A Climate Reckoning Is Already Here — for Gardeners

Thirteen miles isn’t very far: roughly the length of Manhattan or the distance you run in a half marathon. On a freeway, it takes less than 15 minutes to drive.

Multiply 13 by 10, though, and it becomes 130 miles — more than the width of the state of Connecticut. Move the U.S. border 130 miles north, and Whistler Blackcomb becomes an American ski resort; move it south, and Tijuana is the new Los Angeles. If you started walking, it would take you 35 straight hours to cover the distance; if you called an Uber, you’d be looking at a $450 ride.

The temperature regions that determine the local viability of different plants, called plant hardiness zones, are believed to be slipping north at a rate of about 13.3 miles per decade — not a number that sounds especially alarming, but one that will, over a century, add up to dramatically reshape the regional flora of the United States. In addition to being yet another depressing climate statistic, though, that number is also generating a lot of headaches in the surprisingly combustible world of native gardening.

It’s been 16 years (or approximately 21 northward miles) since Douglas Tallamy’s warning in his book Bringing Nature Home that “unless we restore native plants to our suburban ecosystems, the future of biodiversity in the United States is dim.” Though we may still be far from achieving his long-term goal of a “homegrown national park,” in which Americans convert half their yard space to native gardens, Tallamy’s teachings remain hugely influential in gardening and conservation circles (42 states have their own specialized native plant societies promoting these goals).

Tallamy insists that “all plants are not created equal, particularly in their ability to support wildlife.” If we’re to sustain the remaining biodiversity in the U.S., it is essential to feed insects — and in turn, the birds that eat those insects — the foods they’ve evolved to eat. If a plant isn’t native to these ecosystems, then it isn’t worth planting or sustaining. Often, says Tallamy, doing so is actively detrimental to biodiversity goals.

But what even is a native plant in this obviously shifting world? Already, New York City is considered subtropical, capable even of supporting certain hardy palms; by 2040, Seattle could be in the same hardiness zone that central Florida, New Orleans, and parts of Texas are in today. Researchers have seen plants native to the South slowly pushing their ranges north.

Native plants are frequently the species under the most stress from the new weather patterns in their historic ranges. The state tree of Washington, the Western hemlock, for example, is especially susceptible to drought and is struggling to survive in a drier Pacific Northwest. “We’ve found a lot of mortality of trees that should be in the prime in their life,” explained Raymond Larson, an associate director and curator at the University of Washington Botanic Gardens and a contributor to Great Plant Picks, a viability resource for Pacific Northwest gardeners.

As a result, many horticulturalists with an eye on the next century are actively exploring — and recommending — plants that are explicitly not native. Axios Seattle recently published a list of trees that Pete Smith, a program director at the Arbor Day Foundation, believes will be able to tolerate the next 50 to 100 years in the region, and it notably included the Japanese pagoda tree; the pawpaw, a native of the East Coast; and the ginkgo, which is “incredibly tough, very long-lived, and great at tolerating urban stresses” — but an exotic from China that is particularly reviled by Tallamy.

“What honestly most gardeners — many gardeners, anyway — have kind of lost track of is what the word ‘native’ means,” Smith explained to me when I followed up to ask about the globe-spanning range of his recommendations. “It is presumptuous, even, to talk about native plants as if 1492 was some magic date that talks about what is and was native to this continent.”

“Native” doesn’t have a hard and fast definition. In Bringing Nature Home, Tallamy writes that a true native is a plant that interacts “with the community that historically helped shape it,” but he also warns against using too small a timescale when making these determinations: “[A] history measured in centuries is the tiniest drop in the proverbial bucket of evolutionary time.” Native plant purists, Smith added, will argue that “the only quality tree is a tree that was grown from a seed from right underneath the tree that bore that seed. Isn’t that a wonderful ideal? [But] it’s not practical.”

Some native plant proponents have allowed for species that are retreating north (or up) on their own volition since these changes happen slowly and food-chain communities can relocate with them. A number of Southern species in the United States got there in the first place by being pushed down during the last ice age, and have been reclaiming prehistoric ranges as the cold has receded over the last 10,000 years. But ancient forests don’t appear to have migrated as complete ecosystems during these upheavals; it was a race of every-species-for-itself. “There’s a lot more interchangeability among members of an ecosystem than people had thought,” David Jablonski, a paleontologist, told the Smithsonian.

