By Melissa O’Brien 

If you’re an avid gardener or even just someone with a few potted plants on your front stoop or patio, you’ve likely encountered the U.S. Department of Agriculture’s Plant Hardiness Zone Map. This crucial tool is designed to help you determine which plants are most likely to thrive in your specific location based on the weather patterns – specifically how cold it gets in the winter. The zone information from the map is often featured on plant tags at your local nursery along with other important details about how to care for your plant. 

In November, the USDA released an updated Zone Hardiness Map based on average low temperatures from 1991 to 2020. Last updated in 2012, this new map shows a significant shift in hardiness zones due to warmer temperatures across the country. 

What does this change mean for gardeners in the Greater Philadelphia region? We spoke with PHS VP of Horticulture Andrew Bunting, who was a consultant on the new map, to better understand these trends and offer tips for home gardeners choosing what to plant. 

What is the USDA Plant Hardiness Zone Map and Why is It Important? 

The USDA Plant Hardiness Zone Map is a guide you can use to estimate plant hardiness – the coldest temperatures plants can withstand – and determine which plants to grow in your garden. The map is based on average low winter temperatures and is divided into 13 zones, each split into A and B groups. 

Essentially, if a plant isn’t cold-hardy enough for your zone, it likely won’t survive the winter. This makes understanding your zone crucial for planning your garden. For example, if you live in Chicago (Zone 5), and a plant is labeled for Zone 7, it’s unlikely to survive your winters without special care. “While the zone map is a great starting point, other factors like soil drainage, humidity, and microclimates also affect plant survival,” says Bunting. “Sometimes, with excellent soil drainage or protection from wind, plants can survive in slightly colder zones than listed.” 

How is the Map Created? 

The map is updated roughly every 10 years using data from thousands of weather stations across the United States, including Alaska, Hawaii, and Puerto Rico. The most recent update incorporated data from approximately 13,000 weather stations, reflecting changes in climate patterns over time. While these updates historically showed incremental changes, the latest revisions have seen some regions shift by an entire zone due to overall warming trends. 

What these Changes and Trends Mean for Philadelphia-region Gardeners 

Recent updates to the zone map have shown significant shifts. For instance, Philadelphia, previously in Zones 7a and 7b in 2012, has warmed solidly to Zone 7b. Parts of Montgomery County also moved up to 7b, and sections of Delaware County jumped to 8a.  

Andrew notes that this shift allows gardeners to grow plants previously considered too tender for the region, like camellias and crepe myrtles, which are now common sights. “I have friends that grow hearty palms in their yard now, and that's something they’ve only been able to do because of the warmer winter temperatures,” says Andrew. 

The warming climate has extended growing seasons and allowed for the cultivation of a wider variety of plants. However, it also poses risks. Earlier blooming times increase the risk of frost damage to flowers, and fluctuating temperatures can stress plants, leading to potential long-term damage or death. 

Tips for Home Gardeners 

• Stay Updated: Keep an eye on the latest Hardiness Zone Map updates to ensure your plant choices are informed by current data. 
• Observe Your Microclimate: Factors like urban heat islands can create warmer microclimates. For example, urban Philadelphia can be warmer than its surrounding suburbs. 
• Adapt and Experiment: While sticking to your zone is safe, don’t be afraid to experiment with plants from slightly warmer zones, especially if you can provide favorable conditions like good drainage. 
• Monitor Weather Patterns: Be prepared for unusual weather events, such as unexpected droughts or heavy rainfall, which can affect plant health. 

Looking Ahead 

As climate trends continue, we can expect further shifts in hardiness zones. Plants that were once considered annuals might become perennials, and gardeners will have to adapt to both the opportunities and challenges presented by these changes. By staying informed and adapting to changes, you can ensure a flourishing garden year-round. Happy gardening!