Creating a rain garden: capturing water and boosting biodiversity

A rain garden is clever water management and nature combined

A rain garden is a sunken area that collects rainwater, lets it drain slowly and attracts biodiversity. It's not a swamp, not a pond — it's a functional piece of garden that waters itself when it rains. It saves you water bills, reduces soil compaction and gives insects their feast back.

The principle: water flows via downpipes to the low point (through gutters, terraces), drains slowly there, and replenishes groundwater. Plants there get wet sometimes, dry other times — so you choose water-tolerant species.

Where to site a rain garden?

Ideal location: the lowest point in your garden, ideally where water naturally gathers (below gutters, at the end of hard surfaces). If your front garden slopes, place the rain garden at the bottom.

Minimum size: 2-4 metres long, 1.5 metres wide. Deeper than 50 cm is rarely needed unless you're draining an entire roof.

Check the ground: Sandy soil drains quickly (good). Clay holds water longer (also good, but riskier for stagnation). Do a drainage test: dig a 30 cm hole, fill with water, time how fast it disappears. Less than 2.5 cm per hour? You have drainage issues and may need intervention.

Build it: the steps

1. Shape & depth

• Rectangle, oval or organic — shape doesn't matter.
• Depth: 30-50 cm, sloping towards the centre.
• No sharp corners; gentle slopes for safety.

2. Bottom preparation

• Bottom layer (10 cm): coarse gravel or hardcore (drainage).
• Middle layer (15 cm): sand and gravel mixed.
• Top layer (20 cm): good garden soil with compost.

3. Water inlet

• Zinc or plastic gutter from the roof to the rain garden.
• Sand/gravel buffer at the inlet to stop silt blocking the planting zone.
• Pipe that lets water down without erosion.

4. Overflow (outlet)

• High water has to go somewhere. Run a drain pipe 5 cm below the top of the rain garden to carry water to the drain or lawn if it overflows.

Plant combinations for the rain garden

The best plants tolerate both wet and dry periods:

For wet periods (spring):

• Caltha palustris (marsh marigold, yellow, April-May, 30 cm) — classic water plant, very early.
• Iris sibirica (blue Siberian iris, May-June, 70 cm) — elegant, tolerant.
• Lysichitton americanus (yellow skunk cabbage, May, 60 cm) — spectacular, large leaves.

For drier summers:

• Miscanthus x giganteus (giant miscanthus, 200 cm, September-October) — fine foliage, structure.
• Filipendula (meadowsweet, pink or white, June-July, 100 cm) — neat leaves, holds moisture.
• Butomus umbellatus (flowering rush, pink, July-August, 100 cm) — long flowering.

For insects & bees (all season):

• Eupatorium cannabinum (hemp agrimony, pink, July-October, 120 cm) — butterflies love it.
• Scutellaria galericulata (hooded skullcap, purple, June-August, 40 cm).
• Symphyotrichum (aster, many colours, September-October, 80-150 cm).

Edge plants (dry):

• Geranium sanguineum (bloody cranesbill, pink, May-June, 40 cm).
• Nepeta (catmint, lilac, May-September, 40-60 cm).

Watering and maintenance

Water: first summer water regularly (roots aren't deep yet). After that rainwater usually does the job. In dry summers you may need to top up.

Pruning: cut dead stems only in March/April — they protect roots and look good in winter.

Silt: a thin layer accumulates over time. Every couple of years carefully scrape it off and compost it.

Wildlife: rain gardens attract dragonflies, toads and water insects. That's good — they eat midges. Don't add fish.

Why a rain garden pays for itself

• Water savings: for several months of the growing season your plants water themselves.
• Biodiversity: dragonflies, amphibians, water insects — your garden becomes a microhabitat.
• Soil: no hose-dragging compaction, infiltration improves.
• Looks: it looks natural, striking — especially in bloom.

Frequently asked questions

Will mosquitoes be a problem in a rain garden?

No, the opposite. Mosquito larvae need still water for several weeks. A rain garden that dries out weekly or gets blown about is no good for them. Dragonflies — they eat mosquito larvae for free.

How fast does a rain garden dry out?

With 50 cm of good soil and ornamental grasses: 2-3 weeks without rain in June-July. August dries faster. The aim is not constantly wet soil, but periodic watering.

Can I build a rain garden without an overflow?

Technically yes, but risky. Too much water and you get stagnant water and sludge. Overflow gives you control.

Why not a pond instead of a rain garden?

Ponds look lovely but manage differently: deeper, possible pumps, much more maintenance, less suitable for small gardens. A rain garden does 70% of the work with 30% of the complexity.

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