Photography by Mike Miller
Missouri Botanical Garden
“We call this the place where plants go to die,” says Daria McKelvey, the supervisor at the Kemper Center for Home Gardening at the Missouri Botanical Garden. She stops to help a grasshopper cross the sidewalk, scooping it into a nearby bush, on her way to the Climate Garden. “Primarily because we are planting outside of our hardiness zone.”
Plant hardiness zones are standards set by the U.S. Department of Agriculture and used by gardeners and growers to determine where different plant species will thrive. The index ranges from 1 to 13 with colder zones closer to 1, and hot-arid zones closer to 13. The zones are shifting. “Twenty or 30 years ago, St. Louis was probably about a 5, and crape myrtles did not do well here. Winters were too cold,” McKelvey says. “Now they do just fine.” There’s enough heat-absorbing concrete in the city to consider it 7 surrounded by zone 6.
As St. Louis warms, historically healthy plant species will become weaker and smaller. Their ideal hardiness zone will migrate north, and local specimens will die out, removing seeds, berries, nuts, and vegetation from the food chain. Other plants will become more robust, but as zones shift, ecosystems can thin out, simplify, and become vulnerable to collapse. To prevent a breakdown in local ecology, it’s useful to know which plants from warmer southern climates will thrive in St. Louis, which places in the state could become climate refugia, and how we can preserve flora in the worst-case scenario.
In the midst of climate change, the Kemper Center’s Climate Garden is both a laboratory for experimentation and a canary in the coal mine. In the shadow of a weather station, which constantly measures the air’s temperature and moisture, a quarter acre of land is used for study. The question McKelvey is trying to answer is: “Are we able to grow some of those plants that, maybe years ago, were not so hardy and now are sticking around?”
McKelvey holds a bachelor’s degree in biology from the University of Texas at Austin, as well as a master’s in horticulture from Texas Tech University. She’s accustomed to the southern hardiness zones ranging from 7–9, which are headed St. Louis’ way. “This orange jessamine,” McKelvey says, “is a zone 7, it’s going on its third year, and it’s doing just fine.” Colocasia esculenta, or “Pink China” is another zone 8 plant that’s in its third year. And then there’s a yellow version of a red yucca plant that’s native to McKelvey’s home state of Texas. “Those perform beautifully,” she says. “They’re drought tolerant, they can tolerate the full-sun heat.” Plus, yucca attract hummingbirds, as do many of the species in the Climate Garden. The garden beds buzz with honeybees, carpenter bees, bumblebees of all different sizes, and Seraphin wasps.
It all seems peaceful and vibrant (aside from several dead experiments). However, this laboratory could also birth Frankenstein’s monster. “This is a cuphea,” McKelvey says, showing off a vibrant patch of green twisting stems with purple blooms. “It’s called ‘Sriracha violet,’ and it seeds like crazy. It’s a zone 9, and we planted it back in 2019, and it just came back. We found it randomly in other beds.”
On the plus side, that means cuphea could fill in a big area. On the downside, it could be invasive.
Invasive plants grow quickly and spread to the point of disrupting plant ecosystems. “There’s a lot we don’t know when it comes to climate change,” McKelvey says. “What we are concerned about is that diseases that may have been kept at bay by colder temperatures could potentially move up here. And it’s not just diseases, it’s invasive plants, invasive pests…”
The U.S. Department of Agriculture outlines a murderer’s row of invasive pests and plagues not yet in Missouri, including the Asian Gypsy Moth that attacked a wide variety of North American trees and shrubs, or Sudden Oak Death, which is what it sounds like. It’s quarantined to California and Oregon for now.
Invasive pests and diseases can wipe out crops, decimate forests, and stress ecosystems to the point of breakdown. St. Louis already has enough plant plagues and monster invaders, such as the loathsome Bradford Pear Tree, which seeds widely and quickly, overtaking acres and acres. Or the dreaded honeysuckle bush, which displaces native plants and their fruits, which offer better nutrition to birds and wildlife.
Urban forests are becoming a worrying topic of discussion as well. In a recent study, the Chicago Botanic Garden surveyed how 50 different species of urban tree will fare against a warming climate in three urban settings: sidewalk plantings, parks and residential settings, and public gardens. The study then projects out into 2050 and 2080 and found that only 40 species would continue to thrive under “worst-case warming scenarios.”
For 10 species, including the American Linden and the Shagbark Hickory—both common across Missouri—2080 in Chicago will be too hot and too problematic for them to survive.
Trees decarbonize the air, provide shade for people and plantlife, shelter for wildlife, food in the form of nuts, fruit, and sap—but they’re also often home to hundreds of thousands of insects. Those insects, and their larvae, are, in turn, food sources for everything up the chain. Trees are also famously not in a hurry, and operate as organisms on long timescales. Ten years is nothing much to a tree; lifetimes can be 50 or 2,000 years long. If one fifth of a species of tree dies off in an area over the course of 50 years, that’s a fast and drastic change, and a staggering reduction in biodiversity.
