• Skip to main content
  • Skip to footer
  • Garden design
  • Container gardening
  • Food to grow
  • Gardens to visit
  • Events for gardeners
Garden Making

Garden Making

Inspiring ideas and information for great home gardens

Home » Hardiness zones are drifting

Hardiness zones are drifting

By Judith Adam Filed Under: Making a Garden

Corylopsis (Photo by Brendan Zwelling)
Corylopsis (Photo by Brendan Zwelling)

The United States Department of Agriculture has updated its 1990 plant hardiness map, and it reveals that hardiness zones are drifting. The new USDA map reflects average cold temperatures from 1976 through 2005, and illustrates the warming trend we’ve noticed in recent winter seasons. The new data indicates an average 5°F temperature increase, making a half zone movement northward. Although plant hardiness zone designations on the Canadian plant hardiness map are different from those on the USDA map, the warming trend is comparable. That could be good news for those of us pressing the cold hardiness limits of plants in our gardens.

I’ve noticed my garden isn’t so cold as it once was. Ten years ago I planted a Voss’s laburnum tree (Laburnum x watereri ‘Vossii’, Zone 6) in my Zone 6a garden. The laburnum tree was right up against the coldest winter temperature it could endure. The following winter was normally cold, but in spring, half the crown of the laburnum was dead. I pruned out the dead branches, leaving a seriously disfigured tree. However, it has grown and partially filled in the empty spaces, and there has been no further winter injury. Either the tree has increased its cold hardiness (which seems unlikely), or the subsequent winters haven’t been so cold (which seems more the case).

Another hardiness experiment had more dramatic results. I saw a fragrant winter hazel (Corylopsis glabrescens, Zone 6) in full spring bloom in a Zone 6b garden centre. Being entirely seduced by the dangling yellow blossoms, I simply had to purchase the plant and drive it 145 km north to my garden. Knowing it was borderline hardy (and my Zone 6a garden is colder than where the shrub had growing), I put it in a protected position with dense conifers to break the winter winds. The following spring, almost all the branches had died back, and the shrub entirely succumbed during the summer. Two years later I planted another fragrant winter hazel in a location even more protected, with brick walls on two sides. That shrub also declined and died.

Now, you would think the message is clear — winter hazel is comfortable in Zone 6b, but doesn’t have enough cold hardiness to survive in a Zone 6a garden, no matter how much protection is provided. But preferring to ignore the obvious, I planted a third shrub, and this time the location has less protection from wind. Am I tempting fate? This winter hazel has survived two winters with no dead wood, and has increased in size. I don’t have enough precise information to say that shifting hardiness zones have made my Zone 6a garden warmer in winter. But something seems to be changing, and this winter I intend to keep daily temperature records.

Here is the link to the new USDA hardiness map: http://news.aces.illinois.edu/news/new-usda-plant-hardiness-zones

You can look up your zones on the Canadian hardiness map

Print Friendly, PDF & Email
Share this article

Published: July 11, 2012 | Updated: February 6, 2019

About Judith Adam

Judith Adam is a horticulturist, landscape designer and author of several best-selling gardening books, including Landscape Planning. She lives in Toronto.

Reader Interactions

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Footer

Garden Making website

explore

  • Books for gardeners
  • Containers in your garden
  • Events for gardeners
  • Food to grow
  • Garden design
  • Gardens to visit
  • Growing from seed
  • How to
  • Plant ideas
  • Prizes for readers
  • Tips for gardeners

Members

  • Member login
  • Register for free membership
  • Manage your email preferences
  • Add your event listing
  • View events you’ve submitted
  • Backup form to submit event

Events for gardeners

May 25
7:00 pm - 8:00 pm

Peterborough Horticultural Society Monthly Members’ Meeting

Organizer: Peterborough Horticultural Society
416-846-0778
May 27
6:00 pm - 8:00 pm

Selkirk Annual Plant Sale

Organizer: Selkirk & District Horticultural Society
(204) 738-2368
May 28
8:00 am - 12:00 pm

Bronte Annual Plant Sale

Organizer: Bronte Horticultural Society
May 28
8:00 am - 11:00 am

Fergus & District Horticultural Society Plant Sale

Organizer: Fergus Horticultural Society
519-820-6337
May 28
8:00 am - 2:00 pm

Mount Forest Plant Sale

Organizer: Mount Forest Horticultural Society
519-509-6246
View Calendar

Get email updates

Join 9,703 gardeners who get our email updates. We do not share emails.

Check your inbox or spam folder to confirm your subscription.

Copyright © 2022 Inspiring Media Inc. | GARDEN MAKING is a registered trademark | About | Service | Terms of use | Privacy policy