What Your Plant Hardiness Zone Won’t Tell You About Your Garden Climate

Although we can’t predict the weather, by learning more about past weather we can make a more educated guess about the future. By understanding your garden’s microclimate, you’ll be able to choose plants that will thrive in your particular neck of the woods.

Many gardeners rely on the USDA Plant Hardiness Zone Map for a quick and dirty way to decide what to plant based on their “zone”. Zones are also a favorite qualifier of some nurseries and seed companies. There are two main reasons I encourage you to look beyond your zone and more closely at your garden’s microclimate. For one, your zone–is based only on the “average annual extreme minimum temperature.’ For another–it is outdated. It’s developed from data from 1976 to 2005.

A map of Alaska indicating USDA Plant Hardiness Zones by color variation. Much of Northern and central Alaska consists of growing zones 1a to 2b, with coastal regions 2b to 4a and Southeast Alaska contains zones ranging from 4a to 7b.

Alaska USDA Plant Hardiness Zones.

I’m not going to tell you to set up a weather station in your back yard–although that would be the most accurate thing to do and there are some fairly inexpensive digital temperature monitors out there. I’m going to show you a couple of tools that will allow you to zoom in on the particular climactic conditions in your garden–all from the comfort of your armchair.

First, go to NOAA Climate Data Online, then, in the drop-down menu, select, “Annual Summaries.’ Now you should be able to see on the map all of weather stations in and around your area. Find the station closest to your garden. Then click on, “Full Details.’ Note the elevation of the station. Is it very different from the elevation of your garden? Is there another nearby station with a more comparable elevation? Now scroll down and select the year of the data you’re interested in on the drop-down menu. Now click “View Data.’

A map of Alaska indicating where NOAA weather stations are located.

Some of the NOAA Weather Station Locations in Alaska.

I got excited about this web tool because there was a weather station very close to where I lived a few years ago. It showed that in the valley where I lived, there were three days in June and two days in July where the temperature dropped below 32 degrees. I compared that with a nearby, upland station and found that it did not drop below 32 degrees at all in June or July. That was a little depressing even though I already knew that I was in a cold zone, it was interesting to see the data.

The annual summaries are just a start and you may want to explore the NOAA tools further, keep your own records, or note the following characteristics of your garden:

  • the first and last frosts of the season (and ugh mid-season frosts) which in turn determines the length of your growing season;
  • the slope aspect of your garden or farm;
  • hours of direct sun each day–not to be confused with the day length
  • day length;
  • frequency of wind events.

While you’re at it, decide on the best time to plant your garden in the spring by looking at the probability of the last frost in the spring and first frost in the fall in your area using the U.S. Climate Normals Data.  Select Alaska in the drop-down menu.  Plug-in your probably frost-free day to Johnny’s Seed Starting Calculator to know when to start what. Your growing season is the number of days between the first and last frost. Use this number as a way to determine if a crop will mature (based on days to maturity), but be sure to factor in when you start the seeds indoors and whether or not you use season extension technologies.

Although not exactly climate related, the USDA Web Soil Survey is another data tool that can be used to determine your soil types in many but not all areas of Alaska.

Now, you can get most of the data I talked about here with the Alaska Garden Helper, but not in quite as great detail as going directly to the data sources, especially in terms of better characterizing your micro-climate. It is particularly helpful in looking at how our climate has changed and predicting how it might change in the future. It also gives you a good estimate of what your growing season will look like.

Previously published in the Fairbanks Daily Newsminer May 2, 2015.

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