DESIGNING WITH PLANTS IN MIND
5. Air humidity: High relative air humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for broad-leaf evergreens. Consider citrus growers who turn on overhead sprinkling systems in advance of a predicted cold snap (dramatic drop of air temperature) to protect their trees. 6. Soil structure: Soil conditions, such as hardpan,5 and other soil structures can contribute to the success and health of plants. Some plants do not tolerate hardpan soil conditions and may barely thrive or in some cases not survive. Plants have their individual soil preferences. Some species do best in well-drained, sandy, and rocky soils (birch, willow, citrus) while other plants can do well in clay and more fine-grained soils (azaleas, rhododendron, and spruce). Other species perform best in soils that are acidic and contain high concentrations of organic matter. Because of the known soil preference of plants, one can “read” the underlying soil and soil conditions of a landscape by observing the plant types and species. For example, in northern climates, the presence of native birch, alder, and willow is an indicator of well-drained rocky or sandy soil. The presence of native spruce is an indicator of poorly drained organic soils with high moisture content and in some cases the presence of permafrost. While plants may be a reasonable indicator of soil conditions, soil testing is advisable before planting to specify necessary amendments or the need to import supplemental soil. 7. Soil pH and nutrients: Plant species do well under specific soil chemical properties. The pH is a measure of a soil’s acidic or alkaline chemistry reported on a scale of 0–14. The measurement considers the concentration of hydrogen ions in a soil sample. Soil pH is measured on the scale where 7.0 is neutral, 7.1 and above are alkaline, 8.3 is too high a reading for most plants to survive. 6.9 and below are an acidic soil and anything that measures below 4.8 is too acidic for most plants to do well or survive.6 Plants that perform best in acidic soils are referred to as acid-loving plants, for example, azaleas and rhododendrons. Some plants prefer alkaline soils, and with some species a change in the soil pH can modify their flower color (hydrangea, for example). One method of altering the soil pH is by using chemical additives such as lime (to reduce acidity). Lime raises the pH and, depending on the amount added, can reduce the acidity of a soil. Conversely, adding organic material to an alkaline soil can lower its pH. Also, modifying a soil’s pH will reduce the frequency and amount of nutrients (nitrogen, phosphorus, potash and others) added to a soil and thus reduce long-term maintenance costs. Performing a soil test prior to installing the soil will include a recommendation for soil amendments depending on the plants specified.
Plant Selection Based on Climate and Other Ecological Factors Plant species that have been selected considering their range of temperature tolerance (hardiness), their adaptability to existing 149
