Though you might think Tacoma, Washington would be quite cold due to how far north it is on the map, it’s actually only USDA Zone 8a thanks to the thermal mass of the ocean.
That still isn’t tropical, though! Overnight lows in the 10-15F range are to be expected.
That’s more than cold enough to kill a banana tree. Fortunately, these bananas are sheltered by the amazing power of a south-facing wall. Just the fact that they’re alive is amazing, even if they don’t get to keep their fruit. That micro-climate likely puts them solidly into USDA Zone 9.
I have a lot of good ideas on pushing tropical plants beyond their range in Push the Zone: The Good Guide to Growing Tropical Plants Beyond the Tropics. We even managed to grow coffee outdoors in North Florida, where it sailed through nights that hit the low 20s!
Sometimes it’s just a matter of “try and see!” Matt’s Dad went for it and has a little piece of the tropics far, far from the banana plantations of Central America.
Push the Zone: http://amzn.to/2wSPdAh
My post on growing bananas: http://www.thesurvivalgardener.com/bananas-tacoma-good-zone-pushing/