Get your greenhouse started

Close the gap between the seed cataloge and the first shovelful of soil with this attractive, space-saving greenhouse

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Photo by Roger Yip

Like a great vacation, gardening actually starts with anticipation, well before the Canadian climate permits. You can close the gap between the seed catalogue and that first jab of a shovel into soil if you nurture those seeds yourself with a vertical cold frame (or starting greenhouse).

My design can be collapsed and stored after its seasonal work is done, leaving more room for the garden itself. The greenhouse is built of western red cedar; while this wood is pricey, it’s lightweight, warp-resistant and attractive without paint. For durability, exterior-grade screws and nails, stainless-steel staples and outdoor-rated hinges are a must.

Instructions

Time to grow

This project is made entirely out of plastic-covered frames made with 2x2 and 2x8 lumber. Lay out the parts for each panel in your workshop, then use your carpenters' square to ensure each panel is true as it comes together.

 

At each aligned corner, drill and countersink for #8 x 31/2" deck screws. Hold the joints together tightly and drive the screws, being careful not to strip the holes by over tightening. Note in the plans that the end panels need extra crosspieces, placed at intervals to support the shelves.

 

Lumber retailers sell heavy, 6-mil polyethylene vapour barrier by the foot, and it’s perfect for sheathing the panels. (Don’t worry about the text written on the plastic. It rubs off with a little Varsol.) For easier handling, cut a piece of the plastic several inches wider than the width of the panel you’re working on. Lay the plastic across the panel frame with the factory-cut straightedge on a long side. Load your stapler with 3/8" stainless- steel staples (necessary for preventing ugly rust stains), then get ready to think like an upholsterer.

Panels and door assembly

Align the poly sheet and staple the down first corner. Pull the straight edge tight, being careful not to tear the plastic off. Staple the second corner, then staple the rest of that side at 2" intervals. Move to the opposite end and pull the corners taut. Before you staple, check again that the frame is still square. Chances are it will be; the plastic sheeting does a remarkably good job of keeping the wood just the way it is.

 

Pull the centre of the end you’re working on tight, then staple again at 2" intervals. Repeat the process on the other two sides. Use a sharp blade to cut off the excess. Leave about 1/2" excess around the outside edges of the frame.

 

The doors are simply panels themselves, sheeted with poly in the same way as the other frames. They form the entire front face of the greenhouse. And while the doors do add some strength, battens over the edges of the plastic sheeting and staples protect the seams and enhance appearance. While you could choose to cut battens from red cedar lumber, then sand and round all of those edges, I took a slightly more expensive shortcut and purchased pine batten mouldings instead. I opted to protect this wood with a penetrating outdoor stain in a red cedar colour.

 

Centre the battens on the frame, set in 1/4" from the outside edge, along the frame’s inside. I also cut the ends of the battens at a 45º angle for a tidy, mitred fit. Use 11/4" hot-dipped galvanized or stainless-steel finishing nails to apply the battens. Anything less will lead to ugly rust stains over time.

 

Two of the battens involve door hinges, so they need extra preparation before assembly. While the hinges could be recessed into the frame members, it really isn’t necessary to go through the trouble. It’s easiest to cut a recess for the hinges into the back of the battens. I used a mitre saw for this, with the blade depth set to match the width of hinge thickness.

Build the top of the greenhouse

The top of the greenhouse needs to shed rain. I designed a cambered, wing-like profile to create the essential slope, while leaving the front, back and end panels the same height and with square corners—a much easier building job.

 

The ends of the top are best cut from a length of 2x8 red cedar. The longitudinal frame members are 2x2 red cedar, just like the rest of the frame. For no other reason than aesthetics, I cut a leaf pattern through the end pieces. Beauty is also why I used 4"-long galvanized finishing nails rather than the ubiquitous and all too obvious deck screws to hold everything together. Predrill holes slightly smaller than the nail itself to avoid splitting the cedar. The centre frame piece must be notched to accommodate the 2x2 stringers, and these are best fastened with the 31/2" deck screws for maximum strength. No one will see them anyway. As with all other frame members, use your carpenter’s square to be sure the top comes together with 90° corners.

