Tuesday, August 25, 2009

My Latest Project -- A Portable Cold Frame

First, if you are not a gardener, you may be asking "What is a cold frame?" A cold frame is something like a miniature greenhouse, typically used to start plants from seed earlier in the season than out in the open, by providing a warmer, more protected environment. If a heater were used to provide warmth, it would be called a hot frame; but a cold frame just captures the heat of the sun through a window, and a simple enclosure helps to retain the warmth.

Sometimes an old window is simply placed over a bottomless box made of boards set on edge, and seeds are planted in the earth enclosed by these boards. But I wanted a portable cold frame that could be placed over a stone walkway, and moved under the deck when not used. Other suburban gardeners, and city gardeners, with limited space, might want to set up such a cold frame on a patio or paved area, and store it elsewhere. perhaps standing on edge, when it is not used. So I'm publishing my design to share it with others -- here in this blog, and in a Picaso web album.

I decided to use deck planking for the sides, because this lumber is generally treated for weather resistance, and use to plastic for the window -- the kind made as a substitute for window glass. I wanted the cold frame big enough to comfortably hold nine seedling trays (in a 3-by-3 arrangement), the kind that you get from greenhouses or gardening stores. That closely matches a 3 by 4 foot window piece.

The bottom of the cold frame is covered with 'hardware cloth', a wire mesh with 1/2-inch spacing, to allow drainage and support for the seedling trays. But that would allow stray dirt to fall through, which is not good if used over a stone or paved walkway or a patio. So 'weed-block' fabric is lain over the hardware cloth. This kind of tough porous fabric is made for use under a bed of loose stones or paving stones to allow drainage while preventing the soil from mingling with the stones, and blocking access to the soil by any seeds that fall among the stones. For the cold frame, we get drainage without allowing soil to fall through.

All my garden areas are automatically watered, controlled by timers, either by porous 'soaker' hoses for the larger areas, or by a drip system for containers and the raised-bed herb garden. So I naturally wanted a watering system for the cold frame. But this is an optional feature; the cold frame can be built without it, and you can water it with a watering can or a hose with a spray attachment.

To make the cold frame portable, it is made of three stacked frames, each one of which is not too heavy to carry. The frames begin as identical units:
From Portable Cold Frame

One of the frames is cut on an angle, like this:
From Portable Cold Frame

Then the parts are rearranged to form this sloped shape, which becomes the top frame, with a sloping window:
From Portable Cold Frame

The bottom of the bottom frame is covered with hardware cloth, held in place with stop moulding (the kind used as a door stop on a door frame):
From Portable Cold Frame

The "weed-block" fabric is stretched over and stapled to a frame of 1/2" x 1/2" moulding, and placed inside the bottom frame:
From Portable Cold Frame

Nine seedling trays fit inside the bottom frame:
From Portable Cold Frame

The middle frame has a vertical piece fastened in each corner that protrudes one inch above and one inch below the middle frame. These verticals lock the middle frame in place to the frames above and below it, like this:
From Portable Cold Frame

(The aluminum rail in the above photo is an optional support for the watering system.)

Two ventilation holes are formed in the rear wall of the top frame by cutting notches in the facing edges of the two planks of the rear wall:
From Portable Cold Frame

A slot is cut across each notch by dropping a circular saw into the plank edge, then a piece of hardware cloth is inserted into the slot, like this:
From Portable Cold Frame

The piece of hardware cloth is captured in place, with no need for fasteners. I like minimalist designs like this.
From Portable Cold Frame

Two vertical pieces hold the two rear planks of the top frame together. These verticals protrude below the top frame, and together with the rear verticals of the middle frame, form hinges. One hinge is show here, as well as one corner of the watering system at the top of the middle frame:
From Portable Cold Frame

The edge of the plastic window is sandwiched between 'stop' moulding (below) and 'ply cap' moulding (above), which are nailed to the top edge of the top frame:
From Portable Cold Frame

The next photo is looking through the top window toward the inside of the front wall of the cold frame. Here we can see a length of 1/2" x 1/2" moulding fastened to the inside of the front edge of the top frame with two screws. Below that are two pivoting pieces of 1/2" x 1/2" moulding fastened to the inside upper front edge of the middle frame with one screw each. The pivot screws are off-center, so that pointing the short end of the shorter pivot piece upward props the top frame open with a 1-inch gap, and using the longer end of the shorter pivot makes a 2-inch gap. Similarly, using the longer pivot piece provides 3-inch and 4-inch gaps when propping open the top frame.
From Portable Cold Frame

