There are three media-based beds located in the center that are filled with expanded shale. The beds use bell siphons to flood-and-drain the water which drains directly back into the central sump tank.
Along the perimeter wall, there are 2 media beds. These beds are set higher than their neighboring raft-based beds and drain directly into them. This method works very well since the media beds filter all the solids from the system and minimizes the amount of sediments that could accumulate in these raft beds.
I’m undecided on the last two beds and may make them rafts beds and interconnect them with water bridges, or have one (or both) be media-based beds. It all depends on the types of crops I eventually decide to grow.
I drew a chalk outline on the floor that acted as a template for all the beds. The frames are made from 1/2 inch black pipe that I welded together. I’m using steel since it is very strong and doesn’t warp like wood, and is resistant to bugs and rotting.
The sides of the beds are 12 inches high, which seem to be a good depth for both raft and media growing. Leaving for a little extra space along to top so the water and media doesn’t spill out, the growing depth is around 11 inches.
The bottoms have support braces spaced every 12 inches. My original plan was to have the raft beds sit directly on the brick floor, but I had one critical design flaw in the dome… I ran the ducts for the geothermal out through the floor and the beds would block them. I had to prop all the beds up on a brick so the air could pass under them.
When the steel cools off after welding, a little black rustproofing paint is applied which cleans up the look a bit.
This is one of the raft beds that was going to placed directly on the floor but interfered with the heating system. Instead, a few bricks are placed under it as feet and the frame is set into place.
Back in the spring, my neighbor and I milled a bunch of the white pines that I had cut down from the greenhouse site. Some of them are cut to 1 inch thick and are being used to line the grow bed frames. I just laid them into place and let them overhang over the edge, marked a cutting line, and cut off the excess. They lie down nicely and make a very strong decking for holding the liner.
The media bed is assembled the same way but is set on stacks of extra bricks. The bottom elevation is higher than the top of the raft bed so that it can drain into it.
I purchased a large roll of EDPM pond liner. It’s very heavy and difficult to work with, but is hard to puncture and is UV resistant.
The liner covers the entire bed and I carefully make sure it is properly centered so all the edges can be draped over the lip of the bed. I did discover that it’s a lot easier to work with the EDPM if it warms up in the sun…even though it’s hot on the hands. I just keep working it into each corner making sure that there is plenty of material to fit into the corners. If there is a gap between the wood and the liner, it could stretch and eventually tear under the weight of the media and water. I found it was very helpful to set bricks on the liner once I had it in position to hold it in place.
The corners can be a bit tricky. Once I got the liner in place, I cut off some of the excess to make it easier to manipulate. Then I could fold a nice and clean corner. It’s important to remember to make sure the edge of the liner is always going to be over the lip so water won’t leak out!
On the outer edge of the bed, I added a strip to anchor the liner into place. The screws go through the strip and liner, and secure into the side boards. The extra liner is cut off which makes for a clean looking transition from the liner to the bed.
The media beds have bell siphons in them. I’m using a bulkhead fitting through the bottom of the bed for the drain. So that the liner doesn’t get tangled up in the hole saw, I sandwich the liner under a piece of scrap board. These boards a still somewhat damp and the saw is old and worn out so I have to keep cleaning the sawdust from the bit, but eventually I make my way through.
After cleaning the debis out of the area, the bulkhead fitting fits perfectly through the hole. It has a rubber washer that seals against the EDPM liner and fitting. Under the bed, it has a large nut that tightens the fitting in to place.
The stand pipe and drain lines are made from 3/4 inch thin-wall pipe. These bulkhead fittings have a 1-1/8” threaded fitting since it’s what I had in my assorted collection so they need a threaded to slip-fit coupling to reduce it to the right size. I use a temporary stand pipe and fill the grow bed to test it for leaks. I like to fill the beds right up to the rim to test for a worse-case scenario. Plus it’s a great way to check to see if the bed is level and add any shims to the legs for minor adjustments.
Once everything looks good I put in the bell siphon and media guard, and the bed is ready for the expanded shale. Since I recorded this video, I’ve changed the bell siphon design a bit, so click on the link to see these details.
This type of bell siphon uses a trap which helps it to start. The trap assembly is screwed into the bottom of the bulkhead fitting and a section of pipe is attached to the trap and extends to drain into the raft bed.
The liner for the raft bed is installed the same way as the media bed. It was actually was a bit easier setting it up in a larger area. Once the liner was installed, I flooded the bed and removed the temporary holding bricks.
I’m using expanded shale for the growing media. I just give it a quick bath to rinse off the dust, screen out some of the smaller stones, and fill in each bed, one bucket at a time. I like using the shale since it’s about half the weight of stone and easy on the hands while digging in it. Most of the corners are rounded over which will minimize the risk of it puncturing the liner.
Since the raft beds aren’t square, I needed to come up with an efficient design to maximize the space of the bed. Using square rafts would never fit properly. I design a single row raft that was tapered so they would “fan out” in a curved pattern when they were placed in the bed. As the taper became wider, I placed the holes closer together. For a mature plant, the amount of space per plant will be the same, it just is not a perfectly round or square area.
The seedlings will start from one side. Every few weeks, a new batch will be added and the older seedlings will get moved over. At first, the width of each raft is narrower than the space that the plant will need, but as it grows, I add a spacer in between the mature plants to give them a little more room. By doing this, I can compact the new plants together without having to transplant them from a compacted raft into a normal-spaced raft. Each bed will hold about 208 lettuce in about 44 square feet.
The remaining area can be used for seedling starting. Currently the space works well for growing duckweed.
The rafts are made from 1” Dow blue board foam. I made up a template so each raft was cut identical. The template also has holes that marked the location for each hole for the net cup. I used a 1-7/8 hole saw which makes for a snug fit for the 2” cups. I found it was fastest to drill the hole half-way through, then finish drilling from the other side. This left a bit of the plug sticking out so it was easier to remove from the hole saw.
I fill the net pots with a little bit of damp shale and drop in one or two seeds. The dampness makes the seeds stick to the shale so they don’t just drop through. The rafts push right over so the new set can be placed at the beginning of the bed. Once the plants mature and need more space, the filler rafts can be inserted between each row. So far, the lettuce has a good healthy root system and will be ready to harvest in a couple of weeks.
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