Rain Garden! Even the name is evocative and exciting.  But the concept is even more so: the use of garden and landscape features that collect, clean, store and slowly infiltrate rainwater runoff back into the ground following severe rain-storms. It’s an idea that is absolutely of the moment as climate-change becomes an ever-present reality, and elevates gardens from the purely decorative to something that can make a genuine contribution to reducing the seriousness of flash-flooding.  The best thing from a horticultural and ecological viewpoint is that it is the combination of soil and vegetation that enables their effective functioning.

The reason I became interested in the rain garden idea many years ago, is that I could see the potential for using these and other small- to medium-scale garden and landscape features as a means for introducing beautiful and biodiverse plantings, often into places that previously might have been grim and grey.  And yet, it’s ironic that rain gardens have still to be taken up in a big way in the UK in private gardens – although their origins are definitely rooted in domestic and residential contexts in the US, in the UK their application is largely in the public domain (where we use the dispiriting and deeply depressing term: ‘Sustainable Drainage Schemes’ or SuDS).

I’d written the book on Rain Gardens (Timber Press, 2007), and had exhibited a number of Chelsea Flower gardens between 2011 and 2013 that were based on the whole rain garden idea.  So four years ago, when I moved to a new house, a decided to put my money where my mouth is, and convert the front garden into a demonstration of rain garden ideas.  But I wanted this to be a different type of rain garden: do a google image search on ‘rain gardens’ and most of the results will show irregular, informal, amoeboid shapes, often surrounded by lawn (it’s a strange tendency that because a garden or landscape feature has an ecological function, it almost automatically seems to lead to informal, often rustic, design approaches).  Instead, I wanted to take a more formal approach to indicate that these ideas will work in all sorts of settings.  Above all I wanted to investigate the potential of dynamic, long-season plantings as the basis for rain garden design.

Originally the garden was sloping away from the house, and with no central path from the front gate.  Over a very wet winter of 2014 I terraced the garden, using dry stone retaining walls, and put in a straight path, on a diagonal, from the front gate.  The photo above shows the remodelled garden prior to planting.

The diagram above shows the original plan for the front garden.  The central path is flanked on both sides by linear ‘bioswales’ (fancy term for what is basically a ditch!).  As well as taking runoff from the path (which is ‘permeable’ in that it is not set into mortar or pointed between the pavers, thereby allowing water to infiltrate through); the main purpose of the swales is to absorb runoff directly from the roof of the house.

The photos below shows the preliminary shaping of the linear swales along the main path, showing their form as slight depressions along each side of the path.  The available space meant that these had to be relatively shallow in depth.  On the left side, a path leads onto the lawn, and a pipe underneath the path allows water flow beneath.  On the right the water from the house roof is fed first into a pond, and then this has an overflow point into a ‘rain garden’ (a sunken saucer-like area that can puddle up with water following a storm), which itself can then overflow into the swale.

Disconnect your Downpipes!

Disconnect your downpipes is the mantra of the rain garden movement!  Instead of channelling water from the roof in the main drainage system, that water is diverted into the landscape.  The theory is that if sufficient sources of runoff are reduced or eliminated, then overloading of the system, and subsequent flooding pressure downstream is reduced.

It’s a disconcerting thing to do!  In my own example, I cut off two of the plastic downpipes at their base and, using standard connectors and guttering lengths from the local DIY store, continued the pipes through the garden to the beginning of the swales.  The photos below show the disconnection process.

Photos above: the front garden during and soon after planting

Planting

The main planting took place in the spring of 2014.  The planting concept was to use the idea of successional layers to create a continuous flowering display from spring through the autumn, and then to have a good winter structure of stems and seed heads.  Planting design for rain gardens and bioswales is not straight-forward, because the vegetation has to deal with extreme dry periods as well as periodic influxes of water, and the sloping nature of the edges of both rain gardens and swales means that the base will stay relatively moist for longer periods, while the higher areas might dry out completely.  A mix of plants is therefore advisable to cope with these conditions, and to place plants with the greater tolerance of wetter conditions at the base of the features.

