Gardeners who push the zones – planting U.S.-native plants that should be too tender to survive where the gardeners live – may play a key role in saving plant species from extinction.

With climate change, many plants are likely to need saving.

We are in the midst of the sixth mass extinction in the history of the Earth, Jesse Bellemare, an associate professor of biological sciences at Smith College in Northampton, Massachusetts, reminded the audience in a lecture at Coastal Maine Botanical Gardens late last month . Previous mass extinctions were caused by such things as asteroids and volcanoes. But humans are to blame for this one.

People are creating climate change by cutting down forests around the world, especially in Central and South America, he said, and by burning fossil fuels.

The warming of the earth is moving horticultural zones – the places where specific plants can grow and survive – northward (on this side of the Equator) by about 400 meters a year, Bellemare said, the equivalent of a quarter of a mile. The U.S. Department of Agriculture first issued Plant Hardiness Zone maps in 1960. In 2015, the USDA adjusted its 1990 Plant Hardiness Zone map, but Bellemare’s research leads me to believe it may need more adjusting.

Plants can’t simply follow that quarter-mile-a-year movement, Bellemare said.

Take plants like trillium and bloodroot – seeds are transported by ants. The ants move them only a few feet per year – not even close to keeping up.

Geological history provides no hope, either.

After the last glaciers killed all plants in the northeastern United States, the region was again populated by plants. But it took centuries – they moved north at a rate of about six meters a year, Bellemare said. That translates to 600 meters a century, nowhere near the 400 meters a year that climate change is moving the zones.

Consider the white pine – the official state tree of Maine. If winds or birds moved the seeds 400 meters in a year – possible but not likely – it still would take many years for the white pine seedling that sprouted to get mature enough to produce seeds of its own. So the movement of white pines would still be much less than 400 meters annually.

So, how can gardeners help plants keep apace?

By participating in something called assisted migration. Some botanists are recommending transplanting plant species to save them. The poster child for this practice is Torreya taxifolia, or Florida nutmeg, a tree that is native to the Florida panhandle and is now threatened there. Some people want to move it north to save it. Others oppose the idea because it could result in introducing an invasive species to an area where the plant doesn’t naturally exist.

The debate is continuing – but the debate might not matter.

“Might horticulture already be serving as a route of unplanned assisted migration?” Bellemare asked. And being a research botanist, he and some associates conducted a study to find out. They studied the umbrella magnolia, Magnolia tripetala; its native range is in the Gulf Coast, spreading up to southern Pennsylvania.

The tree has been in the horticultural trade since the 1700s. In fact, it was offered for sale by John and William Bartram – nurserymen whose illustrious clients included Benjamin Franklin, George Washington and Thomas Jefferson. In 1822, a catalog from a company in Long Island, New York, shows the magnolia selling for 50 cents a tree, and it came to New England in mid- 1800s – including to the home of poet Emily Dickinson in Amherst, Massachusetts, where some large specimens still grow.

Pierson Nurseries, Maine’s largest plant wholesaler, does not carry umbrella magnolia, and I couldn’t find it in the catalogs of other local nurseries. But the nurseries probably could get it for you if you asked.

It might seem surprising that such a tree could survive and grow beautifully in western Massachusetts, a long way from its native range and where temperatures can get pretty cold. It can survive, but it can’t reproduce unless the conditions are right.

And the conditions are becoming right.

Bellemare and his assistants searched areas around where the umbrella magnolia had been planted at homes and found several stands of umbrella magnolia with healthy seedlings and a few trees mature enough to produce seeds themselves.

Taking cores from the escaped trees, the researchers found that most of the relatively old ones date from 1989 to 1993, at which point the climate had grown warm enough for seeds from the trees in people’s yards to reproduce naturally.

Bellemare then went to the southern edge of the umbrella magnolia’s original native range, and found that the tree is struggling. Big plants were ailing. Many small plants were sprouting from the dying trees’ roots – much as happens with chestnuts in the Northeast – which will produce a tree, sure, but he found no new trees sprouting from seed, an indication the species was neither thriving nor expanding.

So the entire viable habitat for the tree is moving north.

The question now is whether accidental assisted migration is a good thing.

“We have to ask, ‘Is this plant invasive or weedy, or is it something like a climate refugee?’ ” Bellemare said.

Assisted migration has its potential downsides. The plants that are likely to benefit are the attractive ones, with big flowers and showy foliage, because that is what people often look for when they buy for their home gardens. The less attractive plants – which still may be critical to support wildlife – could be left behind, resulting in an overall low diversity of plants.

According to Bellemare, the possibility of these climate refugee plants becoming invasive is low. He cited the New England Wildflower Society’s survey of plants in New England in 2011, which found 376 U.S. (but not New England) natives that are in our fields and forests. Fewer than 10 of these are listed in the Invasive Plant Atlas of New England as invasive or potentially invasive. Plants from other continents have a much higher chance of becoming invasive than those from North America, Bellemare said.

All of this information means gardeners have more than ever to think about when they choose plants. We’ve been thinking about how the plant will look and if it fits our garden conditions for as long as we’ve gardened. We’ve been thinking about how plants we choose will improve the local ecosystem for a few years.

On top of these, we now have to decide if we want to take part in assisted migration.

ABOUT THE WRITER

TOM ATWELL is a freelance writer gardening in Cape Elizabeth. He can be contacted at: tomatwell@me.com.