A cold frame is essential if you intend to start plants from seed for a medium to large garden.  It allows you to grow larger and sturdier plants, ready to set out in the spring, for a lot less than it would cost to buy them ready to plant.

If you are handy with a circular saw, and like building things yourself, you can make a simple, functional cold frame starting with an old aluminum window.

Cold frames in cool climates have glass in the lid to keep seedlings warm.  In the desert southwest, that would be fatal as the heat buildup will kill the plants in one sunny afternoon.  I use heavy weight row cover material instead, and use giant binder clips to secure a piece over the window frame. 

The main concern for the plants inside is making sure they get plenty of sun during the day (but not intensified through glass), proper ventilation, and keeping them warm at night.  To achieve this easily, you will need a greenhouse window opener, which automatically opens and closes the lid as it gets too warm inside, or too cold outside.  They are not terribly expensive, and work great.  You will also need a drop light with a 75 watt bulb to hang inside to keep the plants warm at night.  Use a timer to have the light come on from at least 10 p.m. to 8 a.m.. Keeping the seedlings warm, even it there aren’t freezing temperatures predicted, will keep them growing strong and sturdy.  If it is going to be exceptionally cold, I throw a blanket over the unit.

Greenhouse window opener works hydraulically.

The size of your cold frame depends on how many seedlings you plan on having in there at one time.  My unit holds more than 70 plants and measures 47” x 41” (the size of the old window I used).  I made mine 2’ tall at the back, sloping to 8” in the front.  Make sure to add corner supports using 2 x 4’s.  You don’t need a bottom; just set it on the ground.

You will need hinges at the back so you can open the top and place seedlings inside.  And a 1 x 2 or a piece of pipe to hold the lid open while you are setting them inside or watering them.  They will need water every day, up to 2 times a day, depending on how warm it is outside.

You will want to place the unit to face south, and where it will receive full sun all day.  It may sound like a lot, but since you are setting the plants out in March, the days are not yet that long and you want all the sun you can get to grow strong plants.

See diagram for the basic layout.

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