A prototype seedling propagating tray

3 Apr

A recent project has been building a “flood and drain” bed for raising seedlings. It’s all rather rough and experimental, but showing promise.

S prototype seedling bed 001 - overview

Newspaper pots. Lots and lots (200 here, 350 when complete) of little biodegradable pots. Each group of 50 sits in a “seedling tray” which is has an open grid at the bottom.

The structure is built of 18mm MDF, with two layers of polythene sheet to make it waterproof. The polythene is trapped by a thin strip (18mm x 10mm) of mdf which is screwed down. That applies lots of even clamping pressure to the film so it doesn’t rip. Also visible is the hose that delivers water from the reservoir (currently a 50L “fish bin” just visible below the table).

S prototype seedling bed 003 - right
If the bed fills with water, there will be quite a bit of pressure trying to bend the walls outwards. I was going to put in cross braces, but ended up just using a strong method of fastening the walls. Vertical steel bolts (M6x65) come up through the base and into TAPPED holes in the walls. I recommend this technique for anyone who wants to really lock an mdf construction together. Note: the holes were drilled 5mm, then tapped with an ordinary M6 metal tap in a cordless drill. Very fast and very solid.

S prototype seedling bed 008 - tapped holes

This all sits on a chunk of material from an old office desk, which in turn rests on two homemade trestle legs (made from pallet wood). The tray is 2200mm x 600mm, and the walls are 100mm high. There are seven seed trays along the length, and an area at the extreme left which is where the drain is fitted.

S prototype seedling bed 004 - left

Inside the tray we routed some grooves along the base, hoping that the water would press the plastic down into the grooves and help to drain the bed smoothly. It didn’t. A simple work-around was to string some 4mm poly rope underneath the seed beds. This lifts them up enough that there is no water permanently pooled around the base.

S prototype seedling bed 006 - grooves

The seed trays don’t go all the way to the end. The last 200mm or so is routed down about 4mm deep to act as a drain, and has a “bulkhead fitting” installed to connect to a hosepipe. This was one of the points I was most worried about, as any leak here would get into the mdf and really damage it.

One of the things about polythene is that it’s really hard to glue to. Glue, or silicone caulking, won’t stick to it. I did some experiments with gluing a disk of plastic to the polythene which did work (light sanding, alcohol wipe, blowtorch at a distance to react free radicals with the surface layer) but in the end I decided to just trust the pressure fit of the rubber washer in the bulkhead fitting. So far (though it’s very early days) it’s been fine. If I build another one, I’ll epoxy a few inches of the mdf around the hole so it’s waterproof.

The bulkhead fitting is about 4mm high which prevents draining the last of the water away. Half a dozen pieces of ordinary string (held down by an old 12V battery which proved to work better as a heavy weight than as an electrical device) wicks the last of the water away perfectly. The plastic is dry at the end of the draining period.

S prototype seedling bed 007 - drain

Just in case a leak develops, the whole apparatus is sitting inside a rectangular ring of mdf, again with polythene inside to form a catch basin. The screwed down strips to hold down the plastic on the tray worked well – much better than simply stapling it as we did for the catch basin. Even when stapled through little bits of cardboard, the plastic rips free if you look at unkindly.

For safety, this is a solar powered setup, in this case with a hefty 12V battery intended for a lawn mower or similar ($100 from Super Cheap Auto). Just visible at the back (by the window) is a 1.8W 12V solar charger ($20 from The Warehouse).  I’ve actually got a proper solar panel and controller I can hook up if it needs it.

S prototype seedling bed 005 - wiring

The tray is a fraction longer than the support it rests on, leaving room for the bottom of the bulkhead fitting in the drain section. This leads into some perfectly ordinary domestic hosepipe. Partway down the hose is a tap fitting. This can restrict the speed with which the bed drains. The more it is closed down, the longer the ‘flood’ stage of watering lasts. Currently it takes a few minutes to fill the bed about 10mm deep with water, and about half an hour for that to completely drain away.

S prototype seedling bed 009 -  drain pipe and tap

Inside the reservoir is one of these, a 12V, 840L/hour, BRUSHLESS dc motor. The advantage of a brushless motor is that there are very few moving parts. Basically there’s some circuitry inside that turns DC into AC and uses that to spin the rotor. The supplier claims 30,000 hours operation, compared to some brushed dc motors (e.g. bilge pumps) that often fail after about 200 hours. There are lots of discussions about the problems of brushed dc motors on hydroponics websites. These were about $25 off Ebay.

S prototype seedling bed 010 - pump

If you wondered about the little paper pots, these are made from newspaper strips with the gadget shown here. It’s just a slightly tapered former with a hollow at the bottom and a handle, plus a matching base. I turned these (on a metal lathe) from some branches cut from our eucalyptus trees. There’s various instructions for making them on the net, or you can buy one (e.g. from here). The plan is to plant the seedling complete with the paper pot which should save a lot of fuss.

S prototype seedling bed 011 - maker
Hopefully I can put up an update in a few months to say whether all this worked. I have high hopes, but time will tell.


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