A larger prop is used to hold open the cold frame, as shown in the next photo. This also shows the inside painted black, to increase the heat energy absorbed from the sunlight coming through the window.
From Portable Cold Frame

The above photo also shows the watering system attached to the top frame, where it is lifted up when the top frame is lifted. Earlier, I had mounted the watering system on the middle frame, where it needed to hinge separately, and needed a second prop:
From Portable Cold Frame

The watering system is built on an aluminum frame that supports nine spray heads, each centered over one of the seedling trays:
From Portable Cold Frame

The spray heads, hoses, and connectors are made by RainDrip, which calls the spray heads "misters". Here is a close-up of one of the 'misters', and a wire loop that joins two pieces of the aluminum frame:
From Portable Cold Frame

The 'mister' comes mounted on a plastic stake, which I cut short and fasten to the frame by inserting it through an X-shaped hole in the frame. Each 'mister' head emits eight radial streams of water, and can be rotated to adjust the flow, or even to turn it off.

The 'misters' are connected by quarter-inch tubing to a half-inch supply line, which connects to regular garden hose, as shown here:
From Portable Cold Frame

For more details, you can view the entire Picaso web album by clicking on the link under any of the photos above. Also, we give more details of the cold frame design (except for the watering system) next, including:

Dimensions
Shopping list
Cutting lists
Tools needed/suggested
Construction



Dimensions of the cold frame:

Seedling trays have various designs, but are generally 10 1/2 by 13 1/2 inches, and 3 1/2 inches high. If we allow 11 by 14 inches for each tray, it will allow for some size variation and room for fingers when handling them. So a 3 by 3 configuration of trays will comfortably fit in a 33 by 42 inch space, and a 2 by 4 configuration of trays will fit in a 28 by 44 inch space; and a 33 by 44 inch space will accommodate either configuration. My design adds an extra inch to this.

Width and length: 34 by 45 inches inside, 36 by 47 inches outside.

Inside height is 2 plank-widths plus 1 inch in front, and 4 plank-widths minus 1 inch in back. Some planks are 5 1/2 inches wide; these provide a height of 12 inches in front and 21 inches in back. Allowing 3 1/2 inches for the seed trays and 1 1/2 inches for the watering system, this allows 7 inches of height for seedling growth. Some planks are 5 3/4 inches wide; these provide 1/2 inch more height in front and 1 inch more in back.

For outside height, add the thickness of the "stop" moulding and hardware cloth at the bottom, and the window frame thickness ("stop"and "ply cap" mouldings) on top. This is about an inch.



Shopping list for the cold frame, not including the watering system:

6 8-foot lengths or 4 12-foot lengths of decking planks, weather-treated; 5 1/2 or 5 3/4 inches wide and 1 inch thick. Choose straight, unwarped pieces.

2 8-foot lengths of "ply cap" moulding
(See photo of mouldings.)

5 8-foot lengths of "stop" moulding

2 8-foot lengths of 1/2" x 1/2" (or close to this) moulding

1 3-foot by 5-foot piece of hardware cloth, with 1/2" grid

1 3-foot by 4-foot piece of plexiglass, or plastic window 'glass'

1 roll of 3-foot wide 'weed block' fabric; black is preferred

1-pound box of 2 1/2" screws

1-pound box of 1 1/2" screws

1 1/4 inch nails, about 32

thin 1 inch nails, about 8



Cutting list for 4 12-foot lengths of decking planks:

(1) 47 + 47 + 47 inches
(2) 34 + 34 + 28 + 47 inches
(3) 34 + 34 + 47 inches
(4) 34 + 34 + 7 3/4 + 7 3/4 + 12 1/2 + 47 inches

Cutting list for 6 8-foot lengths of decking planks:

(1) 47 + 47 inches
(2) 34 + 34 + 28 inches
(3) 34 + 34 inches
(4) 34 + 34 + 7 3/4 + 7 3/4 + 12 1/2 inches
(5) 47 + 47 inches
(6) 47 + 47 inches

The 7 3/4 and 12 1/2 and 28 inch pieces are cut in half lengthwise.

Mouldings, hardware cloth, and 'weed block' fabric are cut to fit the frames.

About 2 inches is cut off one short side of the plexiglass so that the window overlaps only half of the window frame all around.



Tools - see photos.



Construction (Also see photos):

The six 34-inch pieces and six 47-inch pieces are used to make three frames, 34 by 45 inches inside, and 36 by 47 inches outside. Use three 2 1/2 screws at each corner, but for the top frame, mark where the front (long side) will be cut one inch from the bottom edge. Two of the screws need to be centered in this one inch so that no screws will be cut. Clamp each corner joint in position, drill three holes with a drill matching the solid core of the screw, insert the screws so that the heads sink into the wood a bit, then remove the clamps.

