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Hoop houses – the fast passive solar greenhouse alternative

At my previous home I had a 10′ x 12′ polycarbonate greenhouse from Harbor Freight.  Now I got it on sale and stacked on a 30% off coupon that brought the price down to about $500 if memory serves correct – a fraction of what I would have had to pay for the panels alone from other vendors.

Building a passive solar modified Harbor Freight Greenhouse

Building a passive solar modified Harbor Freight Greenhouse

The aluminum frame was pretty light, and I think they got away with this by putting in only a few of the clips that would hold the polycarbonate panels in.  The smart engineers must have thought – if we allow the panels to blow out in high winds the remaining frame won’t buckle, which is true enough. All and well for wind but confronted with snow and… well snow presses in rather than blows out… leading to likely failure.

So I ended up building a reinforced internal frame for the greenhouse with post footings below the frost line.  While I was at it I dug down two feed under the footprint of the greenhouse, lines the excavation with Styrofoam insulation and snaked corrugated drainage tile from the middle of the back wall to the middle of the front wall.

Abundance and extended harvests in a passive solar Harbor Freight Greenhouse

Abundance and extended harvests in a passive solar Harbor Freight Greenhouse

I rigged a thermostatically controlled fan located in the peak of the greenhouse to ducting which connected to the drainage line at the back wall.   When the system was on, if the temperature at the peak was greater than the soil temperature the fan turned on and effectively heated the soil mass from the bottom.  If the temperature in the greenhouse dropped to 3C the fan would turn on again in an effort to use the heat stored in the soil to ward off frost.  As added security if the temperature hit 1C a switch would turn on an electric heater – in my case an old hair dryer to provide a thermal boost.  The added boost was usually only required at the start and end of my seasons and for a very brief period before sunrise on cold mornings making it rather thrifty.

This system worked great.  Our safe planting date here is usually the end of May – but I was able to be harvesting salads starting at the beginning of April, and tomatoes in mid-May.  I’d also be eating fresh veggies until Christmas on the other end of the season.

Since moving I’ve missed my greenhouse – but my new yard doesn’t have room for a similar setup.  I have used low tunnels of hooped 1/2 inch pvc pipe to support spunbond fabric as a season extender – but it doesn’t come close to stretching the season near a much.

Frame of the medium height hoop house constructed

Frame of the medium height hoop house constructed

So this weekend I set out to rectify the situation with a couple of mid-height hoop houses over my two raised wood core beds which are 4′ wide and 10′ long.  I took inspiration from the hinged cold frames used by the folks at Wild Gourd Farm. 

My upper frames are heavier than need be 2×6’s rather than 2×4’s but I had all those on hand, left over from the ramps needed to get my two letter-press printing presses home… but that is a story for another day.  Given the heavier materials I added a 2×6 cross piece at the halfway point for rigidity. Four 2″ hinges are positioned a 1′ and 3′ in from each of the ends on one side.

Nice height afforded by 10 foot long conduit segments over a 4' gap

Nice height afforded by 10 foot long conduit segments over a 4′ gap

The 10 foot lengths of 1/2″ plastic conduit give a nice amount of headroom under the curve.  I chose to use 10′ wide building plastic.  I know it will yellow, but like the wild gourd farm folks I used duct tape along the lower portions that I stapled to the frame to preserve it’s integrity and I stapled it a the rigeline to a 1″ x 2″ that I laced to the top of the hoops.

My intent is to be able to install the plastic in the spring – use it until the end of May, remove the lower staples, and roll it up around the ridgeline 1×2 and stow it in the garage attic until fall when I reverse the process.  When the season become too much I can again take it down.

So the weekend saw the project 90% completed – the only remaining task is to build a wax cylinder thermally operated vent for each hoop house to prevent overheating, but that should only be an evening worth of effort.

At the same time, the kids were so enthused with the project that they each wanted their own hoop house to cover their 4’x4′ garden plots.  So together we built the frames, and then on two adjacent sides attached the pvc conduits and bent them over to their respective opposing sides giving a wigwam shape over which we put a piece of dollar store painters plastic drop cloth weighed down by bricks.  The light plastic won’t last super long but it should last long enough to get us well into May.

Wigwam style hoop houses 4' x 4'

Wigwam style hoop houses 4′ x 4′

The project was an easy and pleasant way to spend a portion of a sunny weekend, I’m already looking forward to recapturing some of the season that I gave up when I lost that other greenhouse…  Forget sweet dreams, I have green dreams.

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