We all have different measuring sticks when it comes to how we define the start of spring. Of course, the technical definition is that spring begins on the vernal equinox, which marks the point when the sun’s sub-solar point is directly over the equator. The vernal equinox occurred on March 20 — but I would venture to guess that most people feel like spring missed its deadline.
Human beings are creatures of habit. If we have it in our minds that spring starts on March 20 or when the calendar turns to April, we expect it to occur at that time, year after year. In reality, the point at which the weather “turns” to spring is a dynamic process that varies from year to year. What’s more, with a warming planet, seasonal transitions are becoming even more variable.
For a more accurate proxy, we need look no further than the plant and animal life that surrounds us.
Not surprisingly, plant and animal behavior is more in sync with the environment that we all live in, and more importantly, to the weather conditions that we all have to deal with. While it’s easy enough for humans to wear a winter coat into April, plants and animals do not have this luxury. As such, changes in their annual behavior would be quite useful as a long-term measuring stick for when spring actually begins. That’s where organizations like the National Phenology Network (NPN) step in.
Phenology, the study of cyclical and seasonal plant and animal behavior, has long been used as a proxy for weather and climate. In fact, some of the first weather “forecasts” were born out of folklore related to phenological observations. My grandfather was an adamant believer in a correlation between snowy winters and abundance of autumn acorns. Established in 2007 with support from the USGS, the NPN utilizes a vast collection of volunteer citizen observers, spread across the entire country, to monitor and record seasonal plant and life activity. But they are perhaps best known for their so-called status of spring indexes, which have a habit of showing up on social media around this time of year.
Status of Spring indexes
The NPN makes use of two main indexes, first-leaf and first-bloom, to monitor the start of spring across the contiguous 48 states. The first-leaf index is the date on which leaves first begin to grow beyond winter bud tips. The first-bloom index is the date on which flowers begin to open. Both indexes are limited to observations of only lilacs and honeysuckles, as these are the two types of plants that are most ubiquitous across the continuous United States.
Temperature is the main driver of when wildlife emerges from its winter slumber. “In the eastern U.S., accumulated heat in the spring has the greatest influence over when most plants leaf out,” said Theresa Cummings, assistant director of the NPN. “Other variables like day-length and accumulated wintertime chill (sufficient cold temperatures) also play a role in many species, but by and large, springtime warmth is the primary driver. When this warmth occurs drives when the plants leaf out.”
In addition to the annual spring indexes, Cummings and the staff at NPN have created forecast models using observations of leaf bud-burst and flowering in two species of lilacs and honeysuckles over several decades. “For the most part, the models do a really incredibly good job of predicting these events in lilacs, honeysuckles, and other early-season species very well,” Cummings said.
Status of spring in 2018
So how close has the start of spring been this year to the average spring start of over the last 30 years? “This year, the index was met 1-2+ weeks early in much of VA, WV, and DE, but then flipped to 1-2 weeks late just a bit further north (PA, NY),” Cummings said. “Warmth accumulation was ahead of schedule prior to this threshold in the areas where the index was met early, and vice-versa for the regions just to the north.”
Despite the slow start to spring in much of the northeast, Cummings has good news to report. “Our six-day forecast is projecting spring leaf-out to finally reach much of the NE and a huge swath of the Midwest in the coming week.”
Despite the below-normal temperatures that persisted in March into the beginning of April, much of the southern half of the nation accumulated enough warmth for an early start to spring. And that’s been the trend for much of the Lower 48. Earlier leaf-outs and bloom dates have consequences.
“An earlier start to spring can lead to an earlier and possibly longer allergy season, an earlier start to agricultural and turf pests, and a disruption in cultural events centered around phenological events,” said Cummings. “Last year, when spring arrived very early, there was concern in the golf community that the trademark azaleas would be past their peak bloom by the time the Augusta Masters Tournament took place.”
Let this be a reminder, that despite all of the sophisticated tools, models and observation systems that we have, Mother Nature will always be one step ahead.