The veggies are planted and flower baskets are blooming, perhaps now you’re thinking of planting perennials in the yard. But what can you plant that stands a good chance of thriving year after year? There are two concepts you’ll need to consider in selecting your plants: zones and microclimates.
Zones refers to the guidelines established by the USDA, which gives a rating to areas based on average winter temperatures over 30 years. Each zone has a 10 degree Fahrenheit range, and zones are divided into smaller subcategories of 5 degree ranges. For instance, Lander is positioned straddling zone 4b (-25 to -20F) and 5b (-20 to -15). You’ll want to consult the official USDA map to determine the zone for your land. When you browse perennials, be sure to read the tag to find information about zone hardiness.
Determining the zone is a great first step, but it is just the first step. Zones do not take into account summer high temperature averages, the amount of annual moisture, if that moisture is rain or snow, or comes regularly or seasonally, or prevalence and severity of wind—all of these factors also contribute to whether or not a plant will thrive. My birthplace of Iowa City (GO HAWKEYES!) is Zone 5b, just a little more temperate than Lander. However, there are key differences, such as length of frost-free growing season and that June, July, and August bring more moisture than Lander receives all year. Iowa City averages 37 inches of rain and 28 inches of snow annually- no wonder oak and weeping willows do well in the average backyard. The Matanuska-Susitna area north of Anchorage, Alaska is also 4b, but the 22-24 hours of sunlight yield magnificent crops. I doubt anyone in Wyoming could grow an 89.6 pound cabbage like the one I saw at the Alaska State Fair.
As for micro-climates, they come in large and small scale. Large scale includes significant bodies of water like Boysen Reservoir, or valleys ringed with hills like Jackson and it’s famous inversions. River drainages are a highway for air movement, as anyone who lives along the Upper North Fork can tell you. They have cold mountain air frosting their gardens through June and even into July. Small scale could be the area by your downspout in the shade where mosses and ferns grow, or it could be the “hell strip” of scorched turf between the road and the sidewalk, both of which radiate heat long after the sun has gone down.
What’s a gardener to do? First, keep a sense of humor and adventure. Second, think about ways you can manipulate the environment to either enhance or mitigate micro-climates:
Season extenders like row covers/ walls of water, or raised beds warm the soil faster than it would warm on its own.
Mulches do double-duty in keeping roots cooler and helping hold moisture longer
Structures can be used strategically to buffer wind and block hot summer sun. This could be tool sheds, buildings, or even solid fences. Structures that retain heat, such as a concrete foundation, might provide an advantageous location for heat loving plants.
Paying attention to aspect makes a difference as spring comes more quickly to south facing aspects than it does to north facing
Topography- even slight- can affect plants. The north side of Lander is slightly lower in elevation than the south side, which allows cooler air to flow downhill across town. Gardeners on the north side often report frost when south side gardens received none. Hot air rises and cool air sinks, and pockets of high/low ground can be quite different despite the proximity.
Putting it all together is a creative process, and it can yield great rewards. My hibiscus tree can tolerate light frost. It’s in a pot that can be moved outside in late spring, but it’s on my elevated and covered deck. If a night threatens to be a hard frost, it can easily be pulled inside. Otherwise, it’s a tropical plant that lives outside for several months. Irises are one of my favorite flowers, so I planted a bunch of them in 4 different micro-climates for an extended blooming season. The first to blossom were those planted close to the concrete foundation with a southern aspect. As they were fading, the irises facing the west with a tall fence behind them to the east began to bloom. Just as those were about done, the ones across the yard facing east with a fence behind them to the west kicked into gear. Finally, several weeks after the first irises started blooming, the north facing clumps offered their show.
So far I’ve had terrible results with asparagus, probably because the plants weren’t getting moisture in March and April. Now that I’ve got gutters installed, I think that the next place to try them will be near the downspout- spring snowmelt on the roof might give them the water they need. It’s worth a try, eh?
In the real world, we garden in microclimates, not hardiness zones.
- Charlie Mazza, Senior Extension Associate, Cornell University.