When will spring come? Or has it already? Look up where you live.

All across the east, naturalists are exasperated. As the rest of us luxuriated in this winter’s uncommon mildness, gardeners and wildlife biologists watched with rising pique as one of the earliest springs in recent memory threw nature’s rhythms out of whack.

Ignorant of the human calendar, nature instead responds to the gradual accumulation of heat at the beginning of each year. If the daily average temperature is above freezing, that sends a signal to plants and animals that life is again preparing to grow. Each year, the USA National Phenology Network — phenology is the study of seasonal change — keeps track of when leaves sprout as heat accumulates across the country.

This year’s winter weather pattern cleaved the country in half. As a ridge of high atmospheric pressure warmed the east, a low pressure system kept conditions cooler and wetter than usual across the west, said Michael A. Crimmins, a climate science professor at the University of Arizona.

Minneapolis

Boston

New York

Chicago

Philadelphia

Washington, D.C.

St. Louis

Atlanta

Leaves came

early in the

eastern half

of the country.

Houston

Miami

Seattle

NO LEAVES YET

San Francisco

Denver

Los Angeles

Phoenix

Dallas

Leaves arrived

late in the southwest

and about on schedule in

central California.

Minneapolis

Boston

New York

Chicago

Philadelphia

Washington, D.C.

St. Louis

Atlanta

Leaves came

early in the

eastern half

of the country.

Houston

Miami

Seattle

NO LEAVES YET

San Francisco

Denver

Los Angeles

Phoenix

Dallas

Leaves arrived

late in the southwest

and about on schedule in

central California.

Seattle

Minneapolis

Boston

NO LEAVES YET

New York

Chicago

Philadelphia

San Francisco

Denver

Washington, D.C.

St. Louis

Los Angeles

Atlanta

Phoenix

Leaves came

early in the

eastern half

of the country.

Dallas

Leaves arrived

late in the southwest

and about on schedule in

central California.

Houston

Miami

Seattle

NO LEAVES YET

Minneapolis

Boston

New York

Chicago

Philadelphia

San Francisco

Denver

Washington, D.C.

St. Louis

Los Angeles

Atlanta

Leaves emerged

early in the

eastern half

of the country.

Phoenix

Dallas

Leaves arrived late

in the southwest and

about on schedule in

central California.

Houston

Miami

At River Farm, the headquarters of the American Horticultural Society in Alexandria, Va., Tammy Burke and a dozen volunteers spent weeks last autumn planting 16,000 tulips. They were supposed to bud in late April, when River Farm begins hosting weddings, but they began blooming in March. Burke worries they’ll wither away before the first happy couple has a chance to admire them.

“I don’t think I’ll plant tulips again,” she told me.

The east’s unseasonable balminess did more than disrupt Burke’s tulips. From the mid-Atlantic coast, through the Shenandoah Valley, clear across Appalachia and all the way to southern Illinois, spring leaves sprouted more than 20 days earlier than normal, data from the phenology network shows.

Animals have responded to the east’s winter warmth, too. At the U.S. National Arboretum in Washington, D.C., turtles were basking in January and February instead of March, a red fox was out hunting weeks earlier than usual, and insects, provoked by warmth in the soil, are emerging from the ground too soon.

“We’ve had an influx of critters that people aren’t supposed to be seeing,” Susan Greeley, a wildlife manager at the arboretum, told me. That can complicate things for migratory songbirds, who expect to be treated to a particular diet when they return from the south. “I’m concerned the migrants will come through and not find insects,” Greeley said.

Is climate change to blame for the songbirds predicament? Yes and no.

No, because in any given year, the timing of spring’s arrival has more to do with seasonal variation than long-term global warming, said Mark D. Schwartz, a climatologist at the University of Wisconsin at Milwaukee who has compiled more than one hundred years’ worth of data on the timing of spring’s arrival.

“What I like to tell folks is that you still need to be prepared for considerable variation from year to year. You won’t simply be able to start planting your garden earlier each year,” Schwartz told me in an email.

Yet global warming has tipped the scales in favor of earlier springs, Schwartz said. In the contiguous United States, the leaves’ average arrival date tracks closely with the average temperature at the start of each year.

Leaf appearance and temperature

Compared with 1981-2010 average

Hover on the chart to explore the data

Data for contiguous U.S. Lines of best fit created using LOESS smoothing.

In the decades following World War II, aerosol pollution from increased industrial activity cooled the surface of the earth, offsetting the warming from greenhouse gases. As stronger pollution control measures in the 1970s reduced aerosol emissions, the effect of greenhouse gases began to dominate and warming resumed. Scientists project the current warming trend will continue for decades.

Many gardeners are preparing for a future where springs keep arriving earlier and temperatures continue climbing higher. The U.S. Botanic Garden in D.C. cultivates some plants suited to warmer climates. Cut leaf smooth sumacs and desert willows planted against a south-facing wall were barely hanging on 15 years ago, but they are established and growing comfortably now, executive director Susan Pell told me.

The New York Botanical Garden has started to plant southern oak trees, expecting they will grow more comfortable as the northeast warms. “It’s hard when it’s a constantly moving target to figure out what we should be growing,” said Michael Hagen, curator of the native plant and rock gardens. “We need to be planting for the next hundred years.”

In the shorter term, the gardeners I spoke to said they were less troubled by the prospect of gradually earlier springs than by the season’s increasing irregularity. “People say ‘global warming.’ What I’m finding is it’s not just the warming, it’s the fluctuation,” Burke, of the American Horticultural Society, said.

John Shearin, who tends the Asian gardens at the U.S. National Arboretum, agreed that “things are getting more erratic overall.” After 15 years as a horticulturalist, Shearin has learned to expect the unexpected. “As a gardener,” he said, “you’re always ready to roll with whatever nature throws at you.”