Climate change is changing the zones where plants grow – here’s what it could mean for your garden

WITH the arrival of spring In North America, many people head to the gardening and landscaping sections of home improvement stores, where the displays are bulky with eye-catching seed packets and benches are lined with potted annuals and perennials.

But some plants that once thrived in your garden may not thrive there now. To understand why, check out the latest update from the U.S. Department of Agriculture map of plant frost resistance zoneswhich has long helped gardeners and growers determine which plants will have the best chance of success in a given location.

Comparing the 2023 map to the previous version from 2012 clearly shows that as climate change warms the Earth, plant hardiness zones are shifting north. On average, the coldest days of winter in our current climate, based on temperature records from 1991 to 2020, are 5 degrees Fahrenheit (2.8 Celsius) warmer than they were from 1976 to 2005.

In some areas, including central Appalachia, northern Modern England and north-central Idaho, winter temperatures have risen by 1.5 hardiness zones — 15 degrees Fahrenheit (8.3 C) — over the same 30-year period. That warming is changing the zones in which plants, whether annuals or perennials, will ultimately succeed in a climate in flux.

How phytopathologistI have dedicated my career to understanding and solving plant health problems. Many stresses not only shorten the life of plants, but also affect their growth and productivity.

I am also a gardener who has seen firsthand how warming temperatures, pests, and diseases affect my annual crops. By understanding the impacts of climate change on plant communities, you can lend a hand your garden reach its full potential in a warming world.

Warmer summers, warmer winters

There is no doubt that the temperature trend is rising. From 2014 to 2023, the world has experienced The 10 Hottest Years Ever Recorded in 174 years of climate data. Just a few months of scorching, relentless heat can significantly affect plant health, especially cool season garden crops such as broccoli, carrots, radishes and kale.

Winters are also warming, which has implications for plants. The USDA defines plant hardiness zones based on the lowest average annual winter temperature in a given location. Each zone represents a range of 10 degrees Fahrenheit, with zones numbered 1 (coldest) to 13 (warmest). The zones are divided into 5-degree Fahrenheit half-zones, which are designated with the letters “a” (north) or “b” (south).

For example, the coldest icy hardiness zone in the lower 48 states new mapZone 3a includes tiny enclaves in the northernmost parts of Minnesota, and its winter temperature extremes range from -40°F to -35°F. The warmest zone, 11b, is in Key West, Florida, where annual lows range from 45°F to 50°F.

On Map 2012northern Minnesota had the much more extensive and contiguous Zone 3a. North Dakota also had areas designated in the same zone, but those regions have now shifted completely into Canada. Zone 10b once covered the southern tip of mainland Florida, including Miami and Fort Lauderdale, but has now been pushed north by the rapidly approaching Zone 11a.

Many people buy seeds or seedlings without thinking about hardiness zones, planting dates or disease risks. But when plants have to contend with temperature changes, heat stress and disease, they will ultimately have difficulty surviving in areas where they once thrived.

Successful gardening is still possible, though. Here are some things to consider before planting:

Annuals vs. Perennials

Frost resistance zones are much less essential annual plantsthat germinate, flower and die within one growing season than perennial plants that last several years. Annuals typically avoid the deadly winter temperatures that define plant hardiness zones.

In fact, most annual seed packages don’t even list plant hardiness zones. Instead, they provide planting date guidelines by geographic region. It’s still essential to follow these dates, which lend a hand ensure that frost-sensitive crops aren’t planted too early and warm-season crops aren’t harvested too delayed in the year.

User-friendly perennials have wide frost resistance zones

Many perennials can grow in a wide range of temperatures. For example, the hardy fig and hardy kiwi grow well in zones 4 to 8, an area that includes most of the Northeast, Midwest and Plains states. Raspberries are hardy in zones 3 to 9, and blackberries are hardy in zones 5 to 9. This eliminates a lot of guesswork for most gardeners, since most U.S. states are dominated by two or more of these zones.

However, it is essential to pay attention to plant labels to avoid choosing a variety or cultivar with a circumscribed hardiness zone over another with greater flexibility. Also pay attention to instructions on proper sun exposure and planting date after the last frost in your area.

Fruit trees are sensitive to temperature fluctuations

Fruit trees consist of two parts: the rootstock and the scion, which are grafted together to create one tree. Rootstocks, which consist mainly of the root system, determine the tree’s size, flowering time, and tolerance to pests and soil pathogens. The scion wood, which supports the flowers and fruit, determines the fruit variety.

Most commercially available fruit trees tolerate a wide range of hardiness zones. However, stone fruits such as peaches, plums, and cherries are more sensitive to temperature fluctuations within these zones—particularly the sudden swings in winter temperatures that cause unpredictable freeze-thaw events.

While all fruit tree species are affected by these types of weather fluctuations, stone fruit trees seem to be more susceptible, possibly because they flower earlier in spring, have less hardy rootstocks, or their bark is more susceptible to winter damage.

The strength of perennial plants increases over the season through a process called hardeningwhich accustoms them to higher temperatures, loss of moisture in the sun and wind, and full sun exposure. However, too sudden a drop in temperature in autumn can cause the plants to die in winter, which is known as killing in winterSimilarly, a sudden rise in temperature in spring can lead to premature flowering and subsequent frost damage to the plants.

Pests are also moving north

Plants aren’t the only organisms circumscribed by temperature. Milder winters are causing southern insect pests and plant pathogens to expand their ranges northward.

One example is Southern Plaguea stem and root rot disease that affects 500 species of plants and is caused by a fungus, Agroathelia rolfsiiThis disease is often thought of as affecting heated southern gardens, but has recently become more common in the northeastern United States on tomatoes, pumpkins, squash, and other crops including apples in pennsylvania.

Other plant pathogens can take advantage of milder winter temperatures, which leads to prolonged soil saturation instead of freezing. Both plants and microbes are less busy when the soil is frozen, but in saturated soil, microbes have the opportunity to colonize dormant roots of perennial plants, leading to more disease.

It may be demanding to accept that climate change is putting a strain on some of your favorite garden plants, but there are thousands of plant varieties that suit both your interests and your hardiness zone. Growing plants is a chance to admire their flexibility and the traits that allow many of them to thrive in a world of change.