Rain gardens: manage rainwater the natural way

Roofs, driveways and paths shed water fast. Instead of all of that running straight into overloaded drains, a rain garden gives it somewhere to go — slowly, through the soil, where it does some good on the way.

What is a rain garden?

A rain garden is a shallow, planted depression — typically 15–30 cm deep — positioned to catch runoff from an impermeable surface such as a roof, driveway or path. Water flows into it, pools briefly, and then soaks into the ground over the course of 24–48 hours. It is not a pond. It is not permanently wet. Between rain events it can look completely dry.

The key features that make it work are:

• A clear source of water flowing in — usually a downpipe diverted from a roof gutter, or a low point that already collects runoff.
• A well-drained soil mix in the basin that allows water to infiltrate.
• Plants that can tolerate short-lived flooding as well as dry periods between events.
• An overflow point so that excess water in very heavy rain flows somewhere safe rather than backing up toward buildings.

Why build one?

Urban and suburban drainage systems are under increasing pressure, particularly as more land is paved over. A single rain garden won't solve flood risk at a city scale, but it does several useful things:

• Reduces runoff volume and peak flow. Water soaking into the ground does not immediately reach the drain, reducing the speed and volume of water entering the drainage system after rain.
• Filters pollutants. Rainwater that has run over roads, paths and roofs picks up oil, sediment, heavy metals and other pollutants. As water percolates through soil, many of these are captured or broken down by soil organisms before reaching groundwater.
• Recharges groundwater. Water that soaks into the ground contributes to local groundwater rather than being lost to the sea via the drain.
• Supports wildlife. A planted rain garden provides nectar sources, seed heads, shelter and water access for insects, birds and small mammals. See our wildlife-friendly garden guide for the wider picture.
• Works with rainwater harvesting. If you already collect rain from a downpipe in a barrel or tank (see our rainwater harvesting guide), the rain garden is an excellent destination for overflow from a full tank.

How to plan your rain garden

Identify your catchment and runoff source

Start by deciding where the water will come from. A downpipe from a roof is the most reliable source — it delivers a known volume of water at a predictable point. Measure the area of roof draining to that downpipe: a 50 m² roof section can deliver hundreds of litres in a moderate rainfall event, so size matters.

As a rough rule of thumb, aim for a rain garden that is roughly 20–30% of the area draining into it (for a garden with average-draining soil). Sandy soils can handle a smaller ratio; clay soils may need a larger rain garden or improved drainage in the basin.

Test your drainage

Dig a test hole about 30 cm deep where you plan to put the rain garden, fill it with water and watch how long it takes to drain. If it drains within a few hours, the soil is suitable. If it still has water after 24 hours, the drainage is too slow for a basic rain garden — you may need to improve the soil mix in the basin or reconsider the site.

Depth and size

A depth of 15–30 cm is typical. Shallower is often better — plants establish more easily and the garden drains faster. The overall size should be large enough to hold and absorb the runoff from at least a moderate rain event without overflowing every time it rains lightly.

Siting and safety

Building it step by step

1. Mark out and locate the garden. Use a hose or string to mark the shape. Check underground utilities. Confirm the site is downhill from your downpipe or water source and at least 3 metres from any structure.
2. Dig the basin. Excavate to 20–30 cm depth. Keep the bottom level so water distributes evenly. The sides can slope gently. Keep the excavated soil to use as a berm (low bank) on the downhill edge to help retain water in the basin.
3. Improve the soil if needed. If your native soil drains slowly, mix the excavated soil with sand and compost (roughly equal thirds) and return it to the basin. Good infiltration is more important than rich growing medium — most rain garden plants prefer lean soils.
4. Create the inflow. If connecting to a downpipe, fit a diverter or extend the pipe to direct water into the top of the basin. Create a small channel lined with stones or gravel at the entry point to slow water and prevent erosion as it enters.
5. Create an overflow. At the lowest point of the berm, cut a notch or lay a short length of pipe that allows water to escape safely (to lawn, a planted area, or a further drain) when the basin is full. This prevents waterlogging in extreme events.
6. Plant the basin. Plant wet-tolerant, ideally native species in the lower centre of the basin, and more drought-tolerant plants toward the edges and on the berm. Space generously — rain garden plants typically establish and spread well.
7. Mulch and test. Apply a 5 cm layer of wood chip or bark mulch to suppress weeds and retain moisture during establishment. Then run water in via the inflow and watch how it moves — check the overflow works and that the basin drains within a day or two.

Choosing plants

Rain garden plants need to tolerate two opposite conditions: periodic inundation when it rains, and dry periods between events. Native plants are by far the best choice — they are adapted to local rainfall patterns, support local insects and wildlife, and once established need little care. Species vary hugely by region, so check with a local nursery or native plant society for what suits your climate and soil.

As general types to look for:

• Wet-tolerant perennials for the basin centre: many native irises, sedges, rushes, and wet-meadow wildflowers are well suited.
• Ornamental grasses and grass-like plants that tolerate wet feet intermittently: native species of Carex, Juncus, and similar are invaluable and available in most temperate regions.
• Native shrubs for the margins and berm: many native willows (as shrub forms), dogwoods and viburnums handle wet-to-dry extremes well.
• Wildflowers and pollinator plants for the upper edges, where conditions are drier.

For native plant selection help, see our guide to native plants. For the wider garden water picture, see water-wise gardening.

Avoid plants that need consistently moist conditions (they will struggle in dry spells) or that come from regions with very different rainfall patterns to yours.

Maintenance

Rain gardens are relatively low-maintenance once established, but they do need some attention:

• Water new plants regularly for the first one to two seasons while they establish — ironic as it sounds, a rain garden can go weeks without rain and young plants need support.
• Clear debris from the inflow point after storms — leaves and grit can block the entry channel over time.
• Cut back perennials in late winter or early spring, leaving seed heads through autumn and winter for wildlife.
• Top up mulch annually to keep weed pressure down.
• Check drainage after heavy rain — if the basin takes more than 48 hours to drain, clear any blockages and investigate whether the soil has compacted or become clogged with sediment.
• Remove invasive plants promptly — a well-watered basin can become a launching pad for weedy species.

Rain garden checklist

• Identified the runoff source (downpipe, path, driveway).
• Checked for underground utilities before digging.
• Sited at least 3 metres from any building foundation.
• Confirmed drainage: site drains within a few hours of being filled with water.
• Sized the garden to match the area draining into it.
• Built an overflow point to handle excess water safely.
• Planted with wet-tolerant, preferably native species in the basin centre.
• Mulched to suppress weeds and protect soil during establishment.
• Basin drains completely within 48 hours after a rain event.