How to Build a Cold Frame

Ron's first house call this week took him to Charleston, South Carolina, one of America's most picturesque cities. Louisa Cameron requested that Ron visit her in her elegant and historic Charleston home to help her with a gardening project.

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Louisa had always wanted a cold frame, which is like a miniature greenhouse used to incubate plant cuttings, but she had no idea how to go about building one. Ron certainly knew how to build such a box, but he admitted to not really knowing what it was used for. Together, they were a perfect team!

1. Create design:

• Ron created a simple design-a basic box supported by four legs. On top, he designed a hinged cover made of clear acrylic plastic that would allow sunlight to provide warmth for the growing plants.

2. Build frame:

• Ron began by measuring and cutting the 2x2 wood pieces for the frame.

• Once the wood was cut, they began by building the front and back sections of each base. Ron laid out the cut pieces on a temporary worktable and drilled clearance holes for the screws that were to hold the frame sections together. Ron used a countersink bit for this job so the screw heads would be flush with the surface of the wood.

• Ron used #10 x 2 ½" rust proof, stainless steel screws to secure the frame pieces together.

3. Install braces:

• The next step was to install the braces that connected the front and back frames together.

Tip: Ron created a stop by attaching a length of 2 X 2 to his plywood work surface. This way he had a firm surface to lay the frame down on, but also an edge to work firmly up against while securing the frame together.

• After the frame was assembled, Ron set it on the ground and checked that it was level and plumb and that the height was where Louisa needed it.

4. Attach sides:

• Once the frame was assembled, they added the sides to the planting box. For this, Ron used two rows of ¾ inch cedar tongue and groove planks. By putting the tongue up, he created a joint that would prevent soil and sand from seeping out.
• They made sure that the planks were flush on each end, and then secured them to the frame with #8 X 1 ½" screws.

5. Install bottom planks:

• Ron also selected cedar also for the bottom of the box because it naturally resists decay and insects. This time he chose planks without tongue in groove edges.

• Some of the planks required a little detail work to fit around the box frame. Ron used his favorite Japanese handsaw for this job.

• The planks were left loose on the bottom of the cold frame for two reasons. First, they can easily be replaced if and when necessary and second, there is a small crack between each plank that provides drainage.

6. Trim:

• Next, Ron added a trim strip to cover the top edge of the box. These trim pieces required 45-degree angles cut on the ends. To save time and steps, Ron brought a portable, battery-operated miter box right to the work site.

• Ron used # 6 x 2" rust-resistant, finished head screws to reduce the chance of splitting and to create a finished look.

7. Acrylic top frame:

• With the table saw, Ron cut slots called dadoes on the inside edges of the four frame pieces. These were designed to hold the clear plastic Plexiglas panel in place.

• Next, Ron made an angled miter cut on each end of the four frame pieces. They assembled the Plexiglas frame the same way they assembled the trim piece described above.