Understanding Your USDA Plant Hardiness Zone

One of the first things gardeners should learn is their garden’s plant hardiness zone. The U.S. Department of Agriculture’s plant hardiness zone map is the most widely used zone system in the United States. Nearly every plant you purchase, other than an annual, comes with a tag that includes recommended USDA zones for that plant. By matching your garden’s zone with zone ratings on plant labels, you can know which plants should flourish at your home.

The newest version of the USDA zone map was released in 2012. Based on thirty years of weather data from 1976 through 2005, it’s the most accurate and detailed USDA plant hardiness map yet. The current map divides the country into zones labeled 1 through 13, each spanning 10-degree Fahrenheit increments. The lower the zone number, the colder the winter lows in that zone. Each zone gets refined even further into 5-degree increments labeled “a” or “b.” For example, lows in USDA zone 5a usually drop about 5 degrees colder than in zone 5b.

WHAT USDA ZONES MEAN FOR GARDENERS

It’s important to know that USDA zone information is a guide, not a guarantee. Hardiness zones aren’t based on the lowest possible temperature for an area. Zone information doesn’t reflect the lowest historic minimums or possible future extremes. Instead, they average the annual extreme minimums over the 30-year time frame. Severe cold in any given winter – or unseasonably early or late freezes – can set plant hardiness zone ratings on their ears.

When plant growers determine cold hardiness zones for new introductions, rankings refer to the root hardiness of the plant, not necessarily plant parts above ground. A shrub rose, for example, might be rated cold hardy to USDA zone 3. A normal zone-3 winter could kill that plant back to the ground – or at least down to snow and mulch cover. A hardy rose grown on its roots, not grafted onto another, should survive below ground and spring back from its roots. Canes above ground may or may not survive any given year. Hardiness estimates also assume plants enter winter in excellent health.

PUSHING THE ENVELOPE WITH MICROCLIMATES

Your garden’s annual extreme minimum temperature doesn’t limit you to plants rated for that zone. Gardeners tend to be an adventurous lot. We eventually try plants rated outside our growing zone. Nothing wrong with that, just don’t be upset if a plant doesn’t make it or doesn’t perform as you think it should. Extra mulch or burlap wrapping can do wonders. Remember, zones go both ways – cold and hot. Some plants need cold exposure to bloom or fruit well. They survive winter in warmer zones, but forget about blooms or berries!

When growing plants marginal in your hardiness zone, microclimates come into play. Concrete and asphalt in cities collect and hold heat, creating heat islands and mini zones. The same thing happens around your home. Planting sites against south-facing walls or with concrete-reflected heat stay warmer than exposed areas. Protected, sheltered corners do the same. Higher elevations are cooler, but low-lying areas produce frosty pockets much colder than high spots nearby. By using these individualized microclimates to your advantage, you can grow things your neighbors cannot grow. Use USDA plant hardiness zones as a helpful guide, then go ahead and push that envelope.

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