Urban Site Challenges
Urban Approved plants must be able to reliably grow in harsh urban conditions. These areas are limited mainly by soil, but also by pollution. Urban soils are best described as heterogeneous and extreme. Within a 10-foot by 10-foot square, the soils can vary from compacted and soggy to sandy and excessively drained. With such variation in only a small space, plants must be able to tolerate a wide range of soil conditions. The following is a list of issues that plants face in an urban environment.
Urban soils are often at the extreme ends of moisture availability. They are either too wet or too dry for many plants. These conditions can also vary seasonally, as a site may be excessively wet in spring but dry for the remainder of the season.
Generally, urban soils tend to be more alkaline than forest soils. This is due to the natural breakdown of concrete, releasing calcium carbonate, which raises the pH of surrounding soil. Soil pH is directly related to nutrient availability, meaning that urban plants must be able to function with reduced resources.
Cities, especially those in the north, often have soils contaminated by runoff and industrial use. In Wisconsin, our winters mean urban plantings get a heavy annual dose of salt and road pollution each winter and spring. Urban plantings must be able to tolerate higher levels of aluminum, petroleum products, and salt.
Vehicles, construction, and even foot traffic are constant pressures on urban soils, which as a result are often compacted. Compacted soils are more difficult for roots to penetrate and hold less moisture and nutrients.
The urban heat island effect, where heat is absorbed by manmade structures during the day and released at night, leads to urban soils having higher than normal temperatures and more extreme temperature fluctuations. This leads to accelerated decomposition, reduced moisture and nutrients, and increased difficulty for roots to establish.
Typically urban soils have less available nutrients for plants. This causes reduced growth and even death in plants that aren’t adapted to poor soils. However, in some cases, urban soils can have too many nutrients, or imbalanced quantities of nitrogen, phosphorus, and potassium. In these soils, flowering plants may only grow vegetatively, or roots can burn.
Urban planting spaces are often restricted by impermeable surfaces. This reduces available nutrients, moisture, and aeration. Urban plants must be able to survive with limited resources compared to plants in undisturbed environments.
Urban Approved plants must also show resistance to (or at least tolerance of) urban air pollution. A major culprit of plant injury in urban areas is oxidants, like ozone, that are the result of vehicle and industrial emissions. In areas with heavy industry or high vehicle traffic, pollution levels can damage the foliage of susceptible plants.
Urban Approved plants can tolerate these conditions within reason. A tree that is drought tolerant should not be sited where the soil is always wet. Likewise, a tree requiring full sun should not be planted on the north side of a 10-story building. While a plant may be Urban Approved, the title does not override a plant’s innate Site and Light requirements for survival.