|How cold is too cold for these plants to be left out side in a pot?|
|Wish I could tell you that they'll grow outdoors in the Pacific Northwest, but bougainvilleas are tropical in nature and therefore very sensitive to frost. If the temperature goes below 32F, the plant will die.
I've heard rumors that if a plant can become established for 2-3 years before it is exposed to freezing temperatures, that although the tops will die back in freezing temperatures, the roots will live and produce new vines in the spring. If you decide to plant a bougainvillea, find the warmest spot possible - next to a south or west facing wall where it will receive reflected heat and be protected from the coldest of weather.