Zoned Out, Part 2: What’s Your Plant Hardiness Zone?

Reader Contribution by Toby Grotz

What Zone are You In?

When planting seed in the vegetable garden or picking fruit and nut trees for your yard or the back forty, its best to do some research on local climate conditions. Temperature and rainfall vary widely across the country and around the world. The average annual precipitation or the average temperature determines which types of crops will grow the best in your area. The USDA first published a plant hardiness zone map based on temperature in 1960. Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.

The statistical average, which used to be termed as “Normal” is changing as was shown in the last blog post. By my estimate the Hardiness Zones moved north 100 miles between 1990 and 2006. The change seems to have accelerated. In Kansas we are able to grow Fig trees on the south side of our houses, an impossibility without a greenhouse 20 years ago. Here is a map that shows the Plant Hardiness Zones.

The zones are numbered from 1 – 7, and assigned a color from red, warmest, to blue and pink which are cooler. But first, let’s find your zone. By clicking on the USDA Hardiness Zone Map you can zoom in to your location. Using the USDA website you can zoom in by state and county for a detailed view. A close up view will show the differences between bottom lands along a river and the hilltops and bluffs above the river as shown in the lighter shade of green in the area around the Kaw river from Topeka to Kansas City. (my zones)

As shown below, In my area, the Zone value given is 6a. Zone 6a has a range of -10F to -5F.

Now, let’s look at some historical data from the Kansas City area. Weather Warehouse provides a good tabulation of data shown below. Note the column Lowest Temperature and then look at the Plant Hardiness Zone from the zoom-able USDA website. Although this area has a rating of -10F to -5F, the average lowest temperature between 2001 to 2013 seems to be above zero or about Zone 7a which is 0F to 5F. This correlates with our experience in this area, it gets warmer sooner.

So, When Should You Plant?

The Northern Hemisphere had it’s 12th warmest winter on record. Here is the USA, where parts of the country had the coldest winter in 20 years and Tuscon and Las Vegas had the warmest on record, what should we do in our respective zones? My conclusion:

  1. Use the Plant Hardiness Zone Map as a guide.
  2. Measure soil temperature as detailed in Part 1 of this series.

Post Script

The Politics of the Plant Hardiness Zone Map

This is absolutely fascinating. It is classic case of denial. In 2002 the USDA decided to update the Zone Map. In conjunction with the American Horticultural Society a new map was developed but rejected by the USDA. It was rejected with no comment. It was obvious to the Bush administration that adding new zones due to warmer temperatures would be perceived as evidence of global warming. It was not until 2012 that a new map was issued with the new climate zones.


USDA Agricultural Research Service

The USDA 2012 Hardiness Zone Map and Some of Its Predecessors

The Polar Vortex vs. The Artctic Amplification