Bottom Frame ---

Ihe hardware cloth is probably in a 3 foot wide roll; you will need to unroll a 4 foot section of it and make it approximately flat. Use 1 1/2 inch nails for the following. Lay the hardware cloth over the bottom frame, and begin by nailing down a 36-inch edge of the hardware cloth on a short edge of the bottom frame. Use only enough nails to hold this edge in place. Then stretch it out to reach the other short edge. Cut the hardware cloth to match the length of the bottom frame (47 inches). Cut three pieces of stop moulding to match the width of the bottom frame (36 inches). Nail one of these pieces over each of the short edges of the bottom frame, anchoring the two ends of the hardware cloth. Nail the third piece of stop moulding over the center, from the center of one long side to the center of the other long side. Now cut four pieces of stop moulding to cover the exposed portions of the two long sides of the bottom frame, and nail these in place. This is the bottom of the bottom frame.

Cut four pieces of the 1/2" x 1/2" (or close to this) moulding to make a rectangular frame that will fit inside the bottom frame with a margin of 1/8" all around. Fasten these pieces with two thin 1-inch nails at each corner. Cut a rectangle of 'weed block' fabric that is about 36 by 47 inches. Stretch this fabric over the frame of moulding and staple it to the outer edge of the moulding. Place this inside the bottom frame with the stretched fabric on the hardware cloth and below the frame of moulding.

Middle Frame ---

You should have four small pieces of planking 7 3/4 inches long, with a width one-half the width of a plank. These 'verticals' will be fastened to the long sides of the middle frame, at the inside corners, protruding one inch above and one inch below the frame. (See photos.) Use three 1 1/2 screws for each vertical. Clamp each vertical in position, drill three holes with a drill matching the solid core of the screw, insert the screws so that the heads sink into the wood a bit, then remove the clamp. With a coarse file, taper the outside edges of the protruding parts of each vertical to make it easier to fit the frames together.

Try stacking the frames. If the fit is too tight, try turning one of the frames around, or turning the middle frame up-side-down. If a better fit is found this way, mark the frames to indicate which sides should be aligned. If the fit is still too tight, use the coarse file to remove some wood from the outside edges of some verticals.

While the top frame is stacked on the middle frame, mark on the bottom rear plank of the top frame the location of the inner edges of the rear verticals of the middle frame. This will help to locate the verticals of the top frame.

Top Frame ---

For the top frame, mark where the front (long side) will be cut one inch from the bottom edge. On each side (short side), mark a straight angled line from the center of the top edge to one inch above the bottom of the front corner. First cut each short side on these lines. Then adjust the tilt of the saw blade to match this angle, and cut the long front side lengthwise one inch from the bottom edge.

Ventilation holes

Cut notches for the ventilation holes in the edges of the rear planks of the top frame that will be facing each other. Each notch is 4 inches wide and 1 inch deep. Mark each edge 10 and 14 inches from each rear inside corner, adjust the saw for 1 inch depth, and cut across each mark. Before cutting out the notches, mark the center of the edge across each notch, at least 1 inch past the notch on either side. Then adjust the (circular) saw depth to 2 1/8 inches, and cut a slit in the center of the edge by dropping the saw blade down on the center line. With a general-purpose saw blade, the cut should be wide enough for the hardware cloth to fit. (For a handheld circular saw, you need to lean the front edge of the saw guide on the plank edge, line it up, turn on the saw, and slowly tilt the saw down to a level position.) The center of the saw blade should land over the center of the notch; but to be sure, slide the saw a little to either side of the estimated center position.

Mark the bottom of each notch one inch from the plank edge. Here are two methods of cutting the bottom of each notch:

(1) With a 1/4 inch drill, make holes along the bottom of the notch, including the corners. The holes should be tangent to the line marking the bottom edge of the notch. Use a chisel to break away most of the wood from the notch area, then finish with a rough, then smoother, file.

(2) With a 1/4 inch drill, make holes only at the corners. With a saber saw, cut the bottom edge of the notch from one hole to the other.

Cut two 3 by 5 inch rectangles of hardware cloth, and cut away the four half-inch corners of each piece, as shown in the photos. Make sure that each piece is as flat as possible.

By now, the slit that was cut across each notch is filled with sawdust. Use one of the hardware cloth pieces to scrape out the sawdust. Now insert a hardware cloth piece in each slit of the larger piece of the top frame. The center of the hardware cloth piece should align with the edge of the plank. Now stack the smaller piece of the top frame on top so that its slits fit over the top half of the hardware cloth pieces.

