Growing to the extremes

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September 24, 2012 // UPDATED 5:48 pm - December 27, 2012
By: Dylan Thomas
Dylan Thomas

The Department of Agriculture released the first update to its plant hardiness zone map in two decades this winter, confirming what many area gardeners already suspected: Minnesota winters, on average, just aren’t as cold as they used to be.

The little island of Zone 5a floating south of downtown Minneapolis was welcomed in these parts, even if its arrival didn’t surprise adventurous green thumbs already tending Japanese maples or Lenten roses. But there’s more to Minnesota’s climate story. 

Future chapters are likely to include more weather extremes, including drought. And milder weather is proving more hospitable to both native and non-native pests.

As Tangletown Gardens co-owner Scott Endres put it: “It’s not all a bed of roses with the climate changing.”

The University of Minnesota’s Jeffrey Corney isn’t a climate scientist, but as managing director of the Cedar Creek Ecosystem Science reserve in East Bethel, he keeps close tabs on how native plants respond to climate change.

“All climate model indicators, both global and regional, indicate a warmer Minnesota 100 years from now — potentially as much as 6 degrees Fahrenheit warmer, on average,” Corney said.

Climate projections indicate annual precipitation could increase up to 10 percent over the same period, but that’s not necessarily good news.

Extreme high temperatures boost evaporation and transpiration, sucking moisture from soil and plants. And the additional precipitation expected to fall on Minnesota may come more in the winter and spring, not during the summer growing season went the landscape needs it most. The perplexing result: more rain, but dryer soil.

Corney said the predicted changes were “not a catastrophe by any means.” But for homeowners, it means maintaining a lush, green landscape could take more careful planning and maintenance than ever.

Zone envy

Endres said Minnesota’s slide into a slightly warmer plant hardiness zone produced real excitement this winter, adding, “I think that’s a good thing.”

“We always try to push the envelope a little bit, because as plant people and plant geeks, we always have our fair share of zone envy,” he said.

But horticulturalist Peter Moe of the Minnesota Landscape Arboretum advocated caution in testing warmer-climate plants. It’s still risky to grow plants better suited to conditions in zone 5 and up, including most Japanese maples, flowering dogwood, Stewartia and evergreen magnolia, Moe said.

“Plants like that have never been proven to survive in Minnesota on an ongoing basis, because one of our cold winters will normally damage them,” he said.

Endres said winter damage is always a danger for Zone 5 plants in Minnesota. But by selecting species with a strong track record and planting borderline hardy varieties in protected areas (usually an eastern exposure, out of the wind and, counterintuitively, the southern sun) local gardeners have succeeded with Russian sage, Karl Forester feather reed grass and the super-tough Emporer I Japanese maple.

Calculating 

the variables

Mary Meyer, a University of Minnesota horticulture professor, likes to push the boundaries, too. Most lavender varieties wither in Zone 4, but Meyer grows a patch in sandy, well-drained soil on her property in Plymouth.

A former interim director of the landscape arboretum, what strikes Meyer about the state’s changing climate is the increase in weather variability, both over time and geography — even the 20-mile drive between her home and the arboretum.

“The variability seems to be more normal now, and that makes it much more challenging to garden,” she said. “… We need tougher plants than ever, it seems.”

Meyer said native plant species are often good choices, and in Minnesota, at the confluence of three major biomes — the western prairie, northern coniferous forest and deciduous Big Woods of the east — we have many to choose from.

Endres backed that advice, saying his own yard and garden contain both native cultivars and non-native species. He also offered a few other garden tips for maintaining home landscapes in a shifting climate:

To protect lawns from extreme heat and drought, raise the mower deck as high as it goes before mowing. Longer blades of grass have more surface area for photosynthesis, and they also shade roots and soil, limiting evaporation and transpiration.

Invest in a rain gauge, so you know when the garden is thirsty and when to lay off.

Keep a calendar as a reminder to complete landscaping tasks. Mid-August to mid-September is the best time to fertilize a lawn, for instance, and mulching before the end of November is also important.

Mulching helps plants survive the winter by insulating the soil against temperature fluctuations. In a winter with little snow, like last year, it could make all the difference for your plants.

Finally, Endres encouraged gardeners to embrace the uncertainty.

“There’s nothing for-sure in gardening,” he said. “I like that part of it, personally. I think it’s part of the reward.”