There is also the problem that the climactic zones are moving faster than trees can follow. “The average forest migrates at a rate of roughly 1,640 feet each year,” Wired has written — that is, about three miles in a decade. In order “to outrun climate change,” trees would need to book it north at a rate of “approximately 9,800 to 16,000 feet” a year, or about 10 times as fast. Plenty of foresters aren’t waiting around for that to happen and are seriously exploring the controversial idea of human-assisted migration.

Larson, at the UW Botanic Gardens, meanwhile, said their horticulturalists are looking off-continent for inspiration for the hard years ahead. “We’re experimenting more with plants in Mediterranean climates,” he said, and “also the southern hemisphere: Australia, Chile, New Zealand." Places that have "somewhat similar climates," to the Pacific Northwest, “but tend to get a little bit hotter." And while some of these experiments haven’t panned out as hoped in the past, “we’re going to try them again, because 5 or 10 degrees can make all the difference.”

The conventional wisdom, that introducing or nurturing exotics results in a decline in biodiversity, is also being challenged — often heatedly so. It can seem at times that for every study that expounds on the evils wrought by alien plants, another concludes the exact opposite. The ongoing debate has produced fiery polemics, such as one signed by 19 ecologists and published in Nature in 2011, which announced “it is time … to ditch this preoccupation with the native-alien dichotomy and embrace more dynamic and pragmatic approaches … better suited to our fast-changing planet.” The scientists also swatted down the frequent synonymizing of “nativeness” with “good,” pointing out that “the insect currently suspected to be killing more trees than any other in North America is the native mountain pine beetle.”

(These sorts of back-and-forths are presumably what led former Arnold Arboretum horticulturist Peter Del Tredici, one of the Nature letter’s signatories, to observe, “the use of exotic versus native species … seems to bring out the worst in people, not unlike the debates over gun control and abortion.” Whoever said gardening was boring?)

Arthur Shapiro, a distinguished professor of evolution and ecology at the University of California at Davis, is also among those who have challenged the uncompromising emphasis on the superiority of native plants. “There are many nonnative plants grown in gardens that are immensely useful to butterflies and other pollinators,” Shapiro told me. “And there are many native plants that are completely useless. They might as well be made with rubber or wood.” If you were to uproot every exotic plant in urban California, for instance, you’d “essentially do away with the butterfly fauna.”

That’s partially due to a principle known as ecological fitting, which is “what happens when species with totally disparate histories, that evolved in different parts of the world, come into contact — perhaps as a result of commerce, perhaps as a result of gardening — and they fit together,” said Shapiro. “It’s a marriage made in heaven.” Additionally, oft-vilified “novel ecosystems”, sometimes disparagingly dismissed as “trash ecosystems," arise when exotic species are naturalized due to human influence and/or certain native species recede. Increasingly, though, scientists like Shapiro are viewing these emerging anthropocenic systems as environmental success stories. An unmanaged invasive pine plantation in Puerto Rico, for example, was found to have far more biodiversity than a nearby native-only forest of the same age, Nature recounts; the observation, made in 1979, ran so counter to the established beliefs about the sanctity of native plants that “it took almost a decade" for the resulting paper to pass peer review.

The native/non-native dichotomy is undoubtedly clumsy, so much so that one idea has been to dispense with the unhelpful language altogether. “Neonative,” a term proposed by University of Vienna conservation biologist Franz Essl, for example, could be adapted to describe species that have moved beyond their native ranges and established new foothold populations “due to human-induced changes of the biophysical environment, but not as a result of direct movement by human agency.”

Another idea is to take a step back, put our preconceived notions in check, and learn from what we’re seeing. “As climate changes, communities are going to change, mixtures are going to change,” Shapiro said. “Trying to stop it — except for managing things of economic or medical importance, pests, or disease vectors — is equivalent to trying to plow the sea. It’s futile. So we should actually be paying close attention to what’s happening, because we can learn a lot from it, about how communities self-assemble.”

This isn’t your permission to go plant a bunch of English ivy and scotch broom, though. Two things can potentially both be true: certain native plants have essential ecological functions and some non-native plants can play an important role in shaping future ecosystems. In fact, they’re going to have to, if the climate keeps warming and the hardiness zones continue their upward march.

“We would always tell someone: choose native first,” Smith, of the Arbor Day Foundation, concurred. But at the same time, “Let’s not let the perfect be the enemy of the good.”