Verticals

While the top frame was stacked on the middle frame, you marked on the bottom rear plank of the top frame the location of the inner edges of the rear verticals of the middle frame. This will now be used to locate the verticals of the top frame. Make a vertical line at each mark, that is, parallel to the corner of the top frame.

You should have two pieces of planking 12 1/2 inches long, with a width one-half the width of a plank. These are the verticals for the top frame. Place each piece alongside one of the vertical lines that you made, on the side of the line away from the corner. The top of each vertical piece should be 1/2 inch below the top rear edge of the top frame, and the bottom should protrude below the top frame. Clamp in place, then fasten each vertical piece with six 1 1/2 inch screws, three screws into each rear plank, and alternating the positions of the screws toward the left and right sides of the vertical. (Drill holes for the screws as before.)

Each side of the top layer of the top frame is a triangular piece that extends from a rear corner toward the center of one side. Line up each of these pieces with the plank below it, and drill a hole for a 2 1/2 inch screw 4 inches back from the pointed end down through the pointed piece into the plank edge below it. Then, holding the pieces in alignment, fasten with a 2 1/2 inch screw in the predrilled hole, sinking the head of the screw into the wood a little.

Window

Cut pieces of 'stop' moulding to go on the top sloping edge of the top frame. To make mitered corners, cut these to overlap at the corners, and hold in place, overlapped at the corners, with two temporary nails on each piece, about 10 inches away from the corners, and only partly hammered down. With a hacksaw or other fine-toothed saw, cut across each corner on an angle from the outside corner to the inside corner through both overlapping pieces. Remove the cut-off scrap, and the moulding will lie flat with mitered edges at each corner. Remove the temporary nails, and re-nail with 1 1/4 inch nails with about 10-inch spacing all around.

Lay the window plastic sheet on the top frame and position so that two or three edges of the plastic overlap half the width of the border of 'stop' moulding. Mark the remaining one or two edges of the plastic where it should be cut so that all edges of the plastic will overlap half the border of moulding. using a grease pencil or crayon and a straight-edge. Then cut the plastic as marked, with a hacksaw blade.

Cut pieces of 'ply cap' moulding to go on the top edge of the top frame. The thicker edges of the 'ply cap' moulding go toward the outside, the stepped side down and the curved side up. Make mitered corners as for the 'stop' moulding, but don't nail permanently.

Put the plastic window on the top frame, overlapping half the width of the frame of 'stop' moulding all around. Then put the 'ply cap' moulding pieces in place so that the thin inner edges lie on top of the window and the thick outer edges lie on the 'stop' moulding and trap the plastic window in place. Nail with small nails, piercing the 'ply cap' moulding at the step edge where it meets the window edge.

Props ---

Here we describe devices for propping open the cold frame, either a little bit for ventilation, or high enough to reach in and work with the contents.

Cut three lengths of the 1/2" by 1/2" moulding, 6, 10, and 13 inches long. The 1 1/2 inch screws can be used with these, but 1 1/4 inches would be better. Fasten the 12 inch piece centered on the inside of the front wall of the top frame, with two screws, 2 inches from each end of the piece. Drill a hole (for a screw) in the 6-inch piece 2 1/2 inches from one end. Drill a hole in the 10-inch piece 4 1/2 inches from one end. On the inside of the front wall of the middle frame, make two holes 1 1/2 inches down from the top edge and 4 inches from the center to the left and right, that is, 8 inches apart.

Screw the 6 and 10-inch pieces to the middle frame using these holes. With these pieces each fastened with one screw each, they can pivot on the screws. When not used, these pieces are horizontal with the longest arms pointing toward the corners of the frame. But one of the four ends of these pieces can be turned upward to prop open the top frame. Depending on which of the four ends is chosen, the top frame will be propped open 1, 2, 3, or 4 inches for varying amounts of ventilation.

Take one of the 28-inch pieces that are half the width of a plank, and cut a notch in each end. Cut each notch by making two cuts into the end 3/4 inch from each edge, and 3/4 inch deep. Draw a line for the bottom of the notch, from the bottom of one cut to the other. Cut out the bottom of the notch by either of the methods described for the notches use for the rear ventilation holes.

This prop is used to hold the cold frame wide open, for working inside. The notches keep it from slipping off the edges of the top and middle frames. When not used, it can be stored inside the bottom frame alongside the front wall, or alongside a side wall if you prefer that.

If you like, you can fasten a handle to the front edge of the top frame.