Answer: We all are probably familiar with the United States Department of Agriculture's Hardiness Zones that categorize geographical areas of the country based on average minimum temperature. Hardiness zones indicate the lowest temperatures perennial plants can withstand. However, in many areas of the country, summer growing conditions are just as important, if not more important than winter growing conditions, in determining long-term hardiness and survivability for many plant species. This prompted the American Horticulture Society to produce new guidelines, in the form of a Heat-Zone Map.
The AHS Plant Heat-Zone Map contains 12 different zones in the United States and classifies areas of the country based on the average number of days per year than the temperature is above 86 degrees. Why 86 degrees? This is the temperature where cellular proteins in plants start experiencing damage.
Heat damage is evident by withering buds, drooping discolored leaves with excessive insect damage, or roots that cease growing. Damaged plants might survive for months or even years, but will fail to thrive and perform as expected.
Heat zone ratings assume that basic gardening principles are followed, such as proper soil preparation, mulching, weeding, proper planting location, and, most importantly, adequate proper watering.
These are general guidelines that may vary. For example, if a variety is planted near a heat absorbing brick wall, the heat stress will be more in a hot summer area. Planting in dappled shade would help extend the range beyond the southern limit.
As far as your gardening zone is concerned, you are in USDA Zone 8, with average winter lows of 10 - 20F. Sunset Western Garden Book has taken the standard hardiness zones and factored in not only winter minimum temperatures, but also summer highs, lengths of growing seasons, humidity, and rainfall patterns, to provide a more accurate picture of what will grow in a specific area. You are in Sunset's zone 6.
Hope this information helps clarify things for you!
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