THINKING ABOUT LANDSCAPE ARCHITECTURE
ranked for warmer climates may perform well in a colder zone when planted in a protected area between buildings where a heat island has formed. Likewise, a plant may not do as well as expected by its hardiness rating if placed on the north side of a tall building. The buildings in this case form a shadow, blocking heat gain from the sun, resulting in longer-lasting colder air temperatures.
Other Factors Affecting Plant Growth and Survival In addition to hardiness zones, there are other environmental factors that can contribute to the success or failure of a plant’s performance and survival. Wind, soil type, soil moisture, humidity, air and water pollution, snow and ice, and the sun angle can greatly affect the survival and performance of plants. Where plants are placed in the landscape in relation to buildings and hard surfaces, how they are planted (such as soil preparation), and their size and health at the time of planting can also influence their survival. Briefly, here are some of the other factors that can contribute to successful plant performance and survival: 1. Temperature: Plants grow best within a range of optimum temperatures, both cold and hot. That range may be wide for some species but narrow for others. 2. Duration of exposure to cold: Many plants can survive short periods of exposure to cold but may not tolerate longer periods of cold temperature. Also consider that some cold regions can be extremely dry (low humidity), such as in Alaska, or cold and wet (high humidity), such as in the Northeast US or the United Kingdom. 3. Sunlight: Plants need to be planted in locations where they will receive the proper amount of sunlight. Plants can be damaged by sunburn. Low angle winter sun can cause sunburn on the trunks of trees. The burns or sun scalding not only can damage the tree but also can compromise the plant’s ability to survive. Precautions can be taken such as wrapping or painting the exposed trunk area with sun-protection material. 4. Soil moisture: The amount of water present in the soil is called soil moisture. The water is held within the soil pores and its presence is a major factor in plant growth. If the moisture content of a soil is optimum, the plants can readily absorb water held in the soil. The water dissolves various chemical nutrients in the form of salts that make up the soil solution. Soil containing the optimum range of moisture is important as a medium for supplying the nutrients necessary for growing plants and supporting the process of photosynthesis. Plants have different soil moisture requirements. Soil moisture requirements can vary seasonally. Plants that might otherwise be hardy in one zone might be injured if soil moisture is too low in late autumn and then they enter dormancy while suffering moisture stress. Microorganisms present in the soil require water to carry out their metabolic activities and how well they are able to function can affect plant growth and health. 148
