Anyone with even a small working knowledge of gardening is aware of USDA plant hardiness zones. These 13 zones on the map are based on the average lowest winter temperature in that region. In general, these zones run from coldest (lowest number) to warmest (highest number) from north to south. This makes sense doesn’t it? After all– International Falls, Minnesota is obviously going to run much cooler than Key West, Florida. Though this logic is generally the case, there is a little more to it than just the latitude were you live.
In the Twin Cities, zone 4 is the area we tend to focus on when considering what to plant. Sometimes, however, there is an exception which either occurs naturally or is nudged by human intervention that pushes these “boundaries” of hardiness just a little further apart.
We all know that plants need varying amounts of sunlight in which to thrive. That Black-Eyed Susan is going to be a lot happier in a sunnier location than a Hosta would be. It would only make sense that the area with a lot of sun light is going to be warmer than that spot in the shade. Houses, walls, fences, trees, other plants, and even where you park your car, can affect how much sun an area collects. Logically, the south or south-west side of a stone wall is going to collect more sun during the day, in turn amassing more heat and slowly releasing it at night. If this same wall is protected from wind, this little microclimate can actually be as much as 8 to 10 degrees higher than other spots in your yard!
Temperature is affected by much more than just sunlight. Simple science (the kind we learned back in grade school) tells us that hot air rises. This basic principle can be applied to slopes and valleys. The higher the rise, the warmer the temperature. Down in a valley or below a hill, the thermometer is going to register a slightly cooler reading. The cooler air literally slides down the slope like water and settles at the lowest point. High on a south facing slope is going to get you warmer temps. Just watch out for the drying winds!
In an earlier blog I wrote about the benefit of berms. Even a small berm may have its’ own microclimate. Aside from the factors already discussed, these simple modifications of topography may even alter wind patterns by acting as a buffer.
Water is also a big contributor to dictating the temperature in an area. Water vapor in the air from a pond or lake can trap infrared radiation from the earth like a greenhouse. It’s a form of humidity distribution and can be observed on a large scale all over the world along bodies of water. Just look at how many zones exist in Alaska!
Soil is often overlooked. The fact is, soil can play large roll when the other factors are taken into account. Soil in a shady location will not only be cooler, but will also hold moisture longer. Likewise, soil in a sunny location can play with the temperature a great deal. Much like concrete and pavers, clay soil can radiate heat and provide a warm island of air. Soil with air pockets can trap heat and increase the risk of frost. This is especially true in cold dips and valleys.
I’ve touched on wind before, but it needs to be addressed further. When wind hits an object like your house, it creates turbulence and higher wind speeds along the wall and around corners as it passes around the building. Just be mindful of this when considering the introduction of plants that can easily get dried out. Evergreens planted in a windy and sunny location are especially at risk since they cannot replace moisture lost through their needles or leaves when the ground is frozen.
As with any landscape project involving plant material, the design must take into account the different site conditions that are present. The amount of light and shade, the type of soil present, topography and drainage, are all obvious characteristics that need to be addressed. Sometimes there are other elements that need to be observed or implemented in order to successfully sustain certain plants.
I invite you to take a closer look at a plant hardiness zone map. You may be surprised at what you see when you really examine it!