 

The entire top frame is a little wider than the rest of the structure to shed rain. To continue the flow of the curved top, I wrapped the poly around a piece of pine quarter round, and nailed it with 15/8" galvanized nails to the front edge of the top. This arrangement adds something to the drip edge and means a batten isn’t needed to protect that area of the plastic. With the quarter round in place, stretch the poly over the top and staple it along the lower back edge of the drip edge.

 

At this stage, leave the plastic larger than necessary all around the top for a good grip. The idea is to pull the sheet as taut as possible, back to front and end to end. A tight membrane keeps the roof frame strong and allows water to shed completely. Just be sure to round the top edge of the back stringer and the upper back corners of the end pieces smoothly before you put on the plastic. Sharp corners will puncture the poly. Finish up by reinforcing all stapled edges with more wooden battens.

Greenhouse assembly

The wall panels of this greenhouse are held together by hinges that have removable pins so the structure comes apart for off-season storage. Install the hinges on a large, flat surface. Tap out the hinge pins before assembly, anchor the hinge plates with screws, then apply a little white lithium grease to the pins before putting them almost all the way back in. This strategy makes it easier to remove the hinge pins at the season’s end.

 

With the frame of the structure together, you can easily add the other pieces. The positioning of the shelf support pieces on the end panels and on the centre supports of the unit is arbitrary. You can put them anywhere you like, or add more shelves if you wish. The end supports are permanent. The centre supports, on the other hand, are removable to allow for seasonal storage, and sit atop blocks affixed to the back and front central vertical frame members. The blocks are positioned so that when the cross supports are placed on them, the end and central supports are at the same level. This set-up is key. Dowels keep the centre supports in place.

 

The inside front-to-back distance is 22 1/4", and this becomes the length of the central horizontal pieces. Drill a 1/4"-diameter hole, 1" deep and 3/4" in from the ends of the underside of each cross support. Tap a 2" length of hardwood dowel into each of these holes. Next, drill a 5/16"-diameter hole at least 1" deep into the top centre of each support block, before these are mounted with screws to the vertical members. The pegged cross supports in the blocks add more strength to the structure as a whole; they also support the shelves.

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.

Shelf assembly and extras

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 1 1/2" 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 1 3/4" 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 extrasCatches, 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.

Tools & Materials

Part Material Size (T x W x L*) Qty.

Greenhouse frame

Verticals red cedar 1 1/2" x 1 5/8" x 68 3/4" 10
End panel horizontals red cedar 1 1/2" x 1 5/8" x 22 3/4" 4
Back panel horizontals red cedar 1 1/2" x 1 5/8" x 73" 2
Mid-back horizontals red cedar 1 1/2" x 1 5/8" x 34 1/16" 2
End panel shelf supports red cedar 1 1/2" x 1 5/8" x 19 1/2" 8
Floor stringers red cedar 1 1/2" x 3 1/2" x 72" 2

Door frame

Verticals red cedar 1 1/2" x 1 5/8" x 65 1/8" 4
Top/bottom horizontals red cedar 1 1/2" x 1 5/8" x 33 5/8" 4
Mid-door horizontals red cedar 1 1/2" x 1 5/8" x 30 3/8" 2
Door stop strips red cedar 1/2" x 3 1/2" x 72" 1

Top panel

Stringers red cedar 1 1/2" x 1 5/8" x 70" 3
Ends and centre sections red cedar 1 1/2" x 7 1/2" x 26 1/2" 3
Drip edges pine quarter round 3/4" x 3/4" x 73"** 1
Battens pine moulding 1/4" x 1 1/8" x 98'** 1
Prop rods red cedar 3/8" x 1 1/2" x 13" 2
Dowel pegs hardwood dowel 1/4" x 1 3/4" 2

Shelves

Shelf slats red cedar 3/4" x 1 1/2" x 36" 64
Shelf cleats red cedar 3/4" x 1 1/2" x 19 1/4" 16
Centre shelf supports red cedar 1 1/2" x 1 5/8" x 22 1/4" 4
Centre shelf support blocks red cedar 1 1/2" x 1 5/8" x 4" 8
Dowel pegs red cedar 1/4" x 2" 8

Door latches

Door latches red cedar 3/8" x 1 1 /2" x 4 1/2" 2
Spacers pine batten 1/4" X 1 1/2" X 3" strips 2
**Cut as required



* Length indicates grain direction

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Get your greenhouse started

Illustration by Len Churchill

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