This Organic Process

A couple weeks ago, the United States Department of Agriculture(USDA) released an updated plant hardiness zone map (press release), updating a useful tool for gardeners and researchers for the first time since 1990, complete with greater accuracy and detail. ”Plant hardiness” is a term that is used commonly, in the gardening and planting community; it’s a standard practice for seed sellers and nurseries to label their products according to the USDA Plant Hardiness Zones.

Baltimore - Zones 7a/7b borders

Historically, the maps gave undergone dramatic changes based on new data. So, for someone like me that lives right on the cusp of 7a and 7b, there probably isn’t much of a difference, if any at all (seen above).

1990 Plant Hardiness Zone Map

2006 Plant Hardiness Zone Map

2012 Plant Hardiness Zone Map

Much of the U. S. and the rest of the continent increased in temperature, in recent years, and that affects which trees, plants, and vegetables are right for planting, in your area.

From the press release:

“This is the most sophisticated Plant Hardiness Zone Map yet for the United States,” said Dr. Catherine Woteki, USDA Under Secretary for Research, Education and Economics. “The increases in accuracy and detail that this map represents will be extremely useful for gardeners and researchers.”

It goes on to talk about the changes in the zones, over the last 20 years:

Compared to the 1990 version, zone boundaries…shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.

Some of the changes in the zones, however, are a result of new, more sophisticated methods for mapping zones between weather stations. These include algorithms that considered for the first time such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops. Also, the new map used temperature data from many more stations than did the 1990 map. These advances greatly improved the accuracy and detail of the map, especially in mountainous regions of the western United States. In some cases, advances resulted in changes to cooler, rather than warmer, zones.

Importantly, if your hardiness zone has changed, it does not mean you should start pulling plants out of your garden or change what you are growing. What is thriving in your yard will most likely continue to thrive. It does mean, though, people in Boston, MA, can start growing fig trees.

Learn what this means for you, as a gardener.

A couple weeks ago, the United States Department of Agriculture(USDA) released an updated plant hardiness zone map (press release), updating a useful tool for gardeners and researchers for the first time since 1990, complete with greater accuracy and detail. ”Plant hardiness” is a term that is used commonly, in the gardening and planting community; it’s a standard practice for seed sellers and nurseries to label their products according to the USDA Plant Hardiness Zones.

2012 USDA Plant Hardiness Zone Map

You’re new to gardening; what does this mean? This map refers to a geographically defined area in which a specific category of plant life is capable of growing. This speaks directly to the plants’ ability to survive adverse growing conditions.

Across North America, there are 13 planting zones, in the United States and Canada. These range from zero (0) to twelve (12) and can be broken down into sub-zones (e.g. 7a, 7b). These individual regions are categorized by a 10 degree Fahrenheit difference in the average annual minimum temperature. New, in the 2012 release, the USDA added two new zones (12 and 13) for very warm climates.

And what does that mean? It means that the higher the numbers, the warmer the temperatures for gardening in those areas. Keep in mind that the variables of soil moisture, soil content, winds, temperature, duration of exposure to cold, humidity, and other conditions might affect the viability of individual plants.

These zone breakdowns do not reflect the coldest a region has ever been or ever will be; simply, this is an average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.

Can I make it any simpler? The higher the zone number, the warmer your average low temperature during wintertime.

Why should you care?  These planting zones are a guide to which you’ll be most successful at growing particular plants.

For example, a plant’s suitable hardiness means that plant can be expected to grow in the zone’s temperature extremes, as determined by the lowest average annual temperature.

What’s my zone? You can easily find it, via zip/postal code:

1. Go to http://planthardiness.ars.usda.gov/PHZMWeb/
2. Under “Find Your Plant Hardiness Zone”, enter your zip/postal code. For example, entering 21212 returns “Zone 7b : 5 to 10 (F)”. Entering 15205 returns “Zone 6b : -5 to 0 (F)”.

This information will be helpful in determining which plants and veggies are most suitable to your climate’s growing season. Ultimately, though, we can look to the past to help guide our decisions; it is never a guarantee for future variation in weather (like record warmth and Hurricane Irene).

If you have further questions, local nurseries would be best at helping you determine what plants would be best for your environment.

Read a brief historical perspective.