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making coldframes

February 8, 2011

Summer gardening can suck;  aphids, Mexican bean beetles, molds, fungi, borer worms, droughts, floods, birds, deer, the list goes on … even a successful summer garden is a fight with nature rather than a cooperation with it.   Especially in climates with marginal amounts of sunlight, or late freezes, or simply erratic weather (which will probably be increasing), summer gardening offers high yields but lots of problems.  Winter gardening is the opposite.  Plants grow slowly, but without problems.  No need to check for bugs or blights because these are mostly adapted for warm weather.  Of course you can’t grow tomatoes in cold frames; we are limited mostly to greens and roots, and to giving early starts to spring plants such as peas.

Lots of plants can take freezing temperatures but not ice on their leaves; the cold frame protects from the extreme cold as well as from frost bite.  Kind of like us: cold is survivable but not when wet.  When the snow piles up, just leave it on for insulation; they like the opportunity to nap a bit.

There are at least three ways to use the frames.  First, is to plant in lat summer or fall well before temperatures get cold.  Then as temperatures fall, you’ll have roots and greens in cold storage.  They may grow a bit, but are mainly hanging out.  Second, starting plants in the fall gives them a head start in the spring.  By mid February when the amount of available light starts increasing rapidly, you’ll have well rooted and healthy plants going crazy as they wake up from a long winter slumber.  Roots established and ready to go.   We’ve had greens try to go to seed with snow on the ground!  The third use, is to use the frames as an early starter for peas and other early cold-tolerant crops.  We’ve started ours in early February.



Begin with the most important part of the cold frame, the transparent material on top.  This can be really expensive.  Obviously we need unbreakable, weather resistant material.  If you buy something, one of the many plastics will work, but expect to pay $50 at least for a 3×6 piece.  We used re-cycled patio doors.  They are usually in the range of 6×3, made of tempered glass so strong that even if your kid runs fill tilt at it with a brick in hand, it will not break.  We got ours from a local glass repair store for free.  The main drawback with glass is that it is heavier than plastic; we’ll get to this later.   Note that glass from regular windows is usually not appropriate; it is not shatter proof and if in an old wood frame, the paint may contain lead.

Once you know the dimensions of your frame we can build.  It is pretty simple. Build a box out of 4 pieces of 2×4.  A front, a back, and two identical side pieces

1  The glass wants about a half inch of wood to rest on all around, so design dimensions accordingly.

2  The back of the frame should be taller than the front.  We made ours with the front being 8″ and the back 12″ off the ground.  This gives a slope to the frame, maximizing light and space.  Taller frames means more shadows, but more height; plan according to what you want to plant.  This height is good for greens and roots.

3  Thus the side pieces need to be cut with an angle, starting from 12″ to 8″.  Furthermore we want the glass to sit smoothly on the wood, so the top edge of the front and back pieces need to be at the same angle as the side piece.  There are a variety of ways to do this, including to make the front and back pieces themselves sit at a slight angle (and appropriately cut the side pieces), or to cut  a bit of wood from their tops to create the angle.  You can also ignore it and put in some filler afterwords.  We’ve used all these options and each works ok. But this does have to be addressed because we want the frames fairly sealed. The first option is the best, but also the hardest.  It will take two people, some planning, and some skill.

4  We don’t use treated wood.  The current stuff is not as bad as the old highly toxic treated wood, but we are still dubious.  Untreated wood left outdoors will eventually rot, possibly as fast as 4 years.  We have ours resting on a lower wood bed, so at least our frames are not in contact with the soil.  We also designed our new ones to be dissembled easily for summer storage.  We bought new wood; if you can get clean recycled wood, much better!

5 A cross piece in the middle is useful depending on your material.  We chiseled a groove in the center of the front and back and inserted a small  1×1 middle support.  That way when there is 3ft of snow on top of your frames, the glass doesn’t bend and make the mustard greens too cold.

That is it.  Then there is planning the plantings, and maintaining them (opening and closing).  Get to that next.

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