CO-Horts Blog

Thursday, September 10, 2020

Plant Hardiness is More than a Zone Number

Posted by: Kara Harders, Peaks and Plains Regional Small Acreage Specialist

If you have lived in Colorado for a while you have probably lamented (or heard people lament about) the constant influx of new residents. Colorado really is a state unlike the rest. It isn’t hard to work out why so many people find it appealing to live here: mountains, 300 days of sunshine, low humidity, all four seasons of the year, boating/rafting/kayaking, hunting/fishing, wide open spaces, the big city, I could go on but I think I’ve made my point.

If I had to make a list of drawbacks in Colorado it would be short – wildfires, drought, and home prices come to my mind first, but we are currently (9/9/2020) experiencing one of our phenomenons that is less glamorous: wacky early/late cold snaps.

While Colorado is not unique to this issue, it is among the most common places for it to happen, and it’s hard on plants. It can make plant selection tricky too, have you ever used or seen the Plant Hardiness zone map?

Plant Hardiness Zone Map

Image courtesy of USDA-ARS

This map is super handy for identifying plants that can grow in different areas based on the “Average Annual Extreme Minimum Temperatures”, essentially, how cold can the plants handle. This is useful information and can help guide decisions about what to plant, where to plant it, and if you can expect it to need additional protection.  What this map doesn’t have is a “Areas that Experience Strange Unpredictable Temperature Swings During Odd Parts of the Year” score, I don’t think there is a handy chart for that.

An example of what I am talking about is the Kiwiberry or Cold Hardy Kiwis. The University of Minnesota (in Minnesota, which according to the map, is certainly cooler than Colorado) has been researching the plant, experimenting with species and cultivars, and looking into growing practices for more than 30 years. It does well enough in Minnesota they even looked in possible invasive qualities of the plant. However, when grown in Colorado, the Kiwis failed (even in high tunnels). Why is that?

It is because of days like Tuesday and Wednesday of this week when we experienced a 70 degree temperature swing in 48 hours, or in mid-April this year when it got down to single digits after being in the 70’s the week prior.

While the Kiwis are hardy to -20 degrees, they cannot handle the climactic whiplash that is Colorado. They need an intercontinental climate, an area where it gets cold then stays cold; gets warm then stays warm. In the Kiwi’s case, the warm spells in the late winter and early spring trick them into budding out and blooming.  In experiments done in Hidden Mesa (Douglas County) the late spring frosts kill not only the blossoms, but often the vines too. (Photo to upper right: Kiwiberry, photo by University of Minnesota) 

Similar problems arose in the fall when the first hard frost can happen before the plants have had time to harden off, often killing them. Even in high tunnels the plants slowly died back to nothing, while plants such as raspberries did fine. 

Try to keep in mind the effects late/early freezes can have on perennial plants. Even plants that are designed to handle the swings can suffer when they are too dramatic, and plants not designed for the shifts often don’t stand a chance. To better your odds, try to research plants and varieties proven to do well in our special climate. This is a link to a PowerPoint by Andy Hough (thanks Andy!) from Douglas county with tons of information on hardiness specific to the front range:


  1. I optimistically hope that the CSU Plant Select species will survive the Colorado roller coaster. Is that a well-founded belief? Would love to hear other opinions.

  2. Thank you for an interesting and informative article. I'm a native of Denver and have never heard a more appropriate description of the weather than "climactic whiplash."