The main flowering period is between July and September, and is built around the quartet of Astilbe chinensis ‘Purple Lance’, Lythrum salicaria ‘Zigeurnerblud’, Rudbeckia fulgida deamii, and Crocosmia ‘George Davidson’, with a range of other species playing a supporting role.  The Astilbe is a very long-season plant, with red-flushed foliage in the spring, striking flower buds that are almost as effective as the vibrant purple fluffy flower heads, and then seed heads that last through the winter.  The Lythrum (Purple Loosetrife) is a bold and shining cultivar that almost replicates the colour of the Astilbe, but has a completely different vertical form – it also has good autumn foliage colour.  The Rudbeckia is one of my favourites, with lovely soft greyish green foliage, and rich golden yellow flowers with the black cone, and the Crocosmia again reflects the same rich gold, but with a much more exotic, freesia-like flower head.  In amongst this purple and gold extravaganza, the soft lilac blue flowers of the native Devil’s Bit Scabious, Succisa pratensis, weave their way amongst the more solid framework players.

A little earlier in the year, the upright spikes of Liguaria ‘The Rocket’ match the rising vertical flower stalks of the Astilbe, followed by the pungent acid yellow of Hemerocallis ‘Whichford’ contrast well with the newly blooming Astilbe, and before that, the ghostly lavender grey flowers of Iris sibirica ‘Mrs Rowe’ emerge amongst the expanding foliage of the later-flowering species.

The photos below were all taken in 2017 and show the succession of flowering over the growing season

Above: Astilbe chinensis ‘Purple Lance’ with Hemerocallis ‘Whichford’: July

Below: Astilbe chinensis ‘Purple Lance’ with Crocosmia ‘George Davidson: August

Below: Rudbeckia fulgida deamii and Astilbe chinensis ‘Purple Lance’: September

Earlier in the summer, Ligularia ‘The Rocket’ makes a transitory appearance, contrasting well with the crimson red of the upright buds in the flower spikes of the Ligularia

Photos below: in early summer, Iris sibirica ‘Mrs Rowe’ floats mistily above the foliage of later flowering plants, looking especially good against the black stems of the Ligularia and the rich yellow of the Marsh Marigold, Caltha palustris.

Because the garden was planted originally in the spring, bulbs were not included.  Although most bulbs don’t appreciate sitting in wet cool soils, I will be introducing some Cammassias for 2018.  Another notable omission are primulas, although I fear the tall and dense nature of the plantings later in the year would not promote their long-term survival.  However, I have included some shade-tolerant primulas – Primroses and Polyanthus.  The Polyanthus are of a pale yellow, large-flowered kind. These are not exactly sophisticated subjects, but I have a love of polyanthus since childhood, and it cheers me to see them emerging every spring, and they sit quite happily through the summer beneath the taller foliage canopy.

What hasn’t worked?

In the first year, one of the main components was the white version of our native Ragged Robin, Lychnis flos-cuculi ‘White Robin’.  This was great, and a big favourite of mine, but can be very short-lived, and so it proved.  Unfortunately it hasn’t self-seeded around, but I do want to see it from year to year, so this is something that I will introduce into gaps and spaces every spring.

I was also perhaps overly ambitious in my vision for this formal flanking of planting on each side of the path, and I initially included around 20 Thalictrum ‘Elon’ – my aim was to create a sort of dramatic herbaceous avenue along the path.  Of course, as might have been expected, these were overly dramatic and just too tall for the setting, although they were largely self-supporting.  So I have now moved these out of the plantings.

Photos below: the lovely early summer bronzy foliage of the Thalictrum, with the lime green of Euphorbia palustris behind.

Above: White Ragged Robin at a lower layer amongst the rain garden plantings

Planting Rain Gardens.

Look at most of the North American rain gardens websites and advice, and they will say that native plants should be used in rain gardens.  Again, it’s part of the tendency to think that if a feature has an ecological function, then it only be fully functional if native plants are used.  There really isn’t any proper scientific basis to this assertion.  ‘Right Plant, Right Place’ is by far the best guidance.  Planting design for rain gardens is still in its infancy and there is so much scope for experimentation in gardens.  Recent work done by one of my PhD students indicates that diverse mixes of perennials and grasses are much more effective at promoting the function of rain gardens than simple monocultures or low diversity plantings.  And while ‘Sustainable Urban Drainage’ is often seen as falling within the engineering remit, the actual visible part is the planting, and yet it is what is most often neglected.  Long-term, rain gardens are only going to become a really viable and mainstream idea, if they look good, and if the planting works.  It’s a huge opportunity for garden design and landscape horticulture.

For more technical information about creating rain gardens, see

Rain Gardens: sustainable management of rainwater in the garden and designed landscape, by Nigel Dunnett and Andy Clayden, Timber Press 2007.