Microclimates in your landscape dictate planting material
By Bruce Ross, Master Gardener
Colorado State University Extension, Boulder County,
July 22, 2006
What are the different local weather conditions around your house? A useful exercise is to walk around your yard on a hot summer day and observe the changes in temperature, humidity and wind in different locations along your walk. Conditions feel hot and dry in open areas where the sun is bright. Regions with deep shade under trees are much cooler and may even feel a bit damp compared with the sunny spots. The air close to the ground near a water feature or a recently watered area may feel more humid, particularly if it is in the shade. On a breezy day, unprotected areas will be buffeted by winds.
These features that you observe reflect different small-scale weather conditions that are affected by the many aspects of the local environment, such as sun, exposure to wind, the slope of the ground, shading by nearby trees or buildings and closeness to bodies of water as well as the weather and the season of the year. These local conditions, which may exist in an area as small as a few feet, are known as "microclimates." Since plants in the landscape must live their entire lives within these local conditions, plant health and vigor depends on the microclimate in which it is planted.
The contrasts of microclimates are particularly evident in Colorado where clear skies and high altitude cause the sun to be very intense. New residents who come from humid, maritime regions near sea level are struck by the large change in temperature between sun and shade or when the sun sets in the evening. The differences that they see between sun and shade are largely due to the low water content of the air in this semi-arid environment. The humid air at sea level is not only denser than at 5,000 feet, it also contains much more water vapor that allows it to better maintain a constant temperature. This effect, combined with the intensity of the solar radiation in Colorado, can produce temperature differences of 10 to 40 degrees Fahrenheit between shade and sun. In addition, the strong winds that occur here in the spring can produce some drastic differences in conditions, even in a small yard.
What does this mean for the Colorado gardener? Smart gardeners will think about the different microclimates in their yards when deciding what to plant and where to plant it. Fact sheets 7.831 and 7.832 of the Colorado State University Extension recommend "Right Plant, Right Place," which emphasizes planting the correct plant or tree for the desired landscape location. This recommendation has many aspects to it, including plant size at maturity and soil conditions, but a major consideration for homeowners should be to match the right plant to the right microclimate. Anyone who buys annuals will see recommendations on the plant label concerning whether to plant it in shade, sun or partial sun - each of which relates to one of the conditions for microclimates in your yard. Other recommendations for perennials, such as tolerance to dry conditions or winds, will probably require further research, such as reading relevant Colorado State fact sheets (available on line at www.ext.colostate.edu, or by calling the Colorado State Extension office in your county).
How do gardeners determine the different microclimates in their yards? They could put thermometers and wind gauges around their yard, but a little thought and common sense is probably sufficient to decide what conditions are likely to occur in different locations.
Here are some issues related to this planning process:
Solar heating - Areas on the north and east side of the house will not be exposed to strong afternoon sun in the summer. On the other hand, plants and grass on the southwest side of the house will receive intense afternoon sun in the summer if there are no trees to shade them, particularly next to a light-colored wall that will reflect sunlight. Terrain that slopes down to the south will intensify the solar heating, while terrain that slopes down to the north will lessen the effect. These temperature contrasts remain in the winter, although the sun is lower in the sky and therefore less intense.
Wind - The prevailing winds in Boulder County tend to come from the mountains to the west (although local terrain such as the Foothills may change this direction significantly). Because of this, a homeowner can expect wind effects on plants and trees to be most severe when there are no obstacles to the west to block or reduce the wind. Because the wind speeds up as it passes around buildings, wind effects may be even more severe for plants located on building corners on the west side of structures. The damage to trees and plants due to 50 mile-per-hour winds in Boulder County may be severe if a homeowner chooses a plant too fragile for the windy location.
Moisture loss - Microclimates in areas with strong solar heating and/or higher winds will naturally be drier than areas with shade or wind protection. These areas therefore require plants with lower water demands or larger amounts of irrigation than for plants in more protected areas.
Terrain effects - Cold air collects in low-lying areas of your landscape in the winter. These areas may freeze earlier in the fall or be subject to later frosts in the spring.
All of this demonstrates the great diversity of growing conditions that may be present in the yard of a Colorado homeowner. Gardeners with some understanding of the microclimates in their yards will be able to make more intelligent choices of what to plant and where to plant it. This planning process will go a long way toward producing a healthier garden landscape that is easier to maintain.
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