Extreme Gardening

Pros weigh in on the best plants and gardening practices for our changing climate. 

Extreme Gardening, that’s the name of the reality TV show someone really ought to make about what it’s like to be a northern gardener. We’re already well known for our ability to cope with short growing seasons while making sensible, hardy plant choices and coping with dreadful-sounding issues like frost heave and snow mold. Now, climate trends indicate that we must add excessive heat, humidity, drought and torrential “rain events” to our list of things to think about before putting trowel to dirt. Surely all of that adds up to enough adversity, struggle and tears to make a successful show, right?

Early summer hail storm knocks over family of plastic pigs—top left.

As you no doubt have noticed, our climate is changing. In January, the U.S. National Oceanic and Atmospheric Administration announced that 2012 was the world’s 10^th warmest year since 1880. Closer to home, 2012 was the warmest on record for the United States and the third warmest for Minnesota. But increasing average temperatures are not the only climate trend affecting our region. According to University of Minnesota Climatologist Mark Seeley, the average number of days with a high dew point in also increasing, and we are also experiencing changes in the amount and type of rainfall we get.

Annual precipitation has increased  over the last several decades and is expected to continue to do so. Heavy rain that sometimes leads to flooding is becoming more common. Yet between these events, we are experiencing long periods of drought. Complicating matters further is the rate at which changes are happening, Seeley says. Because it is possible temperatures may rise faster than we, or nature, can adapt.

Extreme Perennials

It is often said that if there is any upside to climate change, it is that northern gardeners may be able to grow more varied plants. But that may not be the case, says Mike Heger, owner of Ambergate Gardens in Chaska. “The media has played into the perception that we can try all of these Zone 5 and 6 plants now,” he says, “but the chances of gardeners getting burned at some point are high because it’s not unusual for us to get a really cold, gut-check winter. Push the Zone if you want to, but know that the risk hasn’t really changed.”

Queen of the prairie can take short-term standing water.

Heger suggests looking at plants that do well a little farther south, say Omaha or St. Louis. “Finding plants that can take extremes on both ends is really kind of a balancing act and we’ll just have to see what works,” he says, adding that some tough plant varieties, such as bishop’s cap (Mitella diphylla) and Bowman’s root (Gillenia trifoliata), may be difficult to find because they aren’t part of commercial plant lines—yet.

Bowman’s root
Credit: North Creek Nurseries

He recommends integrating native plants with exotics that grow in similar conditions. “Natives can be ideal in some situations, but gardeners need to remember that natives are adapted to the conditions they’re in, so before you buy, consider where they grow in the wild and whether your site offers those same things.”

Water Wisely

Mary Meyer, a horticulture professor at the University of Minnesota, agrees that excitement over changes to the USDA Hardiness Zone map should be tempered with the ongoing reality of winter. So while she isn’t filling her garden with Japanese maples and other risky plants, she has long been focused on reducing her water use by growing drought-tolerant plants and conserving water with rain barrels and catchment systems. Meyer recommends grouping plants with similar water needs together. That way, you can water the bed with the queen of the prairie and turtlehead while letting the black-eyed Susan and blazing star fend for themselves.

A few words of caution: Before you enjoy the benefits of drought tolerance, you have to water, water, water because “drought tolerance doesn’t start right when you plant something, you have to take that time to help things get established,” she explains. Meyer suggests following the oft-heard recommendation to give new plants an inch of water each week, more if it’s very hot and dry, for the first year, sometimes two.

If plants are well established, they can often endure long periods of drought if you’re willing to let them whither and go dormant as Meyer does. But even water-wise resolve has been somewhat shaken by recent changes in the weather. “The last two years, from August on, have been a frightening experience because I keep waiting for it to rain and it doesn’t,” Meyer recalls.

