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

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

Climate change is complicating plant choices and care. Early blooms and late freezes can kill flowers such as magnolias.
Casson diedCC BY-ND

Matt casson, West Virginia University

With the arrival of spring in North America, many people gravitate to the gardening and landscaping section of home improvement stores, where displays are filled with attractive seed packets and benches are filled with annuals and perennials.

But some plants that once thrived in your garden may not thrive there now. To understand why, look at the USDA’s recent update to its Plant Hardiness Zone Map, which has long helped gardeners and farmers know which plants are most likely to thrive in a given location.

A US map divided into color-coded geographic regions with a numbered key.
The 2023 USDA Plant Hardiness Zone Map shows the areas where plants are expected to grow, based on extreme winter temperatures. Darker shades (purple to blue) indicate cooler regions, moving southward to temperate (green) and warm (yellow and orange) regions.
US Department of Agriculture

Comparing the 2023 map to the previous version from 2012 clearly shows that as the Earth warms due to climate change, plant hardiness zones are moving northward. On average, the coldest winter days in our current climate, based on temperature records from 1991 through 2020, are 5 degrees F (2.8 degrees C) warmer than they were between 1976 and 2005.

In some regions, including the central Appalachians, northern New England and north-central Idaho, winter temperatures have risen by 1.5 hardness zones — 15 degrees F (8.3 degrees C) — over the same 30-year period. This warming changes the areas where plants, whether annuals or perennials, will eventually thrive in a shifting climate.

Map of the United States showing large areas colored tan, indicating a 5 degree increase in average minimum winter temperatures.
This map shows how plant hardiness zones have shifted north from the 2012 to 2023 USDA maps. The half-zone change corresponds to the tan zone. Areas in white indicate areas that have seen minimal change.
Prism Climate Group, Oregon State University, CC BY-ND

As a plant pathologist, I have dedicated my career to understanding and addressing plant health issues. Many stresses not only shorten the life of plants but also affect their growth and productivity.

I am also a gardener and have seen first hand how high temperatures, pests and diseases affect my annual crop. By understanding the effects of climate change on plant communities, you can help your garden reach its full potential in a warming world.

Hotter summers, warmer winters

There is no doubt that the temperature trend is upward. From 2014 to 2023, the world experienced the 10 hottest summers on record in 174 years of climate data. Just a few months of extreme, unrelenting heat can dramatically impact the health of plants, especially cool-season garden crops like broccoli, carrots, radishes and turnips.

Radishes sprouting in the garden bed.
Radishes are cool-season garden crops and cannot tolerate hot summer days.
Casson diedCC BY-ND

Winters are also warm, which is important for plants. The USDA designates plant hardiness zones based on the coldest average annual winter temperature in a given location. Each zone represents a 10 degree Fahrenheit range, with the zones numbered from 1 (coldest) to 13 (warmest). The regions are divided into 5°F hemizones, which are labeled “A” (north) or “B” (south).

For example, the lower 48 states’ coldest hardiness zone on the new map, 3A, covers small pockets in the northernmost parts of Minnesota and has maximum winter temperatures from -40°F to -35°F. The warmest zone, 11B, is located in Key West, Florida, where the coldest annual temperatures range from 45°F to 50°F.

On the 2012 map, northern Minnesota had a more extensive and continuous 3A region. North Dakota also had designated territories in the same region, but those territories have now moved entirely to Canada. Zone 10B used to cover the southern tip of mainland Florida, including Miami and Fort Lauderdale, but has now been pushed north by the rapidly encroaching Zone 11A.

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

However, successful gardening is still possible. Here are some things to consider before you plant:

Annuals vs. Perennials

Hardiness zones are much less important for annual plants, which germinate, flower and die in a single growing season, than for perennials, which persist for several years. Annuals typically avoid the killer winter temperatures that define plant hardiness zones.

In fact, most annual seed packages do not list plant hardiness zones. Instead, they provide sowing date guidelines by geographic region. It is still important to follow these dates, which helps ensure that frost-hardened crops are not planted too early and cool-season crops are not harvested too late in the year.

Orange flowers bloom with other plants and herbs.
California poppies are typically grown as annuals in cooler regions, but they can survive for several years in hardiness zones 8-10.
The Marmot/Flickr, CC BY

Easy-to-use perennials have wide hardiness zones

Many perennials can grow across wide temperature ranges. For example, hardy figs and hardy kiwifruit grow well in Zones 4-8, a zone that includes most of the Northeast, Midwest and Plains states. Raspberries are hardy in zones 3-9, and blackberries are hardy in zones 5-9. This eliminates a lot of guesswork for most gardeners, as the majority of US states are dominated by two or more of these zones.

However, it is important to pay attention to the plant’s signs to avoid choosing a cultivar or cultivar with a restricted hardiness zone over one that is more resilient. Also pay attention to instructions regarding proper sun exposure and planting dates 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 wood, which are grafted together to form one tree. The rootstock, which consists mainly of the root system, determines the tree’s size, flowering timing, and its ability to tolerate soil-dwelling pests and pathogens. The scion wood, which supports flowers and fruit, determines fruit diversity.

Most commercially available fruit trees can tolerate a wide range of hardiness zones. However, stone fruits such as peaches, plums, and cherries are more sensitive to temperature fluctuations within those regions — especially sudden winter temperature fluctuations that create unpredictable freezing and thawing events.

Packages for hardy fig and kiwi seedlings.
Carefully following planting instructions can increase your plants’ chances of success.
Casson diedCC BY-ND

These weather swings affect all types of fruit trees, but stone fruits appear to be more susceptible, perhaps because they flower earlier in the spring, have fewer vigorous root options, or have bark characteristics that make them more susceptible to infection. in Winter.

Perennials harden over the seasons in a process called hardening off, making them vulnerable to harsher temperatures, moisture loss to sun and wind, and full sun exposure. But a sudden drop in temperatures in the fall can cause plants to die in the winter, an event known as “winterkill.” Likewise, a sudden rise in spring temperature can lead to early flowering and subsequent killing by frost.

Pests are moving north, too

Plants are not the only organisms restricted by temperature. With mild winters, southern insect pests and plant pathogens expand their ranges northward.

An example is southern blight, a stem and root rot disease that affects 500 plant species and is caused by a fungus. Agrothelia rolfsi. It is often thought to affect hot Southern gardens, but has recently become more common in the northeastern United States on tomatoes, squash, squash and other crops, including apples in Pennsylvania.

Spotted stem with small circular growth.
Southern blight (small round fungal structures) at the base of tomato plants.
Purdue University, CC BY-ND

Other plant pathogens may benefit from mild winter temperatures, which cause the soil to become saturated for a prolonged period rather than freezing. Both plants and microbes are less active when the soil is frozen, but in moist soil, microbes have the opportunity to colonize the roots of dormant perennials, leading to more diseases.

It can be difficult to accept that climate change is affecting some of your favorite plants in your garden, but there are thousands of plant species to suit your interests and hardiness zone. Growing plants is an opportunity to admire their resilience and the features that enable many of them to thrive in a changing world.Conversation

Matt Casson, associate professor of mycology and plant pathology, West Virginia University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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