CO-Horts Blog

Wednesday, July 21, 2021

Hardiness Zones: A Cautionary Tale

Posted by Todd Hagenbuch, Routt County Extension

As I closed my website search engine the other day, a pop-up read, “Click Here to see what you can grow in your area!” It came from a reputable, long-time purveyor of seeds and plants so I thought, “I wonder what they think I can grow in Routt County?” I clicked on the link and was fascinated by the results.

Immediately I was taken to a site that said, “Top plants for your zone” and “Phippsburg, Colorado is in zone 5a.” 5a is a USDA hardiness zone.  Hmmm…my faith in the company was fading.

As a brief reminder, USDA hardiness zones are based on the average extreme minimum temperature an area will see in the winter months. The numbers are designed to help gardeners determine what perennials will survive the winter, which is not necessarily the same as what will grow in the summer in your neighborhood.

Another concern is the zone it had us pegged for. Zone 5a has an average extreme minimum temperature range of -20 to -15. While some areas of Routt County are in this zone, Phippsburg is in zone 4b. But I went on, wanting to see what plants the company recommended.

Some of the items recommended for Phippsburg
(identifying characteristics removed to protect the guilty)
Many of the plants recommended for zone 5a were appealing. Spinach and summer squash are great choices. But the next items, including popping corn and Fuji apples? We simply don’t get enough growing degree units (or heat) to grow them well here.

Every plant needs a certain amount of Growing Degree Units to move from one phenological stage to the next. As you can imagine, our mountain valley does not accumulate many growing degree units in our short season. Plants like spinach don’t need many units, and does well here; popping corn, however, needs heat and would not produce well here at all. In fact, the notes about it on the site indicate it needs a minimum of 105 days to mature, many more than our average 59-day growing season.

My biggest concern about this list is that most of the plants listed as ‘popular’ in my area were annuals. Since USDA Hardiness Zones are all about how likely perennials are to survive the winter, there was no connection between the annuals the site is selling and USDA zones. More than that, it confuses the issue of winter hardiness vs. growing season.

Sites like these are why I regularly get calls from people wanting to know what variety of apples, apricots or even peaches will grow here. Even if the plants are going to survive the winter months, that does not mean they will produce here. You can probably plant a zone 4 apple tree and expect it to live here; just don’t expect it to be a big fruit producer.

As for garden company catalogs and websites, use them as a guide but do your research before you order.  Know what will thrive in your location, not just survive. You’ll be a happier gardener and save a lot of money in the long-run, too.


  1. Thank you, Todd, for the Hardiness info. As a newbie gardener it opened my eyes Really Wide! As an added bonus, could you list some resources for the "thriving" part since the Hardiness map takes care of the "surviving" part???

    1. Following. I was thinking the same thing about which ones do thrive..

  2. Thanks for the great info on Growing Degree Units. I like to grow from seeds & have found the 'days to maturity' info inadequate. Where can we learn more about this topic and plant- specific requirements?

  3. Nice article! The other thing I've learned (the hard way) is that plants marked Full Sun do not need full Colorado sun! Especially July sun. A little bit of shade can go a long way even for plants that don't normally need it. I tend to test things out in different spots to see what a plant really likes here.