Dependable Shrubs

If you’re looking for dependable shrubs that can take extreme weather, your best bet is to go with tried-and-true varieties, says University of Minnesota Extension Educator Kathy Zuzek.  While it’s always tempting to try new things, the lack of adequate testing in today’s fast-paced market means gardeners are often doing their own plant trials at home. “I wish we lived in a slower, more cautious world,” says Zuzek, who specializes in woody landscape plants. “Because right now, a lot of species are not being trialed in different parts of the country before they’re released.”

Amsonia Blue Star (Amsonia x ‘Bue Ice’) is a great shrub that can tolerate drought once established.

Recently, Zuzek has been conducting plant trials on shrubs for the North through the Earth-Kind® Environmental Stewardship Program with results to be released later this year.  

Tough Trees

Forget the cold hardiness zone maps, which focus only on average cold temperatures, what gardeners really need in these changing time are eco-region maps, says Gary Johnson, professor of forestry at the University of Minnesota. The maps, which have been developed for many states and can be found online, delineate areas according to ecosystem and environmental resources. Intended to help with the development of ecosystem management strategies, Ecoregion maps do acknowledge how cold a given place gets. But they also provide information about things like heat, precipitation, drought, wind, vegetation, geology, soils, wildlife and hydrology.

This information provides valuable clues as to which plants will grow well in an area. “If you know the original native vegetation, you’ve got a pretty good idea of what you can grow,” he explains. This is particularly helpful when buying trees, an investment that can last centuries or a few short years, depending on what you buy and how you plant it. “Going into a garden center and buying a tree based on the Zone is like going into a clothing store and asking what they have in size large. ‘Well what in size large?’ ‘Oh, anything.’”

Amur maackia can adapt to dry or moist sites.
Credit: Bachman’s

Even popular trees may not good choices. Sugar maple (Acer saccharum), for example, is overplanted despite needing rich soil that holds moisture well. “It’s not that you shouldn’t buy maples,” he says. “But if you plant one, remember they are woodland trees, so they need shelter and good soil. Don’t put them in a parking lot or on a boulevard.” Red oaks (Quercus rubra) are also a bad choice for most urban sites because they can’t tolerate poor drainage or harsh winds. But bur oak (Quercus macrocarpa) and shingle oak (Quercus imbricaria), which is native into northern Iowa, are highly adaptable and dependable. Swamp white oaks (Quercus bicolor), though, do not do well in alkaline soils.

Some of Johnson’s top picks for adaptability, particularly to hot, dry conditions, are: Amur maackia (Maackia amurensis); Black Hill spruce (Picea glauca var. densata); flowering crabapples such as Johnson’s favorite, ‘Chestnut’, (Malus ‘Chestnut’); Hawthorn (Crataegus spp); honeylocust, in particular Northern ‘Acclaim® (Gleditsia triacanthos “Harve’); hybrid elms (Ulmus) such as Princeton®, Discovery®, ‘Accolade ™ and ‘Valley Forge’); Kentucky coffee tree (Gymnocladus dioicus); Linden (Tilia); and Norway spruce (Picea abies).

Once you’ve planted your tree, the most important thing to do to keep it healthy is water, especially during the first year of growth. The amount you water is more important than frequency, says Johnson, who suggests filling a 5-gallon bucket and slowly pouring the water over the root area of a new tree every three or 4 days from planting time until the ground freezes. Even after they’re established, trees still need to be watered.

“Trees are living systems that depend on water to survive,” Johnson says, adding that he pays no attention to weather reports. He just makes sure he keeps the top 6 to 8 inches of soil under all of his trees moist using hoses that he turns on as needed, usually running on a slow trickle from 6 p.m. to 10 p.m. To check the moisture, he uses a tiling probe purchased at a garden center, but an iron rod will also do the trick. “It’s like checking a cake with a toothpick; if there’s good moisture, the soil will stick to the probe.”

For additional lists of perennials and shrubs that can take drought, wind, heat and standing water, see the full-length article on Extreme Gardening that I wrote recently for Northern Gardener magazine.