In a lot of ways the propagation table is the heart of the farm. Nothing moves if you can’t get plants to grow, and giving them a strong start pays dividends later. Luckily plant starts are pretty straightforward; they need light, soil, water, and, this time of year, a little extra heat. Water is easy, soil is simple (though nuanced), and the sun is doing a fine job with the light- heat, though, can be a challenge. There are many different ways to deliver heat to vegetable starts, from the ultra simple electric heat mat, to the ultra challenging (and impressive) compost-powered hotbed. Heat mats would be energetically expensive at my scale (12W/sqft * 120sqft * 24 hours in a day=34.6kWh every day), and I couldn’t dream up how to make a hotbed big enough for all of my starts. The farmer I learned under, Eli at SkyRoot Farm (Clinton, WA) had a cool solution to this problem, he made a propagation table that was heated by a hot water heater, not unlike a small radiant floor heating system. In Eli’s system a pump pushed hot water through pipes buried in a bed of wet sand, heat from the pipes transferred into the sand and from there into the starts sitting on top of the sand bed. What's more, the pump was controlled by a thermostat embedded in the sand, so it only kicked on when the sand dropped below a set temperature, which meant that much of the time the pump and hot water heater didn’t have to work at all. Eli’s system seemed to be more efficient than the heat mat option, and though a bit more complex, I thought I could figure out how to replicate it, so I decided to design my own table, to fit my farm’s needs, using Eli’s concept.
For successful design you always start with the big picture, and hone in the details gradually. What details mattered most for my starts table? It had to be in the center of my farm, I wanted to minimize the distance the starts would be carried out to be planted. At peak production I will need room for 60 trays on heat this year- and I want to have the option to scale up by at least 50% in the next couple of years, so room for at least 90 trays. And I didn’t want to have to shuffle trays around constantly, so the more open the better. I decided that rather than build a small greenhouse just for plant starts, I could use half of my smallest high-tunnel; there I would have room for mixing my potting soil, sewing my seeds, growing my starts on heat, and even room for a table to move them off of heat- all in the center of my farm. This means that from seed to planting out in the field the furthest a vegetable start would have to travel is 150ft (that’s pretty great!). From there the details began to work themselves out. 

I started by building a sturdy platform 34ft long and 4ft deep, plywood over 4x4 legs. I attached 2x6 boards to the edge of this platform to make a 6 inch deep trough cross its top. Then I lined the bottom of this trough with about 3 inches of rigid insulation. I want all of the heat of the table to move upwards through the trays of starts, preventing loss of energy out of the bottom of the table could save me from significant power losses. Finally I lined my trough and table with a plastic vapor barrier sheet- I don’t expect the system to be fully watertight, but keeping water mostly off of the wood frame of the table should make the whole thing last longer. At this point in the project I was thinking to myself, “this is pretty straightforward,” and that’s because I hadn’t yet started on the plumbing.
An engineer friend had helped me work out some details with my plumbing system (heater and pump sizing, heat transfer properties of materials, expected energy consumption, etc), but I was on my own when it came to putting the details into practice. Some details were straightforward; a 10-25G hot water heater should be suitable for this system; no need to use flexible pex tubing, rigid CPVC should be cheaper, easier to work with, and should transfer heat just as effectively as the sand around it; somewhere in the system there should be an air bleed valve and an expansion tank to make it easier to fill the system every year (it will have to be drained when not in use to prevent damage from freezing). 

Other details were a bit more challenging. He explained that if I tried to push water through one very long pipe snaking through the table I would run into problems with friction loss which could at best strain my pump, at worst prevent the flow of water. Instead he suggested that I split the flow of water into multiple lines that flowed in parallel from one end to the other where they would rejoin and flow back. He also cautioned me that in splitting lines I would need to be careful in making the divisions as even as possible so that water would move evenly through the system, this would help minimize hot or cold spots in the table top. What we came up with looked something like a tournament bracket, one line splitting to two, two to four, four to eight- those eight lines traveling in parallel 34ft down the table and then doing the tournament bracket in reverse to join back together into one line that would flow back down the center of the table to the hot water heater. This plan, put into practice, looked liked a lot small plastic fittings and a lot of glue.

Once I was done with the glue up, it was smooth sailing. I filled sand in over the pipes level to the top of my trough, filled the heating system with water, bled out any remaining air, and fired it up. If only. My water heater, which I had purchased used, wasn’t seeming to work so I had to do some youtube-learning to figure out how to diagnose and fix that. I went as far as replacing the thermostats in the water heater before I realized that it wasn’t the problem at all, I had just failed to prime the pump so it wasn’t pushing any water! It was a silly mistake, but hey, it forced me to learn more about how the working components of my system functioned, and that’s probably for the best.

Pump primed and water heater at full function, water began to flow through the pipes across the table and the sand began to warm up; before long I heard the hot water heater kick off and then the pump stop running, and I checked the thermostat and saw that the table had reached my set temperature of 80°F. It was time to start seeding! My goal had to get my first round of seeds sewn the week of February 1st, but with all that went into the table I was a little behind schedule. It was Super Bowl Sunday (Feb 8th) when I got the table online and started my first round of seeding. Some friends arrived shortly before the game and helped me seed a few trays of starts before we went in to cheer on the Seahawks. About a week later the first seedlings started to push their way out of the soil and my first season farming this land began in earnest.