Seed-Starting 101: The Quick-and-Easy Cold Frame

by Doug Muller

Successful seed-starting takes infrastructure, be it a tricked-out heated glass greenhouse or a fluorescent shop-light setup in your basement. Either extreme--or anywhere in between--can work beautifully. However, in my experience, the solutions that are most likely to be implemented by busy gardeners are those that feel accessible and do-able in occasional spare moments.

This post covers one such solution: a cold frame constructed from easy-to-find, fairly inexpensive materials. On the left: the finished cold frame, functional and not too shabby-looking!

I'm a huge fan of cold frames. Not only do they hold miraculous quantities of promising green growth within their simple walls, they also are easy to build and will happily bring through the winter many servings of cold-hardy crops like spinach, scallions, tatsoi, and mache. Here's a cold frame that a reasonably handy person with some power tools can put together for about $100 with materials from a local lumberyard (or, unfortunately, big box store--see below). In one season alone, you can easily produce several hundred dollars worth of seedlings in this frame's roomy 32 square feet.

Materials List:

  • 2 pieces 8-foot-long, 26-inch-wide SUNTUF polycarbonate panels -- $40
  • 2 packs SUNTUF closure strips -- $10
  • 1 box SUNTUF screws -- $6
  • roll of tape sealant (often used for metal roof panel overlap joints and similar) or some silicone caulk -- $10
  • 2 pieces 8-foot 2x12 SPF lumber -- $20
  • 1 piece 8-foot 2x8 SPF lumber -- $8
  • 7 pieces 8-foot 2x2 SPF lumber, as straight as you can find -- $13
  • exterior-grade drywall screws: 1-5/8" and 3" -- $6
  • Hinges - $6

Tools List:

  • Circular Saw
  • Drill with 3/16" drill bit, Philips head driver bit, and 1/4" hex driver bit
  • Optional but makes things a little easier: Chop Saw

All of these materials can be obtained from a local lumberyard, with the probable exception of the SUNTUF items, which can be obtained from Home Depot or Lowe's. I like to give as much of my business as possible to my local lumberyard, Williams Lumber of High Falls, as I appreciate having a locally owned lumberyard so close to home. I want to support them. Unfortunately, they don't stock clear plastic roof panels of any kind, and since the point of this project was to concoct a quick-and-accessible cold frame, I bit the bullet and braved the strip of sprawl on Route 9W outside Kingston to get the polycarbonate cover. (Note that these panels are lightweight and long--they require a truck to be transported--with some sort of bracing to protect them from blowing away in the wind. Or, you can have the staff at the box store cut them each in half to fit them in your car--see below.)

Once you've assembled your materials, here's what to do:

  1. Cut each SUNTUF panel in half so that you end up with four panels that are each 26" wide by 48" tall. This is best accomplished with a circular saw, though tin snips will also do the job.
  2. Arrange the four panels so that they are spread out across a flat surface with the last rib on one panel overlapping the first rib on the next. Try to get them as straight and square as possible.
  3. Measure the distance from the bottom of the first space-between-two-ribs to the bottom of the last space-between-two-ribs. This should be somewhere in the neighborhood of 8 feet. It won't be exact, but that's okay.

Make the Frame for the Lid:

The frame for the lid. The frame for the lid.
  1. Miter cut the ends of two of the 8-foot 2x2s at 45-degree angles, like a picture frame's corners.
  2. Cut one of the other 2x2s in half. Miter cut the ends so that the long edges are 48", like a picture frame's corners.
  3. Attach the 2x2s at the mitered corners by pre-drilling to prevent splitting and then attaching the ends together using 1-5/8" screws or similar. The result should be a giant picture frame, basically.
  4. Cut another 2x2 to about 93" in length. Don't cut it too short! Place it in the center of the frame, centered 24" from top and bottom corners. This creates a middle horizontal support parallel to the other long sides of the frame; this will prevent the frame from sagging under the weight of adhered interior dew or exterior snow loads.

Finish the Lid:

  1. Using the drill bit, pre-drill holes in every other "valley" of each panel's ribbing along the top and bottom edges.
  2. Corner detail. Notice the miter cuts, the tape sealant, and the closure strips. Corner detail. Notice the miter cuts, the tape sealant, and the closure strips.
  3. Place strips of tape sealant along the top surface of the short sides of the frame. (Or, use silicone to seal this seam after step four. Place SUNTUF closure strips along the tops of the long sides of the frame.
  4. Line up the panels on the frame so that they are overlapping and cover the entire frame, setting them on top of the closure strips. Set the final "valleys" set so they are resting on the tape sealant (or, again, you can fill this seam with silicone caulk). This won't be a perfect match--the edges of the valleys will touch the sides of the frames, but they won't rest on it nicely. This is okay. Just be sure this gap is sealed (it may take a few layers of tape sealant, some applied after the cover is attached.
  5. Attach the panels using the SUNTUF fasteners and the hex-head driver bit.

Make the cold frame box:

  1. Cut one of the 8-foot 2x12's into 2 45" lengths.
  2. Using a straight edge, draw a line from the top corner of one end of the length to a mark at 7-1/4" from the bottom corner of the other end. Cutting on this line will create a side to the cold frame that will slope exactly from the rear 2x12 wall to the front 2x8 wall.
  3. Using a circular saw, cut along this line. Be careful--it can be tricky to perform this cut, as it's something of a ripping cut that sort of follows the grain.
  4. Repeat for other 45" length.
  5. Position the pieces of the cold frame. The two 8-foot pieces of lumber are parallel, with the two 45-inch pieces of sloping lumber forming the sides, with the un-ripped side up. These smaller pieces should be "inside" the 8-foot pieces so that, when sandwiched, the entire length of the side is 48" (including the 1-1/2" for the ends of both the rear and front walls).
  6. One sloping side, sandwiched between the longer front and rear walls. One sloping side, sandwiched between the longer front and rear walls.
  7. Pre-drill holes and attach all sides of the frame using the 3" screws.
  8. Half-way down the short sides of the cold frame, attach a spare piece of wood to the inside top edge, flush with the sloping surface of the side.
  9. Flip the cold frame over. Cut one of the three remaining 2x2's into 2 45" lengths. Match these up with the undersides of the lumber that makes the frame and attach with the 3" screws. This will be the "ground floor" of your cold frame that will slowly rot over several years. After it's rotted, simply detach and replace with a new "ground floor." The rest of the cold frame will last for about 20 years or so if left out--maybe more if stored well when not in use. (The ground floor is not shown in the accompanying photos.)

Put the Lid on the Cold Frame:

  1. Set the lid on the cold frame, matching up the corners with the frame.
  2. Attach to the cold frame using a couple of long rectangular hinges and short screws.
  3. If the lid does not sit squarely on the frame, purchase and install a latch to hold it snug.

VOILA! A functional cold frame that can be built in an afternoon for around a hundred bucks. Fill it with trays and go to town! You'll find endless uses for it.

On the right: Spring's-a-comin'! Happy onions, leeks and scallions gain strength and size and fortitude for spring.

This blog is provided by the Hudson Valley Seed Library, a small group of dedicated growers and plant lovers working to provide good seed to gardeners and small farmers. Your purchases support our work. Thanks!