Greetings: There are numerous hardiness zones worldwide and it appears that most define the minimum temperature that a plant may tolerate. I recentlyh was inquiring from a conifer vendor and I was told that Picea pungens gluauca would not survive in my hardiness zone (USDA 9a). it appears to me that the USDA Zone system does not predict survivability other than minimum temp. it would appeat to me that if other attributs necessary for survivability are provided then the USDA Zone is irrelevant unless the hardiness temperature goes below the minimum unless Picea pungens meeds chilling below a specific temperature that is above the minimum.
A better guide for surviveability than the USDA zones, are the Koppen-Geiger zones, which compare the conditions of climates worldwide. To find survivability using them, locate the original area where the plant you're trying to grow is found in nature, then compare that with your own zone on the map. More than four steps difference normally means that survivability is less possible. Koppen-Geiger World Climate Map In the case of Picea pungens of any stripe, I would suspect that in USDA 9a you might not have enough chill hours for it to be truly happy. This said, it's also a fairly common planting in the highlands of Ecuador, so take that with a grain of salt. Members of the Pinaceae are famously adaptable.
Most California nurserymen would look in their Sunset Western Garden Book to find out what zone you are in and then look up the plant that you want to grow. In this case, with you living in Mendocino you are probably in a zone 15 or a zone 17 by the Western Garden Book zonal designations. Then when we look up the plant, Picea pungens 'Glauca', we can see that the recommended growing zones are zones 1-10 and 14-17. You should be able to grow a Colorado Blue Spruce where you are located. Jim
The main problem with growing Blue Spruce in Mendocino's climate is that the winters are not cold enough to kill spruce aphids (Elatobium). The result is that the aphids can build up to populations large enough to defoliate the tree.
Yes, we have the same problem here in the Pacific Northwest: the winters do not kill off bugs that plague them. Nevertheless, very many are sold and planted.
