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

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!

Siberian Kale

Siberian Kale

Hardy, medium-frilled, long-leaved, productive.

Silverado Chard

Silverado Chard

Our favorite green chard; hardy, beautiful, and delicious.

Glacier Tomato

Glacier Tomato

A must for any impatient tomato lover in colder zones!

Utah Tall Celery

Utah Tall Celery

You won't believe how tasty home grown celery is.

16 thoughts on “Seed-Starting 101: The Quick-and-Easy Cold Frame”

  • Naseer

    I'm going to start building my cold frame this weekend, thanks for the detailed instructions!

    Also, I noticed this post is still filed under "uncategorized". You may want to file it under "seed starting 101" like the previous 3 posts so it comes up when you click http://www.seedlibrary.org/wp/?cat=93 .

    Reply
    • doug

      No problem. I'm not sure I've ever written DIY construction directions before, and I guide my own projects by my own semi-expertise... so please let me know if anything is unclear or insufficiently explained.

      Good luck!

      (And thanks for the reminder. Categorizing the posts is a total blindspot of mine!)

      Reply
  • Marjorie@TheCook's Atelier

    Thanks for the DIY. I'm getting ready to start my heirloom bio tomato seeds that I saved from last year's market here in Burgundy. Glad I found your site.

    Reply
  • Naseer

    Ok I've got most of what I need in place. Two questions:
    a) I asked about the roll sealant at my local big box store and they pointed me to the duct tape, which I'm pretty certain was wrong. Do you have any more info on that?
    b) Do you do anything to weather-proof your wood? I'm guessing applying standard weatherproofing is probably a no-no since the chemicals could leach into the soil. How many years do you typically get out of the cold frame that you designed? It's cheap enough that I wouldn't mind replacing the wooden pieces every few years, but I was just wondering.

    Reply
    • doug

      The box store where I found the panels sold tape sealant in the shelving underneath them. If yours didn't stock it, I wouldn't worry: you could definitely use silicon caulk instead. It's cheap and goes on quick after the panels are attached to the frame. Just be sure when you're lining things up that the "valleys" at the ends of the panels are close enough to the wood to allow the silicon to fill the gap easily (say 1/4" or less).

      This sort of cold frame will last a very long time if you do one final thing that I neglected to put in my original instructions: rest it on (and screw it into) a "ground floor" of 2x2's. This is an idea I took from Eliot Coleman. The 2x2's will rot after a few years (say 4 or 5?), but the main cold frame itself does not make ground contact. It will turn gray and slightly weathered looking, but it'll last a long time (20 years?) as long as you keep replacing the bottom layer of 2x2's (at a cost of about, say, $8 every four years). I'd avoid any wood sealing product, as few commercial products are non-toxic (though if you google around you may find a great recipe for a homebrew non-toxic turpentine and linseed oil and paraffin wax concoction that I've found works quite well--but I don't bother to use it on the cold frame). I'll have to update the post itself with this step soon.

      Good luck!

      Reply
  • Naseer

    Thanks, I'll use the silicone caulk solution instead. If you do end up updating the post, you may want to change your silicons to silicones too :). The ground floor is a great idea, I'll do that as well.

    Also, good call on recommending the straightest 2x2's possible. I tried with the slightly warped 2x2's from Lowe's and the lid just didn't sit right at all. I'm going to drive a little further to Page lumber in Poughkeepsie tomorrow and pick up some straight 2x2's (3 for lid, 3 for ground floor). Also, since I don't have a chop saw and my circ saw miter skills leave something to be desired, I may just attach the lid pieces using a butt joint, similar to the 2x12's. I'll cut down the short lengths to 45" and screw them to the insides of the long 2x2's. Hoping to have pictures up this Saturday!

    Reply
    • doug

      Thanks for the spell check.

      The butt joints for the frame should work fine. The miter joints keep the wood frame more square, so just be sure to square up the frame before attaching the roof panels. (Luckily, the panels themselves are square, since they are a manufactured product. But they won't naturally create a perfectly squared form once they're lying on top of one another: there's a bit of play in the overlap.)

      I hear you on the crappy 2x2's--the box stores tend to carry the lowest quality lumber, in my experience. Even a perfectly straight piece may warp in time, though, so I think I'm going to recommend a latch for the front to hold the lid snug. (That said, don't aim for air-tightness: you need a tiny bit of breathing to stave off the stagnant air conditions that invite damping off.)

      Good luck--I look forward to seeing the pictures! If you can think of any improvements as you go, keep us informed!

      Reply
  • Naseer

    Our cold frame is finally done and being used to cover up our peas! First direct sow of the season! Thanks for all the help. Pics and details here:

    http://www.greenthumbgeeks.com/2010/03/cold-frames-galore/

    Time to build some trellises soon and put up a fence.

    Reply
    • doug

      Looks great, Naseer! Glad it worked for you. Good luck for the rest of the season, and thanks for the link to the series! The last installment--Transplanting and Troubleshooting--will be up in the next two days.

      Reply
  • Soi Disant

    In the high subarctic, where I garden, this temperate-zone plan (which is very good), would likely need modification. (These comments might apply to other very cold places, too.) Initially, I'd suggest a second heat-capturing plenum, perhaps in the form of a low hoop-cloche over the entire cold-frame. Secondly, I'd suggest either electrical heating tape or heating pad beneath the soil of the cold frame, or learning how to adapt the pre-electricity compost-heated "hot box" idea to heating a far north, early Spring outdoor starting box. Thirdly, I'd probably play around with a foam insulated side wall, a slightly deeper box, with a cover more steeply pitched, and a foam-sealed, doubled, air-space-insulated Suntuf or Palruf cover.

    Reply
  • Soi Disant

    Once the soil beneath the bed was heated sufficiently, the electricity would only need to be applied intermittently, I would think. A good soil thermometer would help decide this.

    A fourth far-north/cold climate strategy _might_ be, in the course of building the frame, to excavate the ground beneath where the frame is to go, carefully level, lay in a 2" sheet of below-grade-rated foam insulation and backfill a protective layer of soil.

    Reply
  • Jason

    Is it necessary to make sure the panels cover the frame and are sealed shut? I used the base of a small raised bed I have and just built the lid. The panel I got at HD is a bit small and there is about an inch gap on each side. Should I buy another panel and layer them a bit to make sure the gaps are closed and silicone them sealed? I was thinking it would be okay to leave the gaps as air would be needed to circulate.

    Thanks for all the info!

    Reply
  • [...] A Little Protection Goes a Long Way. Have a cold frame or plastic tunnel that you use for seed-starting in the spring? Use it now to keep fresh veggies on the table all winter. An investment of an hour or two in a simple plastic hoop will provide delicious dividends all winter long. Some instructions for an inexpensive, sturdy cold frame are here. [...]

    Reply
  • George Jacobs
    George Jacobs 09/08/2011 at 1:05 pm

    I've made many similar structures over the years and this design is very similar to the one I derived from Eliot Coleman's first book years ago. However, I have always used either UV treated poly and especially like the woven poly because it holds up for years and years. This allows a top that is lightweight enough to be lifted by a Univent solar lifter (it will do 15 lbs.) This is so convenient because you don't have to worry about your plants burning up on a sunny day when you're not around. Don't buy the cheap lifter from Harbor Freight– unfortunately it's junk.

    Reply
  • [...] Valley Seed Library has a tutorial for building a cold frame, and another for starting seeds in one. They suggest using a clear corrugated plastic material for [...]

    Reply
  • [...] can use a cold frame or tunnel to keep temperatures warmer around your crops as the days shorten. This should have [...]

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