Renée Loth

The fragrant barometer

As the climate warms, Lilac Sunday comes earlier and earlier

Visitors fill the the Arnold Arboretum for Lilac Sunday in 1938. Visitors fill the the Arnold Arboretum for Lilac Sunday in 1938. (File 1938/The Boston Globe)
By Renée Loth
May 7, 2011

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LILAC SUNDAY, Boston’s great annual exhale (and inhale!) that marks the height of spring at the Arnold Arboretum, is being held tomorrow — one of the earliest dates ever. Officially it is to coordinate with Mother’s Day, but watchful gardeners know better.

Every year has its own weather character, but the arboretum’s celebrations marking the peak lilac bloom have migrated by three weeks over the last century. In 1920, Lilac Sunday was held on May 30; in 1983 it was May 22; in 2003 it was May 16; and this year it is May 8.Is it global warming? The arboretum is careful not to take a position.

“The truth is we sort of guess’’ at the best date for the celebration, said Stephen Schneider, manager of horticulture at the Arboretum. The lilac blossom is fleeting, and with the arboretum’s astounding collection of over 400 varieties, something is bound to be just budding out or past peak no matter when the date is pegged. But Schneider will admit to a bit of gardener’s intuition. “You just get that feeling that things are probably warming up ever so slightly, not shockingly,’’ he said. “You do notice earlier bloom times.’’

The science of tracking changes in nature’s calendar, whether they be annual bird migrations or the onset of ragweed season, is called phenology. The Arboretum is participating in a long-term study at the National Phenology Network, chaired by Mark D. Schwartz, to record the bud and bloom dates of one particular lilac variety, the Red Rothomagensis.

In a 2006 article in the Journal of Biometeorology, Schwartz and his co-authors tracked the first leaf date and first flower dates of the lilac beginning in 1965, and found “an advance in spring phenology ranging from two to eight days’’ in the northeastern United States.

Bloom times and bird migrations are among the most sensitive biological indicators of climate change, Schwartz notes.

This particular hybrid lilac is common enough for serious backyard gardeners to participate in the survey along with botanical gardens and horticulture labs. The phenology network will sell lilac saplings to homeowners who sign up as observers and provide feedback on detailed data sheets.

In fact, America’s 91 million gardeners are a great untapped resource in the fight against climate change. They are the sentinels of the natural world, noticing everything. “Gardeners are really on the front lines of these issues,’’ said David Mizejewski, a naturalist with the National Wildlife Federation. “They can be a vocal and passionate force for change.’’

The wildlife federation publishes a Gardener’s Guide to Global Warming that includes tips on how to maintain a more sustainable backyard habitat, from planting native species to conserving water to composting. And it recommends broader political action such as pressing for more renewable energy sources.

There is plenty of change to notice. According to the federation, the period between the first and last snow dates in the northeast has decreased by seven days over the past 50 years. Warming trends endanger the mayflower — the iconic Massachusetts state flower — and dozens of other state flowers and trees. And the National Arbor Day Foundation revised its well-known plant hardiness zone map in 2006. Even Kansas is not in Kansas anymore.

Other effects in our personal landscapes could be dire. Sugar maples, which require a long cold winter, could be threatened, and with them the leaf-peeping New England economy. Invasive species, pests, and weeds become more of a problem. Some botanists in the Northeast have noticed an increase in poison ivy. Allergies would be more prolonged and virulent.

Gardening is among the most popular leisure time activities in the country — ahead of golf, hunting, camping, even shopping. Why then, do gardeners lack the political clout of, say, hunters? Could it be the perception that gardeners are just little old ladies in straw hats planting petunias? Would that change if we started calling gardeners what they truly are — citizen scientists — instead? Perhaps policymakers and climate skeptics in Washington just need to get out of their offices and into the dirt more often. It would open their eyes to some worrisome truths. Because the cherry trees at the Tidal Basin are blooming earlier, too.

Renée Loth’s column appears regularly in the Globe.