USDA Releases New Plant Hardiness Zone Map

 Plant Hardiness Zone Map-U. S. Enter zip code for state map.

 

USDA Releases New Plant Hardiness Zone Map

By   Rick Pleggenkuhle

        ISU Extension/Cerro Gordo County

        Agriculture & Horticulture Program Coordinator

The USDA recently released a new Plant Hardiness Zone Map for the Unites States.  This is the first time it has been updated since 1990.  Most of north central Iowa has been reclassified from zone 4b to zone 5a.  Even though your location may now be considered zone 5, I would not recommend replacing all the plants around your home with less hardy trees, shrubs, and perennials.  Temperature extremes can vary dramatically depending on microclimates.  There can be several degrees difference between properties in the middle of a city compared to ones on a hill in the country.  The amount of snow cover or mulch surrounding a plant and the moisture content in the soil often helps determine if a plant survives sustained, extreme cold temperatures.  The average annual minimum temperature in zone 5 is between -10 and -20 below zero.  We can’t forget that Mason City and much of north Iowa has recorded temperatures colder than -20 below in each of the last three years.

 

After monitoring dozens of zone 5 plants over several years here in northern Iowa, I have found that each variety has different levels of success.  Many plants classified as being hardy to zone 5 have performed extremely well, some have survived up until we had a severe winter, and others never even made it through the first year.  Certain trees and shrubs may be “root hardy” to zone 4, but are only “top hardy” to zone 5.  Several Weigela and all Butterfly Bush varieties fit this description.  We also have to take into consideration who gives each plant their zone hardiness classification.  Often times, new plant introductions are sold with a certain zone designation after only being tested for a year or two in that particular climate.  Two separate nurseries may sell the same plant, such as a Bigleaf Hydrangea variety, but each may have it listed as being hardy for a different zone. 

 

Using zone 5 plants opens a whole new spectrum of varieties that we may have only seen in magazines, on gardening shows, or on trips to states south of Iowa.  If you plan to purchase zone 5 plants based the new plant hardiness map; please be selective, research the plant, and be conservative when purchasing them for this part of the state.

 

One last note, there are some benefits to living in the Upper Midwest.  Warmer isn’t always better.   For example, Garden Peonies actually like Iowa’s cold winters.  The colder it gets, the better they perform.  Anyone who has tried growing Peonies down south or along the west coast will tell you that it is nearly impossible to keep them alive for more than a couple of years.  Believe it or not, Peonies are only one in a long list of plants that appreciate being planted in Iowa rather than in a warmer state.

 

 

Rick Pleggenkuhle

ISU Extension/Cerro Gordo County

Agriculture & Horticulture Program Coordinator

2023 S. Federal Avenue; Mason City, IA 50401

Email:  plegg@iastate.edu

Phone:  641-423-0844

www.extension.iastate.edu/cerrogordo

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