Longwood Garden’s Shawn Kister is not the only public garden expert wondering about a shifting climate. “Longer periods of warm weather affect plants in many ways,” says Mac Franklin, the head gardener of the North Carolina Arboretum, who in April is already sporting a farmer’s tan. “It can mean less water in the soil,” he explains, “and then the rain we do get can’t seep into the ground as easily. Longer warm periods can lengthen the pest season and allow for more generations of pests,” such as aphids.
Pests are also able to thrive and spread in forests and other natural landscapes at least in part because of warmer nights. Both the native pine bark beetle, which has ravaged forests from Colorado to Alaska, and the introduced wooly adelgid, which has been sucking the life out of hemlocks in the East, may be benefiting from warmer nights, especially in winter.
Standing dead trees—pine beetle damage—in a patch of forest in the mountains near Granby, Colorado. Warmer winters are allowing the destructive insect to thrive. The pine beetle has infested 1.5 million acres of lodgepole pine in Colorado. (Photo copyright University Corporation for Atmospheric Research. )
With warmer nights, fire season is longer. Pollination patterns may be changing. A study sponsored by Longwood Gardens showed that flowering plants have been blooming an average of one day earlier each decade. Warmer nights and later onset of freezing days also means that plants can generally survive farther north than they used to.
Click blue titles above to change map views. Climate-related planting zones in the United States based on new U.S. Climate Normals (1981-2010); old Normals (1971-2000); and areas that have changed zones as a result of warmer winter minimum temperatures. (Maps by NOAA.)
These maps of climate-related planting zones were created by the National Oceanic and Atmospheric Administration (NOAA) as a special service to the American Public Garden Association. The official Plant Hardiness Zone Map, familiar to any gardener who has looked at a packet of seeds, was prepared by the U.S. Department of Agriculture (USDA) in 1990 using data collected by NOAA. USDA is updating its official map, which will soon be available online.
North Carolina Arboretum Director George Briggs offers this advice to gardeners facing a shifting climate: “If they are climate-literate they can put risk management into their decision making, make their design specifications more conservative.”
“Public gardens have to balance costs, benefits and risk,” says Briggs. “It’s a more complex decision matrix with these shifts. Will gardens have to beef up their sewer systems to account for heavier rain events? How much irrigation to put in? Where once you needed just enough to establish the landscape, now there may be a continuing need for water, or maybe you put in more conservative plants that are more water-efficient.”
Looking to the future
To help garden visitors make sense of all these changes, the American Public Gardens Association is creating a cell phone tour about climate. At Longwood Gardens, the tour will begin in the managed meadow close to a still-surviving grand old sugar maple.
A surviving sugar maple overlooks a managed meadow at Longwood Gardens in Pennsylvania. The tree is the first stop on a cell phone tour of the garden that explains the potential impacts of climate change on plants. (Photo courtesy Longwood Gardens.)
“There is telling evidence that climate change is affecting plant life around the world and here at Longwood,” says Paul Redman, Longwood’s Director. “Sharing the important work of NOAA with our staff, guests, and community is integral to our mission and continues Longwood Gardens’ commitment to environmental stewardship.”
As for the U.S. Climate Normals, the team at the National Climatic Data Center in Asheville will continue to release additional products over the coming months. These will include agricultural normals such as Frost/Freeze Dates, and population-weighted normals such as Heating and Cooling Degree Days.
In the meantime, new data are being collected every day, at thousands of stations nationwide, that will eventually become part of the 1991-2020 U.S. Climate Normals, due for release in another ten years.
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Jennifer Freeman is a science writer with the American Meteorological Society. She wrote this article while on assignment to NOAA’s National Climatic Data Center.
-The New Climate Normals: Gardeners Expect Warmer Nights,
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