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Collapsible Greenhouse
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Give your plants a chance with this portable starting house
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By Don Ross; Photography by Roger Yip
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Water Jugs add weight to the bottom of the collapsible greenhouse
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While you could place stop strips all around the inside of the door frame, it isn't necessary with a structure such as this. All you really need is a single strip along the vertical member at the front of the house. The strip stops the doors from swinging inward as they close. Saw a 6' length of 2x4 cedar vertically with your tablesaw – first along one edge, then along the other – making a piece of wood 5⁄8" thick and 31⁄2" wide. The remainder of this board is destined for shelf slats. A few passes through a thickness planer makes both faces smooth. Refine the edges of the stop strip with a 1⁄4"-diameter roundover bit in a router so your sleeves won't get caught as you work your green thumbs inside the greenhouse. Use countersunk 11⁄4"-long deck screws to anchor the stop strip on the inside of the front vertical member.
My plans call for four levels of shelves in all. For easier handling and more versatility, each shelf level actually includes two shelves side by side. The entire greenhouse measures 73" wide across the inside, while each shelf is 36" long. The extra 1" in width is clearance that makes the job of placing the shelves that much easier.
Each shelf is 191⁄4" wide and is designed to sit upon the end and central supports. Cut the slats from the 2x4 red cedar lumber. With 1" space between each slat, each shelf needs eight slats, for a total of 64 pieces. Pass all slats through a planer for finishing one side and for uniform thickness.
The cleats for the undersides of the shelves are 191⁄4" long, and made the same way as the slats. Mount these at least 1" in from the ends of the underside of the shelves, with 11⁄4" deck screws drilled and countersunk as before.
Two very simple wooden prop rods both serve to lift the hinged top for ventilation and to hold it closed on windy days or cool nights. I used a 13"-long x 11⁄2"-wide x 3⁄8"-thick strip of red cedar for the prop rods. To make the roof opening adjustable, I bored 5⁄16"-diameter holes through these prop rods at 1", 43⁄4" and 12" from one end. A #12 x 11⁄2" pan-head screw driven into the top frame makes a great pivot point for the top ends of both prop rods.
With the prop rods hanging down under their own weight, mark the locations of the middle holes with a pencil on the wall frames. Drill 1⁄4"-diameter dowel holes, 1" deep in these locations. Tap a 13⁄4" length of 1⁄4"-diameter hardwood dowel into each hole to hold the roof closed when its engaged with the middle hole on the prop rod. When you want to hold the roof open, engage the lower prop rod holes with these dowels.
Little extras
Catches, latches and handles for the two doors can be as simple or as fancy as you'd like to make them. I used very basic, homemade wooden pivoting latches, one near the top and the other near the bottom of the doors. The two locations ensure that the twist-prone doors close tightly on frosty nights.
Any greenhouse made in this manner is going to be light in weight and easily blown over by the wind. I included a provision for weighting the bottom of the house that also functions as a heat sink for getting through cool spring nights. You can install 2x4s along the bottom of the structure for this purpose.
Scrounge six or seven 10-litre rectangular water containers, available at the grocery store, fill them with water and lay them down on the bottom stringers. Their weight adds stability and the thermal mass of the water evens out temperatures from day to night.
For a long, rot-free working life, consider placing your greenhouse on a slightly elevated bed of crushed stone. It keeps the wood away from earth made soggy by rain and regular watering.
With seed catalogues bulking up your mailbox, a greenhouse is a quick and easy way to get a head start the gardening season.
Click here to see the materials you'll need to build your greenhouse.
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