Gardeners worry. We love to fret about our plants and the weather conditions they are exposed to throughout the seasons. My friend Henriette Suhr used to call me throughout the darker months of the year, concerned about how a hard frost and high winds in November or ice and extreme low temperatures in February would impact the plants in her garden. Her garden was filled with lovely rhododendrons, including a number from the Pacific Northwest which she risked putting in despite their lack of hardiness in her zone back when she planted them. These plantings caused her endless distress in the winter when the temperatures went below the recommended temperatures for these less hardy rhododendron species. She protected her most prized varieties by installing burlap screens around them in late fall to prevent the desiccation of their evergreen leaves and destruction of their fragile flower buds by winter winds. She was gardening in what she claimed was zone 5, in Chappaqua, New York. And it was zone 5 when she and her husband Billy bought their home in the middle of the last century.
Henriette, who passed away about a decade ago at 98, also worried about something else—humankind’s impact on the environment and what it meant for our future. She hosted one of the earliest fundraising events for the Environmental Defense Fund at her house. And if she were alive to host it today, she would not be worrying about the state of her Pacific Northwest rhododendron species in the winter because the USDA just issued the latest hardiness zone map (https://planthardiness.ars.usda.gov) for gardeners and Chappaqua is now officially in Zone 7, meaning a number of the less hardy rhododendrons that she sheltered and protected in the past will now be fine overwintering in Westchester County.
The new map includes news for us all. My home in Ashley Falls, now sits in zone 6a, which is good news for the cedar of Lebanon I planted a few years back when it was on the border of its hardiness zone, but what does it mean for the plants that are native to our area as the years continue to get hotter throughout the year? The USDA hardiness zone maps calculate the minimum temperatures a species can take, not the maximum. If there is an issue that would worry Henriette if she was alive today, it would be the heat of the summer and its impact on the plants in her garden.
With this being the hottest year on record, it is time we consider at what temperature plants begin to suffer from the heat in summer, or even worse, on an unexpected 90-degree day in April or May before the trees have leafed out to shade the perennials and shrubs that sit beneath them? This other end of the temperature spectrum has not been a part of the calculation of the USDA hardiness zones, but fortunately the American Horticultural Society has developed a Plant Zone Heat map (https://ahsgardening.org/about-us/news-press/cool_timeline/heat-zone-map-developed/) dividing the country into 12 climate zones based on the number of days over 86 degrees F., a temperature above which many plants begin to suffer. The data collected in this map is of interest. In the Southwest, horticulturists may have a sense of a plant’s upper limits in temperature because it has always been a consideration in areas where temperatures can reach 110 degrees, but who can one turn to in the Northeast when considering whether the area is getting too warm to grow a sugar maple, bunchberry, or a conifer that prefers cool summers? Most of us have never really needed to collect such information, other than anecdotally about an occasional species like lupines or the Himalayan blue poppy that melts in the summer heat and prospers in places like Vermont and Maine.
For years, many gardeners have pushed the boundaries of USDA zones, trying plants from warmer climates that are deemed not hardy and planting them in locations protected from winter winds or with good winter drainage. I have had success growing a number of species this way, including a crinum I was told would not grow much above the Mason-Dixon line. By placing such plants near a sun-drenched stone wall that radiated heat or under a thick blanket of mulch in winter to keep the soil from getting too cold, we learned what to do to protect these plants from the lowest temperatures, but what are we going to do to protect plants from the heat? Visions of plants shaded by Victorian parasols come to mind, giving plants a little summer shade like a southern belle in July, to prevent them from succumbing to the heat of the summer sun. However, this seems a little harder for things like trees, which we are used to providing us with shade, not needing it themselves.
I am hopeful that, just as gardeners have shared tips on perennializing cardoons in zone 5 or managing less hardy rhododendron species or a milk lily, we can start sharing advice on how to protect the plants that come to us from cooler climates. Observation and experimentation are essential aspects of gardening, now more than ever. And as Henriette would often say in her gentle Viennese accent, in the only moment where I could say she reminded me of Gilda Radner’s character Rosanne Rosannadanna in philosophy if not tone, “it is always something.”
To find out your new zone information, visit https://planthardiness.ars.usda.gov
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A gardener grows through observation, experimentation, and learning from the failures, triumphs, and hard work of oneself and others. In this sense, all gardeners are self-taught, while at the same time intrinsically connected to a tradition and a community that finds satisfaction through working the soil and sharing their experiences with one another. This column explores those relationships and how we learn about the world around us from plants and our fellow gardeners.
