Our gardening season and our "growing season" are not the same. We were gardening in February this year, planting shrubs we didn't get to in the fall as well as seeds and bulbs we forgot we had. MANY clients were preparing their raised-beds and planting beet and radish seeds. The could have been plating seedlings of chard and kale as well. The best spinach I've ever had was grown from seedlings I planted in early April that then laid covered with snow for 2+ weeks. Our growing season, according to NOAA, is our "frost-free period", when we have less than a 10% chance of ≤ 32°F on any given night, is July 15-August 15. July 31 is the middle of our growing season.
Our "average" temperatures are derived from wildly fluctuating daily temperatures at all times of the year. The average gives us a rough guide with which we make wildly fluctuating guesses at how cold it might get on any given day. That said, it is a tool, much like the USDA zones or the useless-to-mountain-folk Sunset western zones (see Northeastern for a more useful tool). I've posted this pic of our chalkboard before but it "bears repeating".
Here is a graph of an "average" winter (temperature-wise). Jan15, 2013-Jan 14, 2014. For interest, note where the "average" nighttime low is ≥32° and where the average daytime temps average ≥70°F.
I think that, without a greenhouse, our "average" mountain gardening season in Truckee is about March 15 - November 1 (or 15). It is a matter of taking advantage of clear and warm conditions, choosing the right plants and crops and being able to protect the harvests of others (see RowCover). If you have an unheated greenhouse you can add 3-6 weeks on either end of that gardening season for some veggies.
I have planted hardy annuals in February MANY times with great success (pansy, viola, dianthus, calendula, stock, primrose) and I have also planted dormant trees and shrubs in December, January, February and March with excellent success.