Colorado is one of the trickier states to landscape in. It encompasses no less than five USDA hardiness Zones -- from a chilly Zone 3 (-40 degrees F) all the way to Zone 7 (0 degrees F) -- and much of the state receives precious little precipitation. Combined with higher altitude in some parts of the state as well as high-pH soils, it can be frustrating for gardeners of any experience. Fortunately Colorado can be divided into a trio of regions to provide some landscaping guidance.
About 40 percent of the state -- the eastern edge -- is considered part of the Great Plains, says Robert Cox, an extension agent in horticulture with Colorado State University Extension. The greater region stretches from Colorado and into Kansas, Iowa, and Oklahoma. The Colorado section happens to be at a higher elevation than the others, and it has huge yearly temperature swings -- from -20 degrees F in the winter to days on end that approach 100 degrees F in the summer. "What's even weirder is that you'll have days in January where it's 65 degrees F and two days later, it's 12 below zero," Cox says. "That makes it difficult for some trees and shrubs."
Eastern Colorado landscapes also receive only 10 to 15 inches of rainfall all year. With precipitation levels on the low end, minerals that would normally wash out of the soil stay put, resulting in an alkaline planting base. Some plants that do well in other parts of North America, such as pin oak and azaleas, don't work well in eastern Colorado. Successful Colorado landscapes tend to rely on drought-tolerant plants, too.
But Colorado landscaping in the eastern part of the state isn't all troublesome: High sunlight intensity and low humidity equal very few disease and insect problems, Cox says.
Most people picture what's called the Front Range when they picture Colorado: the picturesque stretch of topography from Wyoming to New Mexico, comprising the intersection of the Plains and the mountains, which lie to the west. That includes the Denver metro area and Boulder, as well as two-thirds of the population of the state.
Front Range Colorado landscapes get lots of snow and lots of wind, as well as about the same amount of moisture as the eastern part of the state. But winter weather in the Front Range isn't as severe as elsewhere, which "presents difficulty for some plants, which can't figure out if it's winter or spring," Cox says.
That means successful Colorado gardeners stick with tried-and-true plants. "We don't have a huge palette of landscape plants," Cox says. "Our choices are more limited, and so we often become overly dependent on certain species."
Landscaping on the Front Range -- in USDA hardiness Zone 5 -- is also tricky because many people make the mistake of assuming all Zone 5 plants work well. "Alkaline soil is another consideration that people forget to think about," Cox says.
In fact, hardiness Zones are one of the more misunderstood facets of Colorado landscaping, Cox says. "One of the things we have a problem with is that when people find out a plant is native to Colorado, they think it will do great," he says. "But we are a big state with lots of climate zones, and something native to one part of the state won't grow well in all parts of the state. We've oversold the idea that natives are well adapted to every part of the state."
Take the quaking aspen tree with its spectacular color show in autumn: It's iconic to the mountainous parts of Colorado but in the clay soils of Colorado suburbs, it suffers more disease and insect problems. "But everyone who buys a new home in Colorado thinks they want a landscape with a quaking aspen," Cox says.
Across the Continental Divide, the rest of the state is at a higher elevation, with fewer towns and cities, even when the elevation drops. This area offers an extremely dry climate, particularly in winter and summer, with humidity well below 10 percent at times. The low humidity in those Colorado landscapes is hard on broad-leaf evergreens such boxwood, though less so on green ash and, to a certain extent, cottonwoods.
Although the state's so-called Banana Belt -- home to bumper crops of peaches and apples -- is located on the Western Slope, many of the crops owe their success to irrigation, Cox says. In fact, most Colorado landscapes, including trees, need to be watered, particularly when winter has been dry and warm, he says.
If the list of Colorado landscape plants in the rest of the state is limited, it's even more so in the Western Slope. "The soils are not great and the challenges are really big, so people tend to grow a lot of the same thing," Cox says. "We have our own set of landscape plants that have nice appearance, that work here, and are dependable."