How to build a cold frame

Jump start your garden by 30 to 45 days with a temporary "green house"

Citation
, XML
Authors

Abstract

A cold frame is an old technique to start the spring cool weather vegetables — lettuce, onions, spinach and radishes — up to 45 days before the last frost. They are typically small (4′ x 4′) and can be covered with plastic sheeting, keeping plants 5 to 7 degrees warmer at night — and much warmer with sunshine.

 

What is a cold frame?

 A cold frame is a small wood frame garden box covered in plastic to allow early spring plants to germinate and grow, even before the last frost.   Some restaurants have started to use the

Finished cold frame: ready-to-plant 

technique all year around to make fresh greens available, though the amount of sunlight between November 1 and March 1 will limit growth, depending on how far north you live.

 

By creating this small greenhouse, nighttime temperatures will be 5 to 7 degrees warmer than the outside air – and in sunlight temperatures will be elevated 20 to 40 degrees F.
 
 
In Chicago, using a cold frame allowed me to plant as early as March 1 and remove the frame with mature vegetables in mid- to late April.  In Seattle, I’ve planted as early as February 1.
 
The cold frame is generally 8” to 10” high and uses wood sides.  However many gardens are now using composite or recycled timbers.  Some gardeners scavenge wood and even use old windows for the top of a frame. 

 

60 days after planting 
 

The composite versions use hollow boards made of plastic.  They have the advantage of being much lighter (about 1 ½ lbs.) than wood (a 3’ 2×6 of pressure-treated wood weighs about 9 lbs.) but are much more expensive.

 While excellent for spring vegetables, a cold frame is NOT the best place to start warm-weather vegetables (tomato or pepper plants) and is too short for some spring vegetables such as snow peas.
 
 
 

How does it differ from a raised-bed garden?

 
Raised-bed gardens also use similar boxes to hold the plant bedding but they are likely to be shallower and uncovered.  In addition, the cold frame is usually used for only about 45 days before it is removed and stored, while raised beds are permanent. 

 

Advantages

 
Besides the early start to gardening, the cold frame provides protection from damaging spring winds and pests.  In Seattle slugs are a particular problem but can be controlled easily with a trap inside the box.

 

Materials

 
This cold frame design is for a box that is 4’ wide and 3’ deep, allowing three rows of vegetables with 11” spacing between them.  Besides the size, the most-important decision to make in assembling the frame is how the lower box will be fastened at the corners.  It can be nailed or assembled with 2 ¼” screws but disassembly will be difficult.  You may find a corner bracket or L-bracket sufficient and they are inexpensive.
 
I prefer garden anchor joints such as the “Super Anchor Joint” available from garden stores, Burpee’s or other suppliers.
Plastic anchor joint with stake 

The brown plastic brackets fasten to each board with a pair of ½” screws and are easy to take apart.  In the garden, you’ll simply drive the connecting spike into the ground and the box is formed.  Note that the anchor joints are sold by the pair – and you’ll need four of them.  Stackable joints are NOT necessary for a cold frame but are used with multi-level raised gardens.

The wood, plastic sheeting and metal parts should cost about $50, though the Super Anchor joints are generally about $15 per pair ($30 for an entire cold frame).  Properly stored and maintained, the cold frame will last a decade with little maintenance.
 

Frame materials, with sides cut to size and corner anchors installed on wood. 

 

If you’re using 2 mil plastic sheeting, which is commonly available as a paint drop cloth, you may want to double it – particularly if your springs are windy.  A 3.5 mil plastic would be preferable but is sometimes difficult to find in small sheets.

  • 2” x 6” cedar or pressure-treated lumber – 14’ long
  • 2” x 2” cedar or pressure-treated lumber – 16’ long
  • ¼” x ¾” door strip – 16’ long.  Any small trim can be used here to help hold the plastic sheeting in place and protect it from wind.
  • 2 to 3.5 mil plastic sheeting (Visqueen), 4’W x 3’D
  • 4 L-shaped corner brackets for lid + screws
  • Two 2” hinges
  • 2 hooks & eyes
  • Corner fasteners for base or 2 1/4” screws or Super Anchor joints)

Tools: circular saw, screw-driver, drill, stapler, hammer

 

Assembly

  1.  Cut your 2”x6” timber into two 4’ and two 3’ lengths.  Pre-drill your fastener holes then  fit the pieces together (without fastening the box together) into 4’x3’ box.  This will allow you to measure for your lid – and easily transport the sections to the garden. 
    Frame base assembly in garden & awaiting the lid. 

  2.  Measure the base BEFORE cutting 2”x2” pieces for the lid, as the Super Anchor corners may make lid pieces 4” longer on two of the sides. 
  3. Cut the 2”x2” wood, the assemble using the 4 L-shaped corner brackets.  Screw hinges to the lid. 
  4. Cut your door strip or other furring strip to match the lid.  Tack in place with a light nail or staples that are long enough to penetrate through the stripping.  This layer of stripping prevents tearing of the plastic sheeting in heavy winds.  Using a thin (1/4”) door strip will enable a ½” staple to penetrate the wood and plastic, simplifying the tacking process, which is next. 
    Frame lid, with tacking of plastic sheet in progress. 

  5. Cut plastic sheeting so that it is large enough to lap over the sides of the lid.  This gives you extra material to hold onto while tacking it to the lid.  Staples are excellent for holding the plastic or Visqueen in place.  Tack one side to the lid with three staples, then draw plastic taught and tack down the opposite side.  Repeat with the two remaining sides – then return and double the number of staples on each side, while drawing the plastic sheeting tight.  Now, trim excess plastic from the lid. 
  6. Mount the lid onto the base and attach the hinges.  Add the hook-and-eye latch to each side of the front of the cold frame and keep the lid from lifting in strong wind. 
  7. Save two extra wood blocks to open the lid of the cold frame slightly on hot days.  You may even wish to add supports to the side of the lid to hold it open for watering or other work.

 

After the last frost

 
After the danger of frost is gone, first remove the lid and store.  You may wish to leave the sides in place for a few weeks longer – in case there’s a surprise late freeze and you wish to put the top back on for a night or two (your spring vegetables won’t be bothered by a light frost).  Then disassemble the sides and store them in a dry location.
 
And don’t forget that you can use the cold frame in the autumn if you wish to extend the growing season of the same vegetables.
 

References

 
Google Knol
 
How to Build a Slug Trap (Gary Pilarchik)