Do you need a cold frame? Maybe. Here's how to build one.

Nothing beats a homegrown tomato!

A cold frame is like a mini-greenhouse, essential if you intend to start plants from seed for a medium to large garden. Starting your own tomato seedlings 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. This method also gives you greater flexibility with varieties you can’t find at the plant nursery. Planting heat tolerant tomato varieties is the key to tomato success in the desert southwest.

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” (aka Reemay cloth or season extender) 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 if it gets too warm inside, or too cold outside. They are not terribly expensive, and work great.  You will also need a mechanics 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 make them grow strong and sturdy. If it is going to be exceptionally cold, I throw a blanket over the unit.

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. It helps to have a prop to hold the lid open while you are setting them inside or watering them—a piece of PVC pipe works well. Plants will need water every day, up to 2 times a day, depending on how warm it is outside.

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

Seed starting for tomatoes, peppers and eggplants should take place about 8 weeks before you intent to plant. (Stay tuned for article on how to start seeds.) After the average last frost date (April 1^st for Tucson, April 7^th for elevations 1000 feet higher), you can set out your new plants. I suggest you wait, even it the temps are mild. There is nothing worse then losing all of your hard work by a freak late spring storm.