "heating" layer? [I thought you mean using it instead of "yet more sand" to reduce weight]

I think you would be OK with Sand for heating cable and then immediately (above your permeable membrane) having Perlite for rooting. There isn't any insulation going to happen, and what does will be beneficial. Heat will escape from Sand to Perlite, immediately warming the point where cuttings will me trying to make roots. Heat will not rise, much, through the Perlite because of its insulating properties - ergo you will use less electricity

That's how I see it anyway, so sound fine to me - in fact it sounds like an energy-saving device to me

But if you are only planning to do cuttings in warm weather I think you would be better spending the money on Mist rather than Warming Cables - although it will depend on what you want to strike of course.

Mine cost me £289.20 - which was 3 nozzles (1M separation, so it does a bench about 1M wide by 3M long), and that includes the mist controller and electronic "leaf" (which detects the rate at which the cuttings are drying, and thus when next to "fire" the mist). Sadly the nozzles cost not-much, so having a shorter (or longer ) bench won't make much difference to the price.

 

Will definitely make do with a plastic mister As the propagator is dual-purpose, for seeds as well, can't justify the expense of lots of high-tech wizardry.
Even perlite is a silly price here, especially with postage on top! Thanks for all your advice, K..much appreciated!

 

Indeed. How can an electronic device that: registers when the electrical resistance of a damp, but drying, plate increases to some adjustable value and then opens a solenoid (to allow water to flow up a pipe) for an, adjustable, amount of time cost the earth?

Sadly, I never found a cheaper source.

Actually, I did, but I didn't have the courage to go down that DIY route. I'll look for a link later, but basically it relied on the Mist adding weight to an "arm" made of a light material, which "dropped" as a consequence of the weight of water, and that turned off a micro switch. Once enough water evaporated the "arm" rose up gradually until, at some point, it tripped the micro switch again. Elegantly simple, no electronics (just a micro switch), and cheap as chips. You could use Foggers instead of Mist Nozzles (4 or 5 quid each, only need two or three) and that would give you a misting bench.

It might well be worth installing the mist lines, and just operating them by hand (or a timer), as the mist lines on their own won't be dear. (A timer you would have to adjust to "more often" or "less often" depending on whether it was a sunny day in mid summer or an overcast day / Autumn - but that would still do a 90% job, compared to a fancy "electronic leaf" gadget, I reckon)

I buy 100 Litre bags. Can you find a Horticultural merchant locally?

You could look for a Hydroponic store (flogging kit for home-growing of Cannabis ... probably!) but I find their Perlite quite "coarse". However, I use finer Perlite to mix with potting compost to "open it up", for rooting cuttings the coarse stuff would probably be fine.

You might even find it in a DIY store - sold as an insulating agent.

Maybe getting it shipped from UK would be more carriage, but much less price, and work out cheaper overall?

 

Tu plaisantes! The options on perlite are: Local town - DIY store, garden centre..zilch.
Marmande - DIY store, 3 garden centres...zilch. 150 km round trip to DIY store that (possibly) stocks construction-grade....life's too short.
French mail order - €19 per 100l sac + €36 carriage! UK mail order - £19 + £16 postage = €42. Finally found a French Co who'll deliver for €19 + €12 carriage.
And so it goes here for anything not related to edible gardening You should try sourcing a Pyrus salicifolia 'Pendula' !

 

My Trade price is £13.90 for 100L (£12.90 if you buy 2, £10.50 when you buy 10 - that's quite a lot though!) Sadly they say nothing about overseas delivery ...

I don' think €19 is unreasonable - apart from the carriage - is there no Internet Revolution, Home Shopping and Mail Order in your middle ages country?! The Internet revolution bought about all sorts of courier/carriage company opportunities over here (including drop-ship at petrol stations and the like for subsequent collection from there)