Half the glory of a greenhouse is its profusion of color and foliage, regardless of the weather outdoors.  Sun--and lots of it--is the key to greenhouse success.  Unfortunately, the tilt of the earth in winter means the sun barely slides along the treetops before disappearing below the horizon.  Days are shorter, and the increased distance and widened angle weaken the sun's warming rays.  Therefore, the greenhouse has to be cleverly placed to capture the most sun possible.

Because the sun spends most of its winter days in the southern sky, the ideal direction for a greenhouse to face is south.  Next, in order of preference, are southeast, southwest, and west.  A northern exposure is satisfactory if you're growing only foliage plants.  As a general rule, plants will need at least three hours of sunlight per day.

Keep in mind that trees, buildings, and other obstructions can rob plants of valuable sun.  Deciduous trees (which lose their leaves in the fall) often are valuable allies because they offer filtered shade in the summer.  Wind can be a factor, too.  A greenhouse constantly buffered by frigid winds is difficult to heat and often drafty.  Plan on planting an evergreen screen or constructing a fence if winds are a problem.