Dryer conditions and inconsistency of rain like St. Louis saw in July 2022, when roughly 25 percent of the city’s normal yearly rainfall fell in about 12 hours, is also bad for our photosynthesizing friends. “Sure, we can grow a few more species than we could before, but everything is going to be out of sync,” McKelvey says. Flowers and pollinators have a delicate dance to perform every year, most of which goes unnoticed by humans. “If we’re warming up,” McKevely says, “things might start popping out earlier than when the insects start coming up. And that means the plant can’t set as much seed.” Mismatched pollinator times lead to weaker plants, dying species, weaker insects, and flat and fragile ecosystems. “It just spirals,” McKelvey says. “That’s why we can try to change our ways of gardening. Healthy plants, lower pesticides. But it’s going to take all of us.”
Better Than We Feared, Worse Than We Hoped
“There is some hope that places in Missouri could become climate refugia,” says Adam Smith, associate scientist at the Missouri Botanical Garden’s Center for Conservation and Sustainable Development.
A refuge from the last era of climate change already exists in southern Missouri, in the Jacks Fork bluffs. Plants like harebell are “glacial relicts,” species that are normally found north of here. Harebell was common in the Mississippi River Valley 12,000 years ago, but today it only exists in a microclimate, on the steep and north-facing cliffs alongside more than 450 native plant species, stranded on an island of cool temperatures. “These plants found cool enough areas to stick around as the last ice melted,” Smith says.
Missouri is also home to huge natural water springs, which produce pockets of cool ether, like natural air-conditioning. “I suspect that in the future, those spots will become refugia,” Smith says.
Smith studies climate change using mathematical models to understand how plants will respond to shifting zones. A few times, he’s shoved away from his desk in horror.
“I was doing some mapping for glades,” Smith remembers. Glades are rare enough for Missouri, but, “I did some mapping, and that climate completely disappeared. I zoomed out, and in that climate, these certain species had moved all the way up to Canada.” Smith had been mapping gladecress, which is found in Tennessee, with some here in Missouri. “I know things are going to move,” Smith said, “but it’s one of those horrifying moments.”
Another mappable change is big bluestem, “a grass that’s very common in the prairies around here,” Smith says. Bluestem goes all the way out to eastern Colorado, but in Denver, it’s only about knee-high. By 2050, Missouri’s tall variant will be more common up north, around the Great Lakes. In Missouri, it’ll shrink to be no taller than a boot. That difference has ramifications for the wildlife and insect population that relies on bluestem for cover, food, homes, and hunting grounds.
Some mapping suggests that by the 2050s, St. Louis could have the climate of Eastern Texas, which is terrible news for many of our flora and fauna. But, with more worldwide decarbonization, the local projection could shift north, to southern Oklahoma or northern Arkansas—not great, but preferable.
“It’s not the end of the world,” Smith says. “It’ll be like that saying: Things will be better than we feared, but worse than we hoped.”
Seed Banking on the Future
“One of our big roles in building collections is to tell the story of plants,” says Andrew Wyatt, senior vice president of Horticulture & Living Collections at the Missouri Botanical Garden.
A great success story is karomia gigas of Tanzania. It’s one of the rarest trees on the planet, but in 2018, it sprouted seedlings in a MOBOT greenhouse. Those specimens now represent nearly half of the species’ entire known population. “We’re actively targeting plants with conservation concerns and bringing them into the garden, saving them from extinction,” Wyatt says.
This is called exigency conservation. MOBOT stores seeds in a bank, does propagation in the nursery, and cultivates plants outside, in the gardens. Taking endangered species into the collection is sometimes called an “assisted migration.” “That’s a really key role,” Wyatt says. “Bringing [field species] into the gardens and seed banking them so we can see what’s going to happen over time, and then putting them back into environments that make sense. That’s the great thing about seed banking—you’re holding things temporarily.”
Like the seed bank, the living collection is a place of refuge for plants in danger. Their zone has shifted quickly, and they need a little help getting back on their feet. Later, they’ll be reintroduced in a suitable climate, but some will lose their home forever. Cold, top-of-mountain alpine habitats will become too warm. Things above water now will be underwater tomorrow.
No one knows if the Earth will see 2 or 5 degrees of warming. The long-term hope is that once the Earth’s climate stabilizes, seed banks like the one at MOBOT can help repopulate areas with diverse plant life. Instead of thousands of plant species going completely extinct, seed banks preserve a diverse collection. That way, biodiversity doesn’t take millions of years to develop—it only takes a handful of seeds from the vault to diversify a prairie.
It’s a big dilemma deciding what to save and bring back to St. Louis, and it’s difficult to imagine what that St. Louis will look like as the evergreens disappear, and gargantuan Pin Oak trees migrate north—it all depends on decarbonizing, and it’s not too late.
