DIY Germination Chamber 11

DIY Germination Chamber

On our small farm, projects often come about after some internet research and using odd pieces of “junk” we’ve got hanging around. One of this winter’s projects was a germination chamber.

Some of our seeds require light to germinate but most vegetable seeds don’t. They do need consistent moisture though. So, in an effort to reduce the space that seedling trays take up before the baby plants emerge (at which point they do require light) and in order to reduce the watering these trays need so they don’t dry out, many growers use a germination (or sweat) chamber.

Since we moved to this farm, we’ve started our transplants many different ways. Each year, we’ve made some kind of germination chamber and it’s always looked a bit different. At its most basic, we stacked bread trays near our wood stove and wrapped them in plastic to keep the humidity in. Once seedlings emerged, over the years, they’ve been moved to hot beds, our sun porch, and, for the last few years, electrically heated benches in our high tunnel.

A germination chamber is basically an enclosed area, where the temperature and humidity can be regulated. There is often some sort of tray stacking system where it is easy to see which trays have germinated and be able to easily insert and remove trays. Some people have used old chest freezers, but as our only free chest freezer was a little too small for our purposes, we used an old wire rack and former project leftovers. Farm Hack was a good online resource for helping us design our germination chamber.

It all started like this:

A 3 foot wide by 5 foot tall wire rack (sans shelves) and a stainless steel sink.
Germination Chamber


Ideally I would have taken this in to someone to weld a piece into the bottom of the sink to seal it up (might still do at some point), but opted for the cheaper option of silicone and a sink stopper.

Stainless Steel Sink

Fortunately, the spacing of the supports for the shelves worked with most of the trays that we use (the 50 cell Winstrips are our tallest trays).

Winstrip tray


For the shelving I used ½” EMT, (galvanized electrical conduit), and then made a little notch with an angle grinder at both ends to fit over the wire supports. They are easily removed so that the shelf spacing can be made bigger to accommodate the Winstrip 50 cell trays, or to add more than 3 bars per shelf (4 is better for the flimsier trays).


I decided to recess the sink and slope the sides because of comments other builders had made about water pooling on the bottom.

Pink Board insulation

Copious amounts of tuck tape to ensure most of the water finds its way back to the sink and not through to the floor.

Heated Water Basin


Originally, I was planning on using the Durostat prewired thermostat, purchased online from Farmtek/Growers Supply, but procrastinated a little and needed one faster than what ordering one online would bring. Luckily enough, our nearby plumbing and electrical supply place, Eddy Group, carried this dandy cooling/heating thermostat that mostly gets used for chicken barn applications. It ended up being about $20 cheaper (more if you consider having to pay the US exchange rate and shipping), and pretty simple to wire up.

Germination Chamber

So here is the beast more or less finished. There is good reason that these germination chambers are also called “sweat boxes”. It is quite moist, and there is a lot of dripping. My design proved a little faulty in that all the water that dripped down the door ended up leaking through and onto the floor. A quick improvement for that was to use (again) a bit of tuck tape and make a little baffle to make all the drips go further into the box where they would run into the sink. We also tried to cover the water pan with metal sheeting to see if this would limit the sweating effect while still radiating heat into the box, but removed it because I felt like it was just going to use more power to heat the box.

Although we don’t have to add water to the pan that often to keep the level where it should be, a water line with a float valve would be good (for when someone forgets to refill it and the heater element is no longer submerged).

It’s not perfect but it works pretty well and it saves space and removes our worry over trays drying out pre-germ.

Here’s a re-cap in pictures of how it was built:

germination chamber

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11 thoughts on “DIY Germination Chamber

  • Jeff

    Thanks for posting! Will you guys be monitoring humidity levels in the chamber? I’ve wanted to build one but worries about controlling humidity/damping off while still maintaining a warm enough environment with the bottom water heat design, and whether or not to add supplemental light have kept me from committing.

