Rain gardens, public gardens to collect rainwater

Rain gardens are green areas designed to collect and absorb rainwater , especially from roofs, streets, parking lots, or sidewalks. In practice, they are a sort of small natural drainage system , very useful not only for saving rainwater but also for preventing flooding, even in the case of severe storms.

How do they work

When it rains:

1. the water is conveyed towards the rain Garden;
2. a slight depression in the ground collects it;
3. plants, soil and drainage layers they retain and filter water;
4. The water gradually penetrates the soil, instead of all immediately flowing into the sewer system.

What do they contain?

A rain garden generally contains:

• plants resistant to both humidity and dry periods;
• very permeable soil;
• possibly layers of sand and gravel to promote drainage;
• a water inlet area and, in larger projects, a possible too full to manage very heavy rains.

It is therefore not simply a flowerbed or a mini-urban forest: it is specifically designed to manage water.

The most used plants

They are also the most suitable for the function of rainwater harvesters:

• Iris palustris / Iris pseudacorus – irises for humid areas, very suitable for the wettest areas.
• Carex spp. – sedges; excellent for stabilizing the soil and widely used in rain Garden.
• Juncus spp. – rushes, suitable for areas that remain humid for longer periods.
• A bottle of willow – summer flowering and good tolerance to wet soils.
• Echinacea purpurea – suitable for the driest areas of the rain Garden and very interesting for pollinators.
• Rudbeckia spp. – showy flowering and good resistance once established.
• Asclepias spp. – especially interesting for biodiversity and pollinators.
• panicum virgatum and other ornamental grasses – useful for structure, roots and soil stabilization.

Rain gardens can help:

• reduce the surface runoff and the risk of flooding;
• reduce the amount of water entering the sewer system during storms;
• filter some pollutants present in runoff water;
• promote groundwater recharge;
• create habitats for insects and other organisms;
• increase greenery and improve the urban microclimate.

Rain gardens are particularly widespread in countries where sustainable stormwater management has become part of urban planning.

•  United States: are probably the context in which the rain Garden It is more widespread. They are used both in private gardens and in parks, streets, schools and neighborhoods. The diffusion is particularly significant in Pacific Northwest, in the region of Great lakes and along the east coast.
• Glen: widely used in projects green infrastructure, especially in urban areas.
•  UK: are often integrated into systems Sustainable Drainage Systems (SuDS), along with bioswales, permeable pavements and retention basins.
• Australia: here they are very interesting because they are adapted to very dry climatic conditions; they are often called rain gardens or inserted into the systems of water-sensitive urban design.

In Europe the country where they are most widespread rain gardens It's Denmark. In Copenhagen, the HOFOR, the company that manages water and sewerage in the area of ​​the Danish capital, declared in July 2024 that it had reached nearly 400 rain gardens (“regnbede”) in Copenhagen, in the context of the transformation of some roads into green roadsThe most important one is located at  Enghaveparken, a Copenhagen park transformed into an infrastructure for managing heavy rainfall. The park can handle/store up to approximately 22.600 m³ of rainwater through a combination of reservoirs, underground structures, levees and water regulation systems, equivalent to 10 Olympic-sized swimming pools filled with rainwater.