Building and Using a Precast Septic-Tank Root Cellar

Reader Contribution by Garth And Edmund Brown
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by AdobeStock/martinfredy

How does a precast concrete septic-tank root cellar hold up to a harsh New York winter? See how this DIY root cellar performs when temperatures drop.

I mentioned building a root cellar as part of my plans to source a year’s worth of food locally, but I thought readers of Mother Earth News would be interested in a more detailed description of making and using it. I based the design on these plans for an underground root cellar, which involves modifying an (unused!) precast septic tank. When I decided to join my brother Ed in growing all my own food for 2015, I knew I would need a better place to store our root vegetables than the old farmhouse basement, but using using cinder blocks or building forms to pour concrete in place seemed like a bigger job than I had time for. Using a precast 6-foot x 6-foot x 10-foot tank and some materials I had on hand made the job much easier than it would have been, and I ended up with a root cellar that has maintained a stable temperature and high humidity even in the worst of a harsh New York winter.

One of the biggest potential jobs required to put in a cellar, no matter the design, is digging the hole. Luckily, I already needed to rent an excavator for some other things around the farm, and it made what would have been days or weeks of work with a shovel a matter of a few hours. The location I chose is a bank off of the driveway. It’s south facing, which concerned me a little — I thought it might be prone to overheating — but it was the only suitable site within a reasonable distance of both the houses and the garden.

There were a few tense minutes when it looked like the truck that was delivering the septic tank would not have room to maneuver into position, but the driver finally managed to line it up. I’d been expecting a boom truck, which can drop a tank almost anywhere, but at the last minute I got a call from the concrete company saying the tank weighed so much that only their truck with a slide off of the back could handle it. It ended up working out, but it would have been less stressful if that miscommunication could have been avoided.

The tank already had flanges around the top, and these let me put in vent pipes without drilling new holes. I did need to rent a saw to cut the doorway, and there’s no way to overstate how noisy, messy, and tiring sawing through concrete and rebar is. But the most time consuming part of the project was building retaining walls coming out from the door, for which I used cinder blocks I’d stashed over the years. I did a double layer with mesh going back into the backfill to help hold them in place, and all the tamping and dirt moving seemed endless. Now that I’ve gone through most of a winter, I think that the extra labor was worth it. Having the front of the root cellar as buried as possible helps moderate the temperature, and in a cold winter that’s a must.

I store the vegetables in plastic file holders. I would like to use the six gallon milk crates, but they are much more expensive, and since the cellar fits 72 of them, the cost is considerable. Perhaps over the coming years I’ll find enough at yard sales to transition over to them. I didn’t weigh the total harvest, but it holds an incredible amount of carrots, turnips rutabagas, beets, and Jerusalem artichokes, as well as fresh cabbages and crocks of kimchi and sauerkraut. The rutabagas and turnips that didn’t fit into the crate went into feed bags in the corner of the cellar.

It was a fairly warm fall, so the cellar didn’t get down into the thirties for a while, but once it did it stayed there. The temperature held steady at around 35 until well into January, at which point the weather here, as on much of the east coast, turned arctic. With night routinely dropping to fifteen or twenty below, the temperature in the root cellar started creeping down, getting as low as 32.9 degrees. My wife Alanna put open buckets of water in the cellar moderate the temperature and keep the humidity up, and at the very coldest points of the year Ed shoveled snow against the door, since he figured that was where most of the temperature loss was occurring.

On the whole I’m happy with the performance of the root cellar. The veggies in it have held their quality, even as an extremely cold winter has limited how often I can keep the air vents cracked open. Going forward, it will be interesting to explore which vegetables store best. Lutz winter keeper beets, which I grew for the first time, yielded well and taste wonderful, but they have not lived up to their name in storage, going soft earlier than three root grex. The potato crop was almost a complete failure, which I will write about in the future, so I need to work on the best timing for planting, harvesting, and storing them. I plan on trying endive next year, and in the summer when the root cellar warms up a little I’m going to experiment with using it as a cheese cave. I’m sure in the coming months I’ll think of plenty of other ways to use it to get the most out of my garden and homestead.

Garth and Edmund Brown also blog regularly on the Cairncrest Farm website.

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