Microclimates are small areas that because of a variety of potential influences, differ from the mean climate of a surrounding area. An open courtyard with black stone on the ground & white walls to reflect light might provide a wind-free microclimate that could be a full zone or zone.5 warmer than the surrounding area. That eastern exposure with a window air conditioner that drips water all summer long might provide a perfect microclimate for the alocasia that the surrounding garden is too dry for. The grade level planter whose back wall is the house's foundation wall might also be able to supply warmth in the root zone needed to sustain a zone 5 or 6 plant in zone 4. In certain types of soil that drain poorly (clay, silty clay), you might create a microclimate that is NOT beneficial to a large % of the plants you might like to grow. When you dig a hole and partially fill it with gravel, the air spaces between the gravel particles can quickly fill with water when it rains or you irrigate, which can kill, damage, or set plants back if the surrounding soil drains too poorly. The basement area where I grow under lights & keep the humidity N of 50% is a microclimate, though we usually think of microclimates in terms of the outdoors. Sometimes we take an active part in arranging conditions in order to trump Mother Nature's plan, and at other times, we simply take notice of the variation in how Mother Nature has ordered conditions so as to create her own microclimate, and take advantage of her whimsy. Al

That's funny! My dad's greenhouse growing up was 16' X 40' with two evap coolers at the south end and fans in the ceiling on the north end blowing air back towards the south. The south end was always a good five degrees warmer.

I also have salvia greggi against a south facing wall. I had it for 2 years and its gotten real big; its the cultivar called "Navajo Red". Hummingbirds go crazy for it. I also have Salvia guarenitica in the same spot, I tried overwintering it for the first time so I will see if it comes back or not. On the hummingbird forum some people like using the "bag method" to overwinter their zone pushers. You basically fill black garbage bags with leaves and place them on top of your plants after they go dormant. People often report being able to overwinter plants that are suppose to grow 2 zones warmer. I also have a needle palm that has survived against an east facing wall for 4 years. Just recently I ordered a southern magnolia, no special cultivar, so I am going to see what its capable of in my area.

The climate zone in SW WA is 8b-8a, meaning the winter temperatures can get down to 10-20 degrees. Satsuma is one of the few more cold-hardy varieties of citrus, in fact in Japan they classify it in a semi-separate category from mandarins. That is why I mentioned it. (and more importantly it is really the only cold-hardy variety that tastes good) I generally agree with what gardengal said. However, it is young bananas and citrus that are vulnerable to winter cold. Once they have a few years to become established they are much more hardy. I really think you might be able to grow [certain types of] bananas and citrus in a warm microclimate in SW WA without winter mulching. Just keep them in large pots the first few years so you can bring them inside and protect them until they have a chance to grow bigger. Yes, this is really pushing it, because citrus and bananas are not known to grow in WA, but I think it is possible. I know the OP has not expressed interest in growing bananas, but just thought I would mention here that one of the rarer cold-tolerant varieties is Musa sikkimensis (known as the Darjeeling banana). Not the best tasting variety, but it would grow fine in the western side of WA.