Understanding the land you live on
A Sense of Place: Microclimates in Your Backyard
By Susan Duncan, 5-12-08
 |
|
 | |
|
In the Intermountain West climate varies – by elevation, aspect, within valleys and even within backyards.
In natural landscapes, the varieties of plants (and where they grow) offer clues to microclimates. But man-made landscapes (like wheat fields and blue grass lawns) “mask” the diversity of climate within. The mask leads landowners to assume that the climate on their property is all the same. They discover their mistake when their plantings fail.
Natural features like elevation, aspect, and wind affect local climate, and therefore your backyard is a microclimate.
Elevation. Obviously, high mountain properties are colder, get more snow, and have longer winters. But, warm air rises and cold air sinks. I live near the West Gallatin River. Water flows to the lowest point. Cold air sinks to the river corridors. We get early evening ground fog when the ground is warmer than the air temperature, and sometimes, localized, dense morning fog.
Our place looks flat (only a 1-2% gradient). It is half a mile long (east to west). As I walk the length of the property on warm summer evenings, I can feel noticeably cold drafts in low spots. So, climatically, my property is NOT flat.
Aspect. North facing and east facing slopes are wetter and colder. South facing and west facing slopes are hotter and drier. Much of the Gallatin Valley has a view of the bare west-facing slopes of the Bridger Mountains to the east, and the tree covered north-facing slopes of the Gallatin Range south of Bozeman. Does aspect really make that much difference? You bet!
Perennials (like trees, shrubs, and raspberries) can be harmed by daily temperature fluctuations that induce freezing and thawing in winter. The top six inches of my raspberry canes on the south side of the pump house die back every year. (They need more frequent watering in summer.) Meanwhile, canes on the north and east sides produced most of the 75 cups of berries I harvested last year.
Wind. Wind has mixed effects. It blows away heat radiating from warm soil, but turbulence can protect plants from frost by keeping cold air from sinking. Wind increases evaporation of moisture from soil and plants.
Structures reduce wind speed. Their thermal mass absorbs heat during the day and radiates it back at night. Therefore, densely developed urban landscapes are warmer, and have less dramatic climate variations than rural areas.
Winter winds desiccate and freeze exposed evergreens (wind burn). We had to spray our blue spruce trees with dormant oil each fall until they were well established. Nanking cherries planted on the south side of our garden have never borne fruit in 20 years. Meanwhile, Nanking cherries planted in the windbreak (and sheltered from wind) along the north side of the farmstead have produced fruit.
Prevailing winds reflect the storm track. Our prevailing winds are out of the southwest. We are on the edge of a summer hail belt that runs from Amsterdam to Flathead Pass. Fourth of July hailstorms make lace of my spinach and blow over the corn stalks but so far, have not caused major damage
Man-made structures alter moisture, wind, and temperature regimes even more.
When we moved here in 1986, this property had no trees and no structures. Rain fell and soaked in over the whole property. Winds blew at will from the southwest without impediment. We drew a development plan and moved a mobile home onto a carefully selected portion of 76 acres of flat, fallow fields and pasture. Without realizing it, we had begun to drastically alter the water, wind, and temperature regimes of our property.
Water. Structures and hard surfaces (pavement) are impervious to water. Runoff from roofs is significant in volume and erosive force. We put a gutter on 1/3 of the length of one side of the roof of our 40 by 80 foot barn and directed the runoff into a 150 gallon stock tank. A half inch of rain will fill the tank to overflowing.
Landscaping. Foundation landscaping keeps houses warmer in winter. Evapo-transpiration and shade from trees keeps homes cooler in summer. Our windbreak has altered the climate enough so that we can grow apple trees and Nanking cherries. It also filters dust and noise from the road.
Temperature. Structures create temperature shadows. Forget what the gardening books say about planting by climate zones or in full sun, partial sun, or shade. Get out the thermometers and see what that really means in terms of temperature variation on your property. Draw a map to record your data. Take that information to a nurseryman to help you pick plants to match your habitats.
One winter, before the snows came, I noticed that my bare, brown yard was often covered in hoar frost in the early morning. As the day progressed through mid-morning, noon, to early afternoon, the frost melted off some parts of the yard, but not in others. The frost patterns mapped the hot, warm, and cold places in the yard very specifically.
To follow up on my observations, I set out thermometers around noon on a partly cloudy, twenty-degree day in January. In the hot spot on the south side of the garage (where I grow tomatoes), the temperature read 55 degrees, soaring to over 60 degrees during a brief period of full sun. On the opposite side of the garage, the temperature was a cold, 22 degrees. On the west side of the house, the temperature was 32 degrees, but only 26 degrees on the northwest corner of the house – an 8 degree difference in temperature just by changing the aspect. On the south side of the haystack at ground level the temperature was 42 degrees - on the north side in the shade, 30 degrees. At four to five feet off the ground in a spruce tree, it was 26 degrees. Bird droppings there indicated the sparrows and chickadees preferred being 5 degrees warmer than on the ground.
Try this in your yard before you invest in any more perennials or put in a garden. With a few wall thermometers from the hardware store, you and your kids can become backyard scientists. And, the knowledge you gain about where you live is priceless.
Next time: Garden Layout
Susan Duncan lives on a 76-acre irrigated farm in the Gallatin Valley of Montana that she and her husband Richard built from a fallow grain field since 1976. They raised registered and commercial cattle, sheep, and hay. Now they are niche market entrepreneurs of Dexter cattle and some produce. From 1999-2004 Susan was a country lifestyle columnist for the Bozeman Daily Chronicle “Fencelines” Section. She holds a B.S. Degree in Forestry from the University of Montana. For the last 20 years she has been an active participant in local efforts to envision a viable future and guide exploding development.
Read some of Susan Duncan’s previous columns:
• Sense of Place: Understanding Microclimates in the Gallatin Valley
• Sense of Place: Understanding the Risks of Where You Live
• Can Urban and Rural Develop a Shared Sense of Place?
• Discovering Your Sense of Place
Like this story? Get more! Sign up for our free newsletters.
Comments