The Rain Garden at Knoll

Carex divulsa and Luzula Snowflake, in full spring time flower in the top swale, belong to a group of plants that can tolerate and indeed enjoy the periodically wet and dry regime of a rain garden.

Rain gardens are becoming increasingly popular in our gardens as they provide an attractive aesthetic and diversity of habitat as well as offering a highly practical alternative to the use of water in our gardens and designed spaces. Whats more they can work in virtually any size of garden. With low environmental impact they challenge the more traditional view of water, so often regarded as either as an individual ‘feature’ effectively divorced from the rest of the landscape or, especially in the case of urban stormwater, simply an irritation to be drained or culverted away at the earliest opportunity.

The Rain garden at Knoll consists of four separate but linked lower lying swales that thread their way around the Spring Meadow and Dry Meadow.

In essence rain gardens are planted areas of lower lying ground, swales, dips or depressions, into which water collects during rainfall or flood. The rain garden then acts as a temporary holding area for the water which gradually seeps away into the ground. As with a green roof, a rain garden can be seen as a natural sponge; soaking up excess water in a relatively short space of time to release it more slowly over a longer period.

Created by simply reshaping the existing soil levels, the first of the four lower lying swales that go to make up the Rain garden weaves around some higher ground that when planted will become the Spring Meadow.
Plants in the first Rain Garden swale are encouraged to mingle with the plants at the edge of the Spring Meadow so when in full growth it can be difficult to see the division between the two areas.

Rain gardens are therefore periodically both very wet and very dry, which in horticultural terms may appear as something of a contradiction. But in the natural world such changeable conditions are frequently encountered where water and land coincide and so a range of plants such as carex, juncus, luzula and deschampsia have adapted to take advantage of these sometimes challenging environments.

Acorus Golden Edge planted in the bottom swale seems impervious to the occasional immersion.
Although each individual swale will independently collect from its adjacent surroundings, each swale is also linked by a length of black pipe that allows excess water to flow from one swale to the next, slightly lower, swale.

In our natural systems these plants are generally found in damp and wet areas such as water meadows and stream sides. Such places are frequently wet but as the seasons alter streams and wet areas can become quite the opposite until the moisture returns. To successfully colonise these places some plants have learned to cope with the sometimes wet, sometimes dry environment, and this successful strategy can be used to great effect in our designed spaces and especially when planting rain gardens. 

The third swale runs along the edge of the Dry Meadow and is almost lost to view such is the enthusiastic growth of juncus deschampsia and iris.

Our Rain garden at Knoll consists of four linked swales that run alongside the immediately adjacent Dry meadow. The garden soil is for the most part a very dry sand though with heavy rainfall local flooding can occur and the rain garden was sited in part to help alleviate this problem from over a wider area. With the swales set at a lower level than the surrounding Dry meadow any excess water would flow off the meadow and into the rain garden.

Early in the season, and before the grasses have had time to cover the ground, primroses offer their always welcome flowers along the sides of the top swale.

Each of the four separate but linked swales were planted with a different mix of plants including; Acorus Golden Edge, Carex divulsa, Calamagrostis x varia, Luzula Snowflake, Luzula Marginata, Deschampsia Goldtau, Deschampsia Mill End and Molinia Poul Petersen.

Iris Gerald Darby is a very attractive free flowering iris that will cope with heavier soils and is even happy in shallow water such as in the pond margins of the Water Garden. With this adaptable nature it makes a great plant for rain gardens.

Although the rain garden palette was radically different from that of the Dry meadow, plants from both areas were allowed to mingle at the margins to help merge one area into the other. The winter rainfall soon proved the value of the rain garden. Flooding of nearby pathways was virtually eradicated and while often full the individual swales collectively held the volume of water; releasing it into the ground over a period of time.

Molinias make great rain garden plants, and are especially effective in the autumn when their warm honey brown stems make quite a statement. The upright habit of Molinia Overdam is to the left while the distinctive softer mass of Molinia Poul Petersen is to the right.

Simply by altering the ground levels this project provided a low impact solution to local flooding and created a distinctive growing environment that is as attractive to wildlife as it is to the gardener.