When you shop for your lawn and garden, all nursery plants should have tags that include information about sun and water needs, and about the plant’s hardiness zone. The official zones for the United States are the USDA Plant Hardiness zones. Canada uses the Agriculture Canada Plant Hardiness Zones Map.
Improvements in Hardiness Zones
For the most part, the USDA system is based on the lowest average winter temperature. A few years ago, the USDA updated its zone map because of warming temperatures. The former map relied on data through 1986, and this one is more current. The USDA web site also has added an interactive map for users. You can enter your ZIP code and get the zone instead of guessing whether your town is light yellow or nearly light yellow on the map. So it’s now GIS-based and pretty precise; the USDA map ranges from zones 1 through 13, with 13 as the warmest zone.
Canada’s map is based on climate data and information about plants in the area. The formula to determine zones include mean frost period length, snow depth, high temperatures and wind data, among other information. The zones range from 0 to 9, with zone 0 as the coldest. Canada’s site also features a terrific search that gives zone-specific information for plants.
Sunset Zones a Bonus for Western States
It’s important to know whether a tree, shrub or other perennial will survive your zone’s winter before you plan or purchase; the USDA map helps with gardeners’ decisions. Sunset (Sunset Western Garden Guides) takes the information a few steps further – by telling gardeners about a plant’s chances all year long. For example, we’re in USDA zone 6B, but so are parts of Virginia and other Atlantic areas, as well as parts of Washington and Oregon. Although El Nino has disrupted normal patters, it’s safe to say that plants get lots more rain in Washington and Virginia.
Sunset looks at distance from the equator to help calculate length and severity of winters. The zone map also separates areas with similar temperature ranges according to coastal location or elevation. At more than a mile high, the elevation matters. Our days might warm as much as a comparable location at 2,000 feet altitude, but our nights cool considerably more. Most of all, Sunset knows the western part of the country. Sunset has 24 zones for the American West, divided by geography and climate. Our small town isn’t notated on the map, so it’s a little tough to determine our exact zone. But Sunset places us in the high desert/intermountain region, and the information still is helpful.
Interestingly, Sunset also provides zone information for the rest of the country. It would be well worth it for gardeners east of the Rocky Mountains to review the Sunset zones for information such as wind and climate extremes.
Gardeners can avoid confusion about zones in plant purchasing and care information by knowing both USDA and Sunset numbers for their locale. Typically, all plants except Sunset’s own will use the USDA zone on the tag.
Use Zone Data with Microclimates
If you’re still uncertain about zone, you can check with local Master Gardeners. Another reason to ask a seasoned gardener about climate and zone is microclimates. In many Western states, zone varies within a metropolitan area. Think of the areas above and below southern California’s thermal belts. The ocean has little to do with the climate in those distinct areas. In Albuquerque, temperature and wind can vary markedly from the foothills of the Sandia Mountains to the valley floors.
Knowing particulars for your area of a town or region helps, but you can take it a step further if you want to push the edges of a plant’s hardiness zone or care needs. If you’re half a zone colder than the plant’s care information recommends, plant it against a south-facing wall. Plants that need protection from heat can go on the northeast side of your home, where the house shades them on blazing afternoons.
And since we’re all about the water out here, microclimates help with water use as well. For example, I might be able to place a shrub that needs a little more water at the bottom of a hill, welling on the downside to catch some of the rainwater.
So remember to pay attention to zone, sun and water needs when buying plants, especially in chain stores, which might stock plants too far out of your zone to keep alive all year. Your plants will be healthier and you’ll save money and time.