It was a lovely day on the University of Pennsylvania campus: sunny and 70 degrees. Students sunbathed and tossed Frisbees on College Green, near cherry trees on the verge of blooming. The only incongruity? It was still winter.

It’s been hard to avoid remarking on the weather, with repeated swings from 70 degrees one day to snow and below-freezing temperatures the next. Though climate scientists have thus far resisted attributing any one weather event to climate change, the earlier arrival of spring-like temperatures and increasing occurrence of extreme rain and snow fit the pattern.

Plant experts at Penn have been among those pondering what the unpredictable weather means for native, ornamental and food plants. Their research and experiences shed light on why, for example, cheery daffodills appeared in February, only to be pummeled under ice and snow in March, and what these phenomena might mean for plant life moving forward.

Plants don’t consult the calendar to determine the season. Instead they rely on adaptations that allow them to “guess” the right time to develop their buds, to bloom and to leaf out.

“Plants have to be immensely sensitive to their environment because they’re sessile,” that is, fixed in place, says Doris Wagner, professor of biology in Penn’s School of Arts & Sciences. “Their strategy is to wait until the environment is good — to sense moisture, temperature and light — and only then put their resources into reproduction.”

Wagner, who studies how plants switch between different types of development, such as from vegetative growth to flowering, has identified some of the molecular pathways involved in this sensing, which she says depends on three main cues.

“One is exposure to prolonged cold, and by prolonged cold I mean months.”

During a typical winter, a type of protein called a transcription factor works to clamp down on flowering, but exposure to cold progressively shuts down the protein until the clamp “releases,” allowing flowering to occur.

The second cue, Wagner says, is day length, or, to be more accurate, the duration of the night. A key transcription factor that promotes flowering is degraded during the night. This means it can only accumulate in the spring when the days get longer and the night gets shorter to promote flowering.  

“If you’re a plant, once you’ve had a long period of cold and now your days are getting longer, you’re pretty safe to assume that it’s spring,” Wagner says. “It’s at this point, the gates are released for flowering, that ambient temperature plays a role, and that’s what fouls them up.”

A couple of days in the 60s and 70s can “trick” a plant into blooming. Once a bud has gotten to a certain stage of development, it becomes vulnerable to frosts, so a warm spell in February followed by a March freeze could mean the plant won’t set fruit that year.

Despite the opportunities for confusion, plants in the mid-Atlantic region are used to a certain degree of unpredictability in temperature and precipitation.

“These sorts of swings are not atypical,” says Anthony Aiello, the Gayle E. Maloney Director of Horticulture and Curator at Penn’s Morris Arboretum. “It happens pretty regularly because of the way our climate is, so it’s something that, as horticulturists or gardeners, you come to accept or at least expect.”

Yet Aiello says the frequency of these weather swings seem to be increasing while overall temperatures have been warming up. The USA National Phenology Network, an organization that tracks observations on the timing of biological events and activity, found that signs of spring arrived up to three weeks earlier this year than normal.

Botanists at the Morris Arboretum have the data to back this up. Through the Pennsylvania Flora Project, an effort to track the distribution of plants across the state, researchers there and at many other institutions have assessed how plant populations have shifted over the years and decades.

Tim Block, the John J. Willaman Chair of Botany at the Arboretum, has been a long-time contributor to the project. One trend he attributes to climate change is the recent northward expansion of particular plant species.

“One that comes to mind immediately is a plant called Carolina silverbell, Halesia carolina,” he says. “It was historically known in Pennsylvania only at the extreme southern edge of the Susquehanna Valley, but it’s now made its way all the way up to the State College area up in central Pennsylvania.”

What’s more difficult to track is whether some plants are disappearing from this area as conditions change.

“Plants have evolved to have some amount of flexibility in their ability to respond to changes in the weather,” Block says. “But what we’re seeing of course is a really rapid acceleration of change that plants are just not evolutionarily programmed to handle.”

Cultivated plants, many of which are not native to the eastern U.S., are of course subject to the same fluctuations, with both benefits and challenges for avid gardeners.

Aiello notes that one popular non-native tree, the saucer magnolia, beloved for its ample pink blooms, is almost always hit hard by capricious spring weather.

“They are notorious for flowering in late March and then inevitably getting hit by a late frost,” he says. “Our native magnolias are much more in tune with the climate and tend to flower much later, starting in late April and early May.”

​​​​​​​​​​​​​​And the quintessential spring bloomer, the cherry tree, was also a victim of this year’s weather.

“Early in March it was predicted that the peak bloom would be in mid-March, a good two or two and a half weeks earlier than average,” Aiello says. “Then we had that really cold weekend, so they all got frosted.”

In a place like Washington, D.C., this has a real economic impact, as tourists may decide to flock, or not, to the Tidal Basin to witness the mass blooms.

Farmers are even more vulnerable to these weather events. New Jersey’s peach crops, for example, can be harmed by late frosts and in some cases completely destroyed. In Florida and California, citrus crops are often lost during cold spells.

For most non-food plants, a late frost may prevent fruit from forming in a given year but will carry few other long-term consequences. In some cases, however, a frost could kill young shoots, harming the overall look of a plant. And Aiello says that he’s witnessed some cases at the Arboretum where a whole plant has been lost if a late hard frost kills the cambium, the layer of living tissue under the bark of a woody plant.

Earlier springs have more subtle effects on plants as well. If a warm spell triggers plants to flower too early, before bees, birds, bats, butterflies and other pollinators are present, it can throw a whole network of ecological relationships off balance.

“Plants flower at a time when they are, if I can be anthropomorphic, ‘hoping’ that pollination can occur,” says Block. “And I think one of the effects of climate change that’s not so well understood is the relationship between plants and their pollinators.”

In addition, milder winters can mean that insects considered plant pests can reproduce throughout the year, causing more damage. The same is true of fungal diseases that can strike plants.

As a result, humans are in some cases stepping in to help plants out where they cannot help themselves.

At the Arboretum, Vince Marrocco, who manages the rose garden, spread giant row covers over many of the rose bushes in an attempt to protect young shoots that had already begun growing in the early warmth. Aiello says if home gardeners want to protect certain tender plants or trees, simply covering them with a row cover, sheet or blanket can be effective.

On a grander scale, conservation managers are beginning to incorporate climate change into their management plants, down to the level of what plant species they may include in a restoration effort.

Roger Latham, who earned his Ph.D. in Penn’s Department of Biology and pursued post-doctoral work in what is now the Department of Earth and Environmental Science, is an environmental consultant. He has already seen the impact of climate change on the natural communities where he works, changes that are in some cases striking in their scale.

​​​​​​​“The most dramatic impacts of climate change that I have personally seen are the massive die-offs of trees in the lower Delaware Bay,” he says.

There, once-forested hummocks of land have had to contend with rising sea levels and increasing exposure to saltwater.

“Now they are inundated several times a year instead of a couple of times a century,” says Latham. “It’s striking because this happened so recently that most of the trees are still standing — forests of ghostly, barkless, leafless, weatherbeaten tree corpses.”

​​​​​​​When restoring land, Latham says he and his colleagues think about what species are likely to stand the tests that climate change throws at them, including warmer temperatures and increasingly vigorous diseases and pests.

“Everywhere we work, we’re trying to build in resilience to a community,” he says. “You want to make sure you can do it in a way that the effects will be lasting through all kind of changes and all kinds of wild swings in weather, because that’s what we think we’re in store for.”