Plant zones shifting north as winters warm

Sep 18, 2012

If you’re thinking of planting trees or shrubs in your yard... the U.S. Department of Agriculture has guidelines for what to plant depending on where you live. It’s called the Plant Hardiness Zone Map.  It’s based on average minimum winter temperatures.  So you can use it to decide if the kind of tree you want to plant will make it through the winter without freezing to death.

This past January, the USDA updated this map for the first time since 1990. 

But one researcher argues it’s already out of date.

Nir Krakauer is an assistant professor of civil engineering at the City College of New York. He says the USDA used the annual minimum temperatures between 1976 and 2005 to make their map.  He updated that map with more recent data.

“In general, a lot of Michigan might be a half zone less cold than the USDA map would show.”

In other words – he says – our winters have been warming faster than other seasons... and that’s shifting those plant zones north.  That means plants that used to only survive in warm southern climates are doing well farther north.

Krakauer says you might be able to experiment with growing plants that do better in warmer places.

Experts say one of the things that’s striking is how much warmer our winters have gotten on average.

"Minimum temperatures have increased in some cases 5 to 10 degrees Fahrenheit in the winter in just the last 30 years. Those are very, very significant changes."

Jeff Andresen is the state’s climatologist and an associate professor of geography at Michigan State University.

“If we look at our changing temperatures, Michigan is warmer now than it has been in the past, but it turns out much of the warming that has occurred, especially in the last few decades, has occurred during the winter season and the spring season as well, and overnight: minimum temperatures have increased in some cases 5 to 10 degrees Fahrenheit in the winter in just the last 30 years. Those are very, very significant changes.”

He says the USDA plant zone map is still a reliable guide for what to plant in your yard.

Unless you really love taking risks.

“It’s like dating in high school, they want what they can’t have, they want to go for that next one up.”

Bert Cregg is an associate professor of horticulture and forestry at MSU. (You can read his blog post to find out why he's excited about the new hardiness zone map). He says a lot of people want to try to plant things that really won’t survive in Michigan.

"It's like dating in high school, they want what they can't have, they want to go for that next one up."

“I go to these gardening shows and whatnot and see T-shirts: Been There Killed That or You Don’t Really Know a Plant Until You’ve Killed it At Least Three Times.  People are always wanting to push their hardiness zones.”

Cregg says the trouble is... even in a warming climate, we can still get very cold winter events.

“Yeah, on average, the winters are getting warmer, but that extreme event is still potential and of course that’s what’s going to take your plants out.”

Cregg says you might’ve heard fall is the best time to plant trees.  But he actually recommends waiting until spring... because if it gets cold early this fall, it could be hard for those young trees to get established.