In 2012, the U.S. Department of Agriculture (USDA) updated its cold hardiness zones. Farmers, suppliers and home gardeners use the zone designations to help standardize how well a plant can survive at average extreme minimum temperatures. In the new iteration, the USDA added two new warm zones to reflect areas in which the average low stays above 50 degrees (if only!). Of course, the zones are restricted to tropical areas of the country, such as Hawaii. Several zone boundaries also shifted to reflect warmer temperatures in several zones from the 30 years of data collection.
Read what you want from the shifts in zones, though the USDA is quick to point out that climate change trends require 50 to 100 years of data collection, and zone changes are not reliable evidence of whether there is global warming.
At any rate, if you haven’t checked your USDA zone in a few years, it’s a good idea to look again and see if it’s shifted. The USDA has given you another reason to check. The scale has fine resolution and GIS technology. This means that between your ZIP code and GIS location, you can have more accurate data to reflect the effects of, say, urban streets and buildings on your townhouse patio. It’s huge for someone like me who lives in a rural area and seldom receives accurate weather data, for instance, unless I collect it in my own back yard. The map recognized my location and pinpointed my zone accordingly as 6B.
Cold hardiness is not the only factor to consider, though, especially for xeriscaping. Altitude affects temperature, but also contributes to drying of plants. And wind, well, don’t get me started. I will say that wind dries a plant out and does plenty of damage to new plantings or houseplants you’re beginning to bring outdoors. You also have to consider soil and microclimates, such as those on the city balcony. On a broader scale, valley or riverbank climates differ from those of mountains or open plains.
Within your xeric garden, you can push a zone slightly by adding plenty of warmth to a plant when you place light-colored gravel under it or plant it near a south-facing wall. Or help out a plant that needs a zone cooler by putting it under the shade of a tree or on the north side of your home. Just be sure to check tags for plant hardiness zone before buying, especially from catalogs, online or in big-box stores.