@Audi_o_phile asked about a mist propagation bed I built 20 years ago, then abandoned when moving to a new location. Keep in mind that I built it once, then having seriously underestimated the weight, rebuilt it with some help.
For the location I recommend a cement pad (or pavers that won’t sink) that is sloped to drain – the usual 1" every 8’ used in patio construction. The bed is going to shed water and you don’t want to end up walking around in mud. You’ll position the drain hole of the bed according to the pad slope. Also, you’ll want it near a hot water source. My kitchen was nearby so I ran a bleeder line from under the sink out the exterior wall, along with 12-gauge grounded AC electric cable tapped from the garbage disposal circuit. In addition to hot water you’ll want cold water at irrigation pressure to mix with it. I’ve since thought about using an small inline hot water supply – but it gets complicated (warmup time) and expensive.
Now you’re going to need pond liner or a rectangular prefab with durable walls about 16" high. Square would be ok I suppose. This is going to determine the dimensions of your platform. For the latter I recommend heavy duty floorboard painted with weather seal on top of square cinder blocks at 30" intervals in multiple rows at the same interval. You’ll want it ~28" off the ground for ease of access of the interior. Access to the interior is also going to dictate the width of the bed. It’s no fun if you can’t reach the middle – or nearly so. You’ll also want to fit an array of plant trays in there that will sit on top of the heating cable. Further, there’s also going to be some irrigation pipe in there too so read to the end before planning the foot print. Mine was 10’ x 5’, although I’m 6’ tall.
I built walls to sit on the treated floorboard, using painted 1/2" siding and corner braces. I made this choice based on the superstructure I would build. You’ll want to drill the hole for your drain before putting on pond liner, and while you’re at it grind the top perimeter so it can be a sunken drain hole. If you build walls, you’ll also want to prepare a cut-out at the upper end for an outdoor electric junction box at least 12" x 6" x 6" deep for the stuff you’ll put in it. Choose one with a hinged door and turnkey latch. Also at that end adjacent to the electric box you’ll need holes drilled side-by-side at about 4" apart for the hot and cold water. On the inside you’ll want a mixing valve which by trial and error you’ll tune to “lukewarm”, about 70F depending on season – but install later in construction.
So next build the superstructure from 2" schedule 40 PVC pipe that sits on the walls. The bottom runs the entire perimeter. At two ends you construct a 35° to 45° A-frame with a vertical support coming up from a T-fitting in the center of the base. The top of those posts have 90° elbows that point at the other end. The side pieces of the A-frame have diagonal cuts and are connected by metal screws to the upper elbow and bottom perimeter corners. Place a horizontal pipe between the two elbows with temporary vertical support about 1/3 and 2/3 the distance, and don’t glue it. Along each spare side of the perimeter insert a T-fitting almost halfway, in fact one 3" off center in one direction and the 3" off center in the other direction. (Note: if your bed is 5’ long or less you won’t need these, and if your bed is greater than 10’ you’ll need multiples). Now point them up at the cross pipe between the two A-frames, then install two more T-fittings in the cross pipe to complement them. Finish this step by inserting pipe into the T’s you just installed (easiest by first disconnecting from the A-frame, then reconnecting). Now you no longer need the temporary supports. Next affix the frame to the walls with flexible zinc or aluminum tie-down straps. I recommend nuts and bolts in case you have to move them.
Next you’ll build 4 (or 2 for a short bed, or more for a long bed) hinged windows that fit in the openings between the diagonal struts. Use PVC with 90° elbows, but 1" to 1.5" diameter is ok depending on window size. Attach these with hinges to the top horizontal pipe using nuts and bolts. Note they need to be staggered from the ones on the opposite side. You’ll want these to close even with the perimeter pipe, so insert some rounded stops into the inside of the perimeter pipe. Also, you’re going to want to prop these open when accessing the inside, so create some car-hood style wands you can rotate up for that purpose. They can be your “stops” when not in use.
So next you’ll install the pond liner with a staple gun, only stapling to the walls. You’ll need to do it in sections. There will be places of raised overlap and you want water to flow towards the drain. This takes most people several hours. Afterwards cut a hole for the drain in the liner to match your fittings. The exterior of the drain pipe preferably has a couple slightly downhill 4" length bends at 90° as a temperature air trap. The end of the pipe should have an interchangeable coarse mesh to keep critters out.
Now slice out the liner piece over your utility box hole and install it – having first drilled access holes for incoming and outgoing conduit of various sizes. Seal the sliced liner to the box. Also cut liner holes for the incoming water lines and install stubs for these. Up in the A-frame above the electric box install a thermostat controlled exhaust fan with the thermostat on the inside (typically made for equipment sheds, some rated for wet environments).
Next install the mist irrigation line. I recommend 1/2" PVC. Run it down the middle of the floor lengthwise with 24" risers at the beginning, end, and every 2’ or so. You will also want dead-end side struts about 4" long for stabilization. Use 4-way (cross) couplings for this. Space them with the nursery trays in mind. The pipe will need to be elevated an inch or two where it passes over the drain. Use Netafim mist emitters for greenhouses, half pattern at the ends and full elsewhere.
The irrigation line needs an electronic control valve right after the hot/cold mixer. I like the Rainbird inline for compactness an longevity (at least here!). The valve can be activated by a simple timer if you wish – if so only operate for 1 minute max each time. A better choice is an electronic leaf (pictured below). It will need power from the electric box. It also needs to be level left-to-right.
Almost done. You will want a heating cable, probably 100’ or more. The reasonably priced ones are sold to hog farmers. The heat mats used by livestock farmers are also worth a look, but the kind sold to indoor pot farmers are a poor choice in this environment. Be sure to size these up before you buy or build anything. The controller for the heat element goes in your electric box and the wired probe into a pot with soil.
Finally, you want coverings for your hinged windows and A-frames. The independent makers of boat sails are the best choice I’ve found. You want clear sail with grommets in them. In my experience they want you to bring the window(s) plus a picture and stencils of the A-frames, including exhaust vents. You’ll attach them with zip ties. There’s cheaper solutions for material but the sail is the best insulator. Of course, consider I’m all worked up about insulating from 40°F temperatures!
