May 16, 2020
Match Maker: Picking the Right Tree for Your Area
Question:
After I bought a ‘Sea Green’ juniper I noticed the tag said “Hardy to 20°F.” Well, I live at almost 7,000 feet in Torrance County. When I looked online it said my zone is 6. I think maybe the tag is wrong. Do you agree?
- Carolyn M., Torrance County
Answer:
I’m not familiar with the ‘Sea Green’ juniper, so I did a quick search and confirmed that the recommended planting zones for that tree are USDA Hardiness Zones 4–9. I also double checked the USDA Hardiness Zones for your county, which tend to be in Zones 6–7. You can find the USDA Hardiness Zone for your location. As described on that website, the USDA Plant Hardiness Zone Map is “the standard by which gardeners and growers can determine which plants are most likely to thrive at a location.” The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. For example, USDA Hardiness Zone 6 has average annual extreme minimum temperatures from -10°F to 0°F, and Zone 7 is slightly warmer with average annual extreme minimums from 0°F to 10°F. Since the plant you bought is recorded as being cold hardy to a safe minimum of Zone 4, with average extreme cold temps down to -30F, there’s a really good chance it will survive winters in your general area.
The average extreme low temperatures vary based on which years the data averages came from. For the 2012 USDA Plant Hardiness Zone Map, the zone assignments are based on data from 1976–2005. Because average temperatures are going up with climate change, we can expect the assigned zones for different regions to change too, but not as uniformly as you might think. It’s not that simple. Even as average temperatures rise, we’re still expected to get cold snaps and polar vortexes. So while the extreme minimum temperature data used to assign USDA Hardiness Zones are changing slightly with global warming, it’s those high temperatures that keep breaking records and will continue to do so.
Lately, I’ve gotten many questions about ailing trees around the state, and I tend to get more worried about plants not being heat hardy enough for where they’re planted. Usually when a range of zones is given for a plant (e.g., USDA Zones 4–9), that higher number implies the hottest cold hardiness zone it can tolerate. So it seems you’re in the clear on that spectrum too with your new juniper tree. In the next 30 years, high temperatures in the Albuquerque area are expected to resemble the current high temperatures in Las Cruces or El Paso, and in 80 years, by 2100, closer to the highs currently experienced in Tucson. For trees to live and thrive as long as possible, we need to consider how cold hardy they are today and how heat hardy they will be in warmer decades to come.
It’s always important to double check these planting details. In this case, the label seems to have been misprinted, but you’re still in the clear. Thank you for planting trees and for paying attention to these important details.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.
Links:
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.
Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!