Rain gardens are a neat, innovative way to reduce the all-too-frequent flooding after intense rain storms that we’re getting now.  I need hardly remind you that this July was the wettest since 2009 and the sixth worst on record, according to the Met Office.  Downpours lead to much greater flooding because the rain falls either on ground that’s as hard as concrete after a prolonged dry spell or on soil that’s already soggy and can’t absorb any more water.

Two years ago the Royal Botanic Garden Edinburgh [RBGE] endured a month’s rain in an hour in the Edinburgh garden. Then paths were eroded, compost washed out of flower beds and the plants themselves flattened. Sound familiar in your garden?  According to David Knott, curator of the living collection at RBGE, things had been getting noticeably worse over the last 10 to15 years.  “You couldn’t walk on our Birch Lawn at all in the winter months because it was so wet. Even by Scotland’s standards, we’re getting wetter.”

So the Botanics have taken the bull by the horns and built a special rain garden on the site of the Birch Lawn.  The site has been designed to capture and slow down water accumulating during a rain storm and to gradually release it into the ground beneath.  RBGE is working with Heriot Watt University to monitor and assess the effectiveness of the site.

The RBGE rain garden measures 20m by 7m. A pool is surrounded by a strip of grass which should absorb some of the rainwater. Run-off pools in the middle are 450mm deep and the accumulated water then soaks slowly into soil that has been improved to enhance drainage.  The soil mix is 30 per cent existing soil, 45 per cent fine sand, 10 per cent fine gravel and 15 per cent compost.

Plantings are a mixture of Scottish native and non-native perennials and grasses that can cope with extreme conditions.  Plantings such as Filipendula ulmaria, Cicerbita alpina, Ligularia fischeri and Aruncus gombalanus have been selected for the wettest areas.  Species including Aquilegia formosa and Anthyllis vulneraria were picked for the drier edges.

The design is really working and a number of organisations, including Scottish Water, are adopting the idea.  Why not visit the site and see whether you could.

Plant of the week

Amelanchier lamarckii is a smallish tree, maximum 10m, that has excellent autumn colour, the leaves turning shades of crimson and scarlet. The colours are most intense when the tree is grown in slightly acidic soil.

In spring it produces drooping heads of white, star-shaped flowers and the new foliage is a soft copper colour that turns green by early summer.d copy some of the ideas for your own garden.