    • broadforkfarm Post author

      Hi Jeff, Thanks for your question and comments! We have not been monitoring humidity levels thus far. From the amount of moisture on the walls of the chamber I suspect it would be approaching 100%. We don’t really worry about watering the trays going in because of this, as long as the seeding mix isn’t bone dry. We also take the trays out as soon as we see any sign of emergence. I think there would be a lot of issues with damping off if trays were to stay in there with supplementary lighting. Because we don’t have a heated greenhouse and our heated benches don’t maintain optimal germination temperatures, we opted to go with the germ chamber to give our seeds a quick start.

  • Kelly

    Thank you so much for sharing this information! I’m starting a farm in Michigan and have been looking for good DIY germ chamber designs. This was very helpful! 🙂

  • samuel oslund

    Hey guys,

    we love your design. We’re wondering if you could give us an idea about the wiring and perhaps what sort of power draw this has. Do you have a photo of how you connected the thermostat with the element?

    We don’t have a ton of juice in our greenhouse, so we’re a bit concerned with how much electricity that little heater would need.

    thanks for the help!



    • broadforkfarm Post author

      To connect the heating element to the thermostat I used standard 14/2 wire (what is commonly used for wiring in houses). The element is 1500w, which is what most small space heaters are, so if it is the only thing you are running on that breaker, 15 amps should be adequate without any issues.

  • Edgar Estrada

    Hi! I based on your design to do something similar but smaller (1 plastic container only)

    My seeds germinated great, however one day, all of them died. Super fast. all the little plants just went down.

    Do you know what could’ve been wrong? was it temp? a lot of moisture? lack of air?

    • broadforkfarm Post author

      Sorry to hear your seedlings died! Was your plastic container clear or dark? If it was clear (and in the sun), it could have overheated. If it was dark, did you remove the seedlings as soon as they popped up out if the soil? Once they’re up, they need sunlight right away or else they’ll get “leggy” and reach up to look for sunlight. Hope this helps, Shannon

      • Edgar Estrada

        Hi Shannon
        Thanks for your reply.

        It is a plastic container, transparent, not directly in the sun (only gets sunlight after noon, until 5 pm)

        I put a growing light outside on top of it that turns on by nights.

        I put a small electric water heater inside, turning on every 3 hours for half an hour.

        The bottom of the plastic container has water enough to completely cover the water heater.

        I made a metallic inox structure inside to hold 2 trays of seeds.

        I had the lid closed, just briefly opened it for a few minutes every day.

        Like I said in my previous msg, what most intrigues me is that all of them (even different species) died at the same time — and now that I remember, specifically the only 4 peppers I planted did not grow at all, they just remained as seeds; the rest (tomatoes and some other herbs) did germinate.

  • D.

    Hey Broadfork,

    Nice job on the chamber. I built a similar one a couple of years ago except it is much bigger. Fits between 90 trays, 100 trays if I push it. I also am using a heating element submerged in stainless steel restaurant pan. The heat element is always submerged. I use a float valve to regulate the level. I am using the Duro Stat from Farmtek. I want to know if you have experienced any issue with wiring. My Thermostat burned out last week and I am not sure why. I recently changed the cord that is connected to the heat element and plugs into the thermostat, so that is my first suspicion, but the rating of cord seems appropriate. I am also not well versed in electric matters and it seems as though from my internet searches, using the heat element this way is not common. I am using an extension cord that I have cut off the female end and have wired to the heat element. The hot and neutral are connected properly and I have a ground connected as well. The cord then plugs into the thermostat. My business partner is the one who originally wired this two years ago and has since left and is not available. So now I am trouble shooting on my own. How do you have your heat element wired? What type of wires/cord are you using?

    • broadforkfarm Post author

      Thanks Rocksteady,

      Everything is using 14/2 wire (common household grade used for most circuits that don’t draw a lot of power), which is heavy enough for a 1500W heating element. I would make sure the extension cord you’re using is at least 14 gauge, and that your heating element is 1500W. If it is more than that, you should have a heavier gauge wire, and probably a 20 amp circuit. I don’t think the duro stat thermometers would be rated for much over 1500W. Hope that is a help.

      Good luck, Happy Farming,

      Bryan and Shannon