Rain Garden

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Demonstration sites at RBGE include green roofs, stormwater planters, living lawns, bioretention ponds, and more. Our flagship project is a stunning , planted in 2019 to solve persistent flooding. The paths by the raingarden no longer flood, the garden is irrigated only by rain, and five years later the plants are still thriving. Our raingarden is a collaboration between RBGE scientists, horticulturists, and staff at Heriot Watt University. This has become a shared outdoor space to facilitate discussion about urban flooding.

A rain garden is a garden designed to collect and use stormwater runoff, intentionally employing the functional traits of plants to solve flooding issues and manage water. Problematic floodwaters are transformed into important irrigation, the plants provide all the benefits of biodiversity and a greener world, and humans have a new and beautiful garden space. The RBGE rain garden is our flagship NBS site, created to solve local flooding in 2019 in partnership with Heriot Watt University, who have monitored the hydrology of the site ever since. The garden is a success! We do not irrigate, plants are thriving, and the area has not flooded since, including during a 150-year storm in July 2021.

What are we learning? How are we adapting?

Soil moisture sensors and leaf wetness sensors have been collecting data in the rain garden for the past 5 years, and that data is used to make planting and maintenance decisions to continuously improve. Two different soil types were originally used in the rain garden: a standard SUDS system high infiltration soil, and a soil mix informed by the expertise of our horticulturists: more organic matter, which at the outset will be slower to infiltrate but give the plants what they need to establish and thrive. We have found that in the higher organic matter soil, the robusticity of the flora allows the plants to play a larger role in water movement: they actively soak up more water, matching in net effect the less organic soil’s infiltration while also providing all the many benefits of plants.

Though designed to handle a 100 year storm, the rain garden successfully absorbed a 150 year storm in 2021 without overflowing. We have learned that the power of plants is underestimated, and that the whole of the connections within the ecosystem are greater than the sum of its parts. PhD student Madeleine Geddes-Barton is currently studying how root system development influences water flow, finding intriguing evidence that plants given the space they need create water access pathways: routes that allow water to move directly from surface to groundwater along the roots, making over time a stronger and more effective living system.

Selected native plants included meadow saxifrage (Saxifraga granulata), devil’s-bit scabious (Succisa pratensis), kidney vetch (Anthyllis vulneraria), meadowsweet (Filipendula ulmaria), alpine blue-sow-thistle (Cicerbita alpina), field scabious (Knautia arvensis) and teasel (Dipsacus fullonum). The non-native plants included Aruncus gombalanus; China, Fischer’s ragwort (Ligularia fischeri; East Asia), crimson columbine (Aquilegia formosa; western North America), Poisson’s primrose (Primula poissonii; China) and Siebold’s plantain lily (Hosta sieboldiana; Japan). All were grown onsite from seed at the RBGE Nursery.

Recent adaptive management changes include the Horticulture and Science teams working together to improve biodiversity value, making changes to the plant species composition, and modifying management techniques to provide winter habitat for insects and wildlife. In 2025, we:

·       Removed some of the potentially invasive Gunnera cryptica (giant rhubarb) from the raingarden, allowing the Darmera peltata (Indian rhubarb / umbrella plant), Dipsacus fullonum (native teasel), and other plants that better hold water within their leaves to fill the space.

·       Modified our management practices to provide winter cover and food sources. We practice ‘chop and drop’ in the back of the rain garden, and we now cut back to a height of 20cm to provide hollow stems for solitary native cavity-nesting bees to overwinter and other insects to shelter within, and we are leaving seedheads standing for both bird forage and winter interest.

·       Added trees and an understory of ferns in place of the former strip of grass at the back of the rain garden. The trees and leaf litter in the upper rain garden provide a first level of filtration, slowing the flow of stormwater and capturing some of the suspended particles.

Key elements that make the rain garden successful are the diversity of plants, the diversity of plant heights, and the upper garden that helps filter sediment runoff before it gets to the amended soil in the bowl of the garden. Learn more about the rain garden and its creation at https://journals.rbge.org.uk/rbgesib/article/view/298/242.

Publications and Further Information

Kelly, D.A. and Kalaichelvam, A., 2026. Evaluation of the hydrological performance of a raingarden designed for stormwater management in Edinburgh, Scotland. Frontiers in Environmental Science, 14, p.1703556. LINK: Frontiers | Evaluation of the hydrological performance of a raingarden designed for stormwater management in Edinburgh, Scotland

Kalaichelvam, A., 2025. Long-term performance assessment and future proofing of a for stormwater management (Doctoral dissertation, Heriot-Watt University). LINK: Long-term performance assessment and future proofing of a raingarden for stormwater management

Hughes, K. and Foulkes, J., 2022. Reducing Environmental Impacts at the Royal Botanic Garden Edinburgh. Sustainability, 14(14), p.8793. LINK: Reducing Environmental Impacts at the Royal Botanic Garden Edinburgh

Kelly, D.A., Wilson, K., Kalaichelvam, A. and Knott, D., 2020. Hydrological and planting design of an experimental raingarden at the Royal Botanic Garden Edinburgh. Sibbaldia: the International Journal of Botanic Garden Horticulture, (19), pp.69-84. LINK: Hydrological and planting design of an experimental raingarden at the Royal Botanic Garden Edinburgh | Sibbaldia: the International Journal of Botanic Garden Horticulture