Warmer in the Cape Cod garden: New hardiness zone map released

Cape Cod gardeners are a wily lot. It takes guile and planning to wrest success from our sandy soil, and keeping a close eye on the weather is an essential part of the toolkit.

So when the United States Department of Agriculture (USDA) released a new version of its Plant Hardiness Zone Map in November, those with dirt under their fingernails took notice. According to the USDA, the map is "the standard by which gardeners and growers can determine which plants are most likely to thrive at a location."

The previous map was issued in 2012, and things have changed since then.

"When compared to the 2012 map, the 2023 version reveals that about half of the country shifted to the next warmer half zone, and the other half of the country remained in the same half zone," according to the USDA.

A section of the U.S. Department of Agriculture's 2023 Plant Hardiness Zone Map showing Cape Cod and the Islands.
A section of the U.S. Department of Agriculture's 2023 Plant Hardiness Zone Map showing Cape Cod and the Islands.

This shift played out on Cape Cod, where the Outer Cape and portions of the Lower Cape moved from zone 7a to the warmer zone 7b, when compared with the 2012 map.

But Cape gardeners may want to think twice before filling their yards with less hardy tropical flowers and palm trees. Though it's not a technical term, the phrase "a scooch warmer" might sum up the hardiness zone shift in some parts of Cape Cod.

"There's no doubt that it's getting warmer, but the weather is very variable," said Cape gardening guru C.L Fornari. "If you plant hardy to Zone 7a plants on Cape Cod, you're in for heartache."

According to the USDA, plant hardiness zone designations, "represent what’s known as the 'average annual extreme minimum temperature' at a given location during a particular time period." The 2023 map is based on 30-year averages.

The average annual extreme minimum temperature for Zone 7a is 0 to 5 degrees Fahrenheit, while Zone 7b checks in at 5 to 10 degrees.

Of course, colder-than-average weather descends on Cape Cod every once in a while, with repercussions that can affect the entire growing season for certain plants. Fornari recalled the cold snap that occurred in February, when temperatures plummeted below zero, wreaking havoc on plants, including hydrangeas, roses and butterfly bushes.

So it may not be worth it to fly too close to the sun when it comes to pushing the plant hardiness angle. Plus, "there's a big difference between a plant surviving and a plant thriving," said Fornari.

There are still ways for Cape gardeners to stay in the game during the colder seasons. With the assistance of a cold frame or tunnel, you can grow cold hardy greens "well into January," said Fornari. Thrill seekers might try a cold hardy rosemary plant outside if they feel lucky.

But Fornari thinks it would be unwise to invest in plants that push the edge of our hardiness zone.

"Don't bet the farm on it," she said.

Eric Williams, when not solving Curious Cape Cod mysteries, writes about a variety of ways to enjoy the Cape, the weather, wildlife, and other subjects. Contact him at ewilliams@capecodonline.com. Follow him on X: @capecast.

Thanks to our subscribers, who help make this coverage possible. If you are not a subscriber, please consider supporting quality local journalism with a Cape Cod Times subscription. Here are our subscription plans.

This article originally appeared on Cape Cod Times: Cape Cod gardening: New USDA hardiness zone map shows changes