Rain Gardens

Climate change has resulted in increased urban flooding due to more impermeable surfaces and reduced interception. Consequently, cities have experienced an increase in polluted water runoff and sudden flooding. One solution to this has been the creation of a unique nature-based urban infrastructure, specifically designed rain gardens. These rain gardens are planted bioswales that filter and retain rainwater, slowing its flow into the stormwater system. This study examines rain garden design and what impact this has on their function.?

The rain gardens in this study in Lancaster, PA (USA) were predominantly planted with shrubs or trees. This is usual for this type of urban green infrastructure, where the most common planting scheme is shrubs, tall grasses or herbaceous species. Until now, there has been limited research into the influence of different plant mixes on the rain gardens ability to filter pollution and retain water.

Plant species selection can influence the performance of rain gardens due to a number of factors, including root structure, water needs, pollution tolerance, and purification abilities. In Lancaster, the rain gardens featured a range of different plant groups with herbaceous species most common, and on average the gardens contained 14 different species. The researchers found significant seasonal differences with the summer infiltration rate almost twice that of winter infiltration. In the winter, gardens displayed lower compaction, higher soil water content, and higher soil electrical conductivity.?

Additionally, plant species diversity significantly impacted infiltration. Grass species provide physical filtration of particulate matter and in this study specific species demonstrated good performance. In particular, it found species such as Vernonia baldwinii (Ironweed), Nepeta cataria (Catmint), Itea virginica (Virginia Sweetspire), Rudbeckia sp. (Coneflower), and Eragrostis pectinacea (Purple Lovegrass) were associated with greater infiltration rates. These plant species exhibit varied root and shoot structures, but have shown adaptability to diverse environmental conditions. Overall the researchers concluded that effective rain garden performance is influenced by both soil and plant characteristics, including soil compaction, pollution levels, and planting composition.

Nature-based solutions offer extensive benefits to urban environments. Research such as the above demonstrates how selecting and diversifying plant species can enhance these benefits. It also highlights the potential to improve green infrastructure performance by understanding the soil and plant characteristics.

Written by Dr Francesca Boyd , AIPH Green City Consultant.

Review of the article: Do plants matter?: Determining what drives variation in urban rain garden performance

Dudrick, R., Hoffman, M., Antoine, J., Austin, K., Bedoya, L., Clark, S., … & Gotsch, S. G. (2024). Do plants matter?: Determining what drives variation in urban rain garden performance. Ecological Engineering, 201, 107208. https://www.sciencedirect.com/science/article/abs/pii/S0925857424000326

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