“Intoxicating” is a word that comes to mind in springtime to describe scents wafting on the air – lilac, lily-of-the-valley, honeysuckle. “Intoxicated” comes to mind this spring, as many flowering plants crowd the party far too early, having tossed back record warmth in March and been on a tear ever since.
Frankly, I’m a little disillusioned. Plants are supposed to be wired with a certain sobriety; resisting the lure of this year’s March heat, surely they’d only go so far. But what I’ve seen this spring in a number of trees, shrubs and flowers I thought I knew – from seed in some cases – is a complete lack of moderation. And what a new study relays is that these plants may not be the reliable, slow-to-change backdrop we once thought, but a far more temptable lot.
Quite contrary, that’s how my garden grows
Out my back door, the landscape is a gorgeous muddle. A few things, like forsythia and redbud, bloomed at their usual time. Others, like magnolias, fruit trees, lilacs and rhododendrons are weeks ahead of schedule. On May Day my pear tree was sporting fruit! It reminded me of my “tween” daughter asking to wear dangly earrings – I’ll admit, they look pretty good, but it’s just too soon.
When “Lilac Sunday” started in Boston 90 years ago, it was held close to June 1; these days it’s mid-May. But this year, common lilacs (these from my yard) were turning brown before their typical first bloom date.
If you pay attention to these things in your landscape, if you expect them to do what they’ve always done, then this spring is seriously disorienting. It’s like watching a school play gone awry, when the otherwise darling actors keep coming onstage in the wrong order, overlapping their lines, making it impossible to follow along. Individually, they’re precious, but could someone make them stop?
We know this year’s record-smashing March heat prompted lots of plants to kick into their spring growth mode very early. And with last month’s temperature data in, it’s clear that April egged them on. According to the National Climate Data Center, this April was the third warmest on record for the U.S., with above average temperatures across large swaths. And while this year has been dramatic so far, it’s not an isolated blip, but part of a larger trend (see map).
Here in Massachusetts, the Arnold Arboretum hosts “Lilac Sunday” each May. This year they added a disclaimer to the event, recognizing the main attraction would have passed its peak by then. Lilacs are a poster child for spring; their bloom time is widely seen as a key spring indicator and has long been tracked. Studies have documented how, in recent decades, the flowering time of lilacs has shifted earlier in the season – a total of roughly 4 days earlier here in the Northeast. But three weeks earlier than normal? This is what’s so eye opening for me about this spring: seeing how dramatically plants can respond when weather conditions give them unusual cues.
“Spring Creep”: Still Creepy, Not Creeping
For this particular gardener, trying to go about my normal spring yard work surrounded by all this misbehaving vegetation, that new study on plant phenology (when in the year they do what they do) provided some useful explanation. Based on extensive real-world observations, the response of plants to warmer temperatures appears to be a lot stronger than indicated by the many previous controlled experiments (the results of which have been used extensively to drive ecosystem models and inform planning). In the real world, plants are apparently blooming and leafing out 5 to 6 days earlier per degree Celsius increase in local annual temperature, versus roughly a day or two earlier in the laboratory.
In the U.S., few April high temperature records remain from the 20th century; most have been broken since 2000. Source: NOAA
In my garden, this is illuminating; I can hope my leafy party-goers sober up later in the season with no ill effects, and I can stop standing around looking puzzled in my gloves and rubber boots. In the wider world, it’s troubling. So far, we’ve witnessed about 1.5 degrees F (0.75 degrees C) of warming, globally, but many regional temperature increases have been even higher. We’ve already committed ourselves, globally, to about another degree F (0.6 degree C) of warming given past emissions and, at the moment, we’re on track to drive temperatures dangerously higher this century. If the responsiveness of plants – the foundation of ecosystems – to higher local temperatures is greater than we realized, and if many of the tools we’ve been using to plan for ecosystem change may be poorly suited to the task, then we have some catch-up to do.
We could start here: no more spiking the climate punch if we don’t want the natural world getting unruly.