Extension Communications 3614 Administrative Services Building Ames, Iowa 50011-3614 (515) 294-9915

9/1/03

Contacts:
Linda Naeve, Reiman Gardens, (515) 294-8946, lnaeve@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning Sept. 5, 2003

Consider Microclimates When Matching Plants to the Site

Linda Naeve
Extension Coordinator
Reiman Gardens

It won't be long until planting season. Fall planting season, that is. Many herbaceous perennials and trees do well when planted in early fall. The warm soil and cool air during the eight to 12 weeks before the ground freezes enables many landscape plants to become well-established and develop new roots.

As gardeners look for the best plants for their landscapes, they need to first determine whether the plants are hardy. The USDA Hardiness Zone Map provides the best assistance. The zones are determined by the average minimum temperature in each location. The map divides Iowa in half, with northern Iowa in Zone 4 and southern Iowa in Zone 5. Consult the labels on perennials to determine if it is suitable to your zone.

Recently, the American Horticulture Society developed a Heat Zone Map that divides the country into 12 zones and indicates the average number of days each year that a given region experiences "heat days" -- temperatures over 86 degrees. This is the temperature at which plants begin suffering physiological damage. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). This information is valuable for gardeners in regions where high temperatures are common for several months each year. In the future, this zone number may be listed on plant labels next to the USDA Hardiness Zone.

These two indexes should be used as guidelines for selecting plants. However, in a home landscape, there are also many small areas with variable climates, known as microclimates. A microclimate is an area that differs in climate from the areas immediately surrounding it. For example, it may be warmer or colder, wetter or drier, or more or less prone to frost.

Urban areas tend to have many microclimates. Man-made barriers, such as buildings, walls, fences, windbreaks, paved surfaces and terrain changes create several different environments. Buildings and paved surfaces absorb heat during the day and radiate it back into the air at night, moderating low temperatures during the winter. As a result, an urban environment may be a zone warmer than a rural area just a few miles away.

The warming effect of an urban environment also needs to be considered in the summer. Urban microclimates can trap heat, creating a scorching environment that can damage plants.

Buildings, like your house, create microclimates. At least two different climatic conditions are found around a building, one on the north and one on the south. The north side of the house is noticeably cooler and moister. The sunny south side will be warmer and drier. Each side supports a distinctly different group of plants. Plants selected for the south side of a house or wall should be able to tolerate full sun and a potentially hot, dry environment. Perennials on the south side will emerge earlier in the spring due to the warming effect. Thus, it's important to select plants that won't be damaged by a late snowfall or unseasonable low temperatures.

Topography also has an effect on microclimates. Cold air is heavier than warm air. When frost threatens, the cold air flows downhill and settles in the lowest points, creating "frost pockets" where frost forms first. Some low areas may be 10 degrees or more colder than neighboring slopes. Keep this in mind when selecting a site for a perennial garden on a hilly lot or undulating terrain. Plants that produce cold-sensitive, early spring flowers should not be grown in the lowest area.

By carefully matching plants with their specific environment and microclimate, gardeners can successfully grow species that are considered only marginally hardy for their area.

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Editors: Three color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 448K; the middle picture's fullsize photo is 552K; and the bottom picture's fullsize photo is 464K. Caption: Outdoor Room
Caption: Walled garden 1
Caption: Walled garden 2

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