Yes, a cold frame in full sun is the only setup that works well through winter. Sun is the heat source for the whole box, so any shade cuts your plant survival and growth fast on cold days.
A south-facing cold frame soaks up the most rays during the short, weak winter days you face. North or east sides of a yard get fewer hours of direct light, and that gap can be the difference between live and dead plants.
I walked my own yard at noon on a late December day before I picked my frame spot. I stood with a watch in hand and timed the direct sun on each patch of ground I had in mind.
The winner held six full hours of sun from 10 a.m. to 4 p.m. with no shadow from the house. That site became my permanent frame zone, and the lettuce inside has thrived through three winters since.
Solar math explains why this test matters so much at our latitude. At 40°N, the December sun sits only 26 degrees above the horizon at noon.
That low angle means a tall fence or shed to your east, west, or south will cast a long shadow. Any morning or afternoon shade can cripple your frame for half the day.
Iowa State Extension and Nebraska Extension both call for a south or southeast exposure in their cold frame guides. They also stress a windbreak to the north that blocks cold gusts but does not cast a shadow on the box.
Good cold frame placement starts with a list of three needs in priority order. Most sun, least wind, and no shade from trees that leaf out later in spring.
I tested a second frame on the east side of my garage one year as a fun side test. Plants grew but at half the speed of my south frame, and the inside ran 8°F (4°C) cooler at night.
Slope the box lid south at a 30 to 45 degree angle for max winter sun capture inside. The back wall should be taller than the front, which creates that slope and faces the glass right at the low winter sun.
Strong cold frame solar exposure drives every other gain you get from the box. More light means more heat at noon, more stored warmth in the soil, and more growth in your winter greens through cold nights.
Take these steps to find your best spot today. Walk your property at noon on a sunny winter day. Mark the brightest patch with a wood stake. Then check spring trees nearby to confirm no leaf-out will shade your frame later in the year.
When I first built my frame, I picked a shady corner just because it had open ground. My plants struggled all winter, so I learned the hard way that sun trumps every other factor for site choice.
Read the full article: Cold Frame Gardening: Complete Guide
