You know your street address and ZIP code. You know your town, village or city. You know — maybe — your ward. But do you know your gardening zone? How about your microclimate?
Knowing if a plant will survive winter temperatures is essential to gardening, and that’s what a zone tells you. The U.S. Department of Agriculture publishes maps of national hardiness zones determined by data collected from weather stations. The most recent is based on data collected over 30 years, from 1976 to 2005. So, for example, if you are told a plant is “cold-hardy” to Zone 3, it means it can survive temperatures as low as minus 35 degrees.
To find your hardiness zone and its average annual low temperature, enter your zip code at planthardiness.ars.usda.gov. Beacon is in Zone 6a (minus 5 to zero degrees) and Philipstown is in Zone 6b (minus 10 to minus 5 degrees). Microclimates within the zones might shift those numbers slightly.
The USDA has a newer interactive map that is supposed to give much finer detail and account for microclimates, but over the course of several days and browsers, I never got it to work. The good news is that there’s an effort to show more accurate local data. I’ll keep trying.
Why does it matter? Because if you’re growing a plant at the edge of its zone, as the climate warms and our winters aren’t as cold and our summers are hotter, plants that once thrived won’t and new plants will. That may seem like it’s expanding the range of what plants can grow here but it’s also associated with fewer days of freezing temperatures. Trees that need a certain number of chill hours to produce fruit, such as apples, will be challenged in milder winters.
A fig tree, Ficus carica, is an excellent example of a plant that might thrive in the right microclimate. While the fig grows best in Zones 8, 9 and 10, it can grow in Zones 6 and 7 under the right conditions. Jennifer Lerner, the senior resource educator at the Cornell Cooperative Extension of Putnam County, helped me understand how microclimates function.
“The closer you are to the river,” she said, “the warmer it will be. Water has a buffering effect and the temperature doesn’t swing as fast.”
I know this to be true because of the temperature panel on the dashboard of my car. My Philipstown property is about 2 degrees cooler than the top of East Mountain Road and 5 to 7 degrees cooler than the riverfront in Cold Spring and Beacon. When it’s snowing at my house, it might be dry a mile away. My growing season is about two weeks shorter in spring and fall for vegetables than near the river, and the bud break is later. When I’m choosing what to plant in my yard, I take all this into consideration.
Elevation, wind and ground surface also create microclimates. In the Highlands, we have many, and your property could even contain multiples. South facing slopes are slightly warmer than north facing slopes, for example. That fig tree might be able to grow in a south-facing courtyard that is protected from wind.
All is lost, though, with just a single day when temperatures drop below that plant’s cold-hardiness zone. With temperatures becoming more unpredictable, it is possible that we can have a mild winter but a period when the temperature drops below average.
And what about hotter average temperatures since we’ve had record-breaking heat worldwide in the past five years? As warming temperatures impact plants with hotter summer days, there’s more research to be done on the upper limits of plants’ tolerance. Until it was taken down recently for updates, the American Horticultural Society displayed a heat-zone map on its website. If you saw a plant listed as “3-8, 6-1,” it indicated it would be cold-hardy in Zones 3 to 8 and heat-tolerant in Zones 6 to 1.
Pamela Doan, a garden coach with One Nature, has grown ferns in Seattle, corn on a Brooklyn rooftop and is now trying to cultivate shitake mushrooms on logs. Email her at firstname.lastname@example.org.