Where spring leaves are already out — and what it means for the season

Spring may start on a different day depending on whom you ask. A meteorologist may say March 1, while an astronomer may say March 20.

But what if you ask … the plants?

Unfortunately, they won’t reply directly, but their behavior may provide clues about the transition into spring. In fact, the “spring leaf out” is the phrase for gauging the season’s arrival based on the emergence of leaves.

This year, about 19 percent of counties in the United States are experiencing spring leaf out at least a week early — while 9 percent of counties are registering a spring leaf out that’s more than a week late. Timelines are affected by seasonal temperature and rainfall patterns, with leaves emerging early in many northern states and late in some southern states this spring.

That’s according to data from the USA National Phenology Network, which tracks this phenomenon every spring. The data can be used to determine if a particular place is leafing out early or late, compared to average.

You can explore just how early or late spring leaf out is occurring in your county with the table below.

The early leaf out in many northern states could contribute to an early start to allergy season — especially considering that most states have experienced stronger than average, pollen-blowing winds during March and that much warmer temperatures will soon arrive.

Where leaf out is early or late

This year, leaf out patterns have been diverse. Strands of earlier and later-than-average conditions line the country, tied in part to temperatures and snowfall during recent months.

Parts of the Gulf Coast, Florida, southern Plains, Tennessee Valley, northern North Carolina and the mid-Atlantic have experienced a later-than-average emergence of leaves. The phenomenon in coastal areas of the Pacific Northwest is also delayed.

Leaf out in the nation’s capital has been about two days later than average, while Childress and Hall counties in northern Texas have been 12 days late — the latest in the contiguous states.

The process in the Williamsburg area of Virginia has been 11 days late, probably linked to unusually cold winter temperatures and a heavy late-season snowstorm.

To the contrary, a swath of the country from Texas to southern North Carolina has experienced an early leaf out, as have parts of the Rockies, central Plains, Midwest, Appalachians and Northeast.

Marion County, Indiana, home to Indianapolis, has experienced leaf out about 10 days early, as has Dauphin County, Pennsylvania, where Harrisburg is located. Des Moines, in Polk County, Iowa, is leafing out nearly two weeks earlier than average. Parts of western Maryland are running about a week ahead of what’s typical.

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The New York metropolitan region is also leafing out earlier than what is typical, extending north and west into northern New Jersey and the lower Hudson Valley.

The estimations of leaf out are based upon mathematical models that predict the timing of leaf out for species active in early spring.

The emergence of lilacs and honeysuckles, typically among the first plants to show their leaves in the spring, is estimated based on temperature relationships.

To understand if leaf out is occurring early or late, historical relationships can be compared to the current year.

Factors influencing leaf out

Seasonal temperatures and precipitation play a crucial role in determining when spring leaf out occurs.

March has been considerably warmer than average across the country, especially across the northern tier where temperatures have been 5 to 10 degrees above average. Iowa, Wisconsin, Missouri and Michigan have been the four most unusually warm states. Leaf out is running ahead of schedule in Iowa and Missouri and will soon reach Wisconsin and Michigan.

A later-than-average leaf out along the Gulf Coast and in parts of northern North Carolina and southeastern Virginia coincide with areas that experienced some of winter’s biggest and most unusual snowstorms.

Well-above-average winter snowfall in these areas may have played a role in the slower pace of the leaf out.

A swath of earlier-than-average leaf out from the central Plains to the Northeast corresponds with snowfall that was below the seasonal average. Perhaps the ground in this region wasn’t as cold or frozen as it typically is, giving plant growth a head start.

What’s to come

Windier-than-average conditions this March, coupled with an early leaf out, could present challenging conditions for allergy sufferers. Pollen exposure is lasting longer and intensifying nationwide, and strong winds can spread pollen over longer distances.

The windiest states compared to average have been New Mexico, Oklahoma, Michigan and Texas.

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Blustery winds are expected to continue in the weeks ahead due to frequent temperature clashes.

Surges of warmer-than-average air during late March and early April could kick spring leaf out into high gear for many parts of the country, but especially the north-central states, Northeast and New England, where it is advancing.

Temperatures from the Plains to the mid-Atlantic will probably soar to summerlike heights later this week into early next week as southerly air flows from the Gulf of Mexico become more persistent.

So if it hasn’t leafed out in your town yet, it won’t be long now.

And with rising temperatures, increasing pollen counts and the inevitable spring sneezes, it’s clear that the season is in full swing.