Finding plants that can survive our weather extremes can be tricky

Touting a United Nations recommendation for combating climate change, yesterday's front page of the Daily Herald asked, "Could planting 1 trillion trees be the answer?"

If so, we still need to figure out what trees will grow best in the suburbs. In 1990, the U.S. Department of Agriculture's Plant Hardiness Zone map for the United States, ranging from a frigid Zone 1 to a tropical Zone 11, put the suburbs solidly in the middle of Zone 5.

When authorities updated the map in 2012 to reflect rising temperatures, many suburbanites found themselves rezoned into the warmer Zone 6a. This year has given us record rain, heat waves, droughts and record cold. Is the USDA going to update the map again? And what's the deal with all these extremes?

"Your first question is simple," says Jerry Hatfield, the USDA's laboratory director for the National Laboratory for Agriculture and the Environment, in Ames, Iowa, who says there are no plans in the works to update the plant hardiness map. As for the deal with all these extremes, "We don't know," Hatfield admits.

When our old mulberry tree fell after a storm a year ago, we agonized over what trees we'd plant to fill the void. After reading a story headlined "Palm Trees Are Moving North," about how tree species were affected by warming temperatures, I briefly entertained the idea of buying a palm tree. That tree might have sparked conversations in July, but it wouldn't have survived a normal winter, let alone January's polar vortex, says Boyce Tankersley, director of living plant documentation at the Chicago Botanic Garden.

Even sturdier boxwoods, the evergreen shrub popular with suburban landscapers, had a tough time with those below-zero temperatures last winter. Walking his dog around his Grayslake neighborhood, Tankersley passes a yard with boxwoods. "Three of the four boxwoods are dead, and the fourth is pretty and green," he says.

"I planted some boxwoods last summer," Hatfield says of the evergreen rated to grow in Zones 5 through 8. "And over the winter, with those frigid temperatures, we lost a few."

Summers aren't that reliable, either. Our basil pots and annuals marked "full sun" blistered in the heat of July.

"In that period with full sun, we didn't have a lot of rainfall," Hatfield notes. "We talk a lot about drought-tolerant plants, but drought-tolerant plants still need water."

Tankersley says suburban gardeners need to give nature help at times. "In my case, I actually pay some of the neighborhood kids to water our plants when we're on vacation," Tankersley says.

Of course, our May was the wettest in recorded history.

"We're shifting to more spring rainfall and more intense rainfall," Hatfield says. "In our climate across the Midwest, we're expecting to get more extremes. It just adds complications to your life. Those are things we have to live with."

Tankersley pushes the envelope at work and at home. "Horticulturists are constantly testing the boundaries of hardiness. We experiment here at the Garden," Tankersley says, noting they have the freedom to grow a plant from the South, or a tree from the Northwest. "We never give up until we kill something three times."

Some beautiful conifers effectively said, "This is not Seattle and we're out of here," Tankersley says of a planting that didn't appreciate our hot and dry July.

"I live in the zone of climate denial," Tankersley says with a chuckle, admitting his yard features plants from zones with much milder winters. "But I also know I'm going to have to dig them up and put them in my basement for the winter."

Our general trend of rising temperatures "is punctuated with spikes of really hot weather and spikes of really cold weather," Tankersley says.

The Chicago Botanic Garden website at chicagobotanic.org lets you search for plants and ask expert horticulturists about what plants and trees would work best for your yard. Plants from continental climates "are made for extremes," Tankersley says.

But don't expect to find a plant that blooms year-round, thrives in floods and droughts, and withstands 100-degree days with blistering sun and nights where whipping winds make it seem colder than minus 30 degrees. That plant, unlike climate change, doesn't exist.