How do I build a rain garden in New Brunswick?

A rain garden in New Brunswick is a shallow, planted depression designed to capture and filter stormwater runoff from your roof, driveway, or lawn, reducing erosion, preventing basement flooding, and cleaning water before it reaches NB's rivers and streams. With annual rainfall of 1,100–1,200mm plus significant spring snowmelt, NB properties generate substantial runoff that a rain garden can manage naturally and beautifully.

Choose the right location. Position your rain garden at least 10 feet from your house foundation to prevent water from seeping toward the basement. It should sit in a natural low point where water already flows or collects, or at the bottom of a slope. Avoid placing it over septic systems, utility lines, or areas where bedrock is close to the surface. The ideal spot receives runoff from a downspout, driveway, or sump pump discharge. Full sun to part shade is best for the widest plant selection.

Size the garden based on your drainage area. A general rule for NB's rainfall volume is that the rain garden should be approximately 20–30% of the impervious area draining into it. If you're capturing runoff from a 200-square-foot roof section, plan a rain garden of 40–60 square feet. Depth should be 4 to 8 inches at the deepest point — deep enough to hold water temporarily but shallow enough to drain within 24–48 hours. Water should never stand for more than 48 hours, which prevents mosquito breeding.

Excavate and amend the soil. Dig the garden to your planned depth with gently sloping sides (3:1 slope ratio). NB's native clay-heavy soils (particularly in the Fredericton area and river valleys) drain too slowly for a rain garden, so replace or amend the bottom 12–18 inches of soil with a mix of 60% sand, 20% compost, and 20% topsoil. This engineered soil drains quickly while retaining enough moisture for plants. Sandy coastal soils near Moncton and Shediac may only need minimal amendment. Create a shallow berm on the downhill side to retain water, with a small overflow outlet for heavy storms.

Plant with NB-adapted moisture-tolerant species. The centre (wettest zone) handles plants like blue flag iris, Joe-Pye weed, and cardinal flower. The middle zone suits New England aster, bee balm, and switchgrass. The edges (drier zone) support black-eyed Susan, wild bergamot, and native sedges. All these are native or adapted to New Brunswick and tolerate both the periodic flooding and dry periods a rain garden experiences. Plant densely — 1 plant per square foot — to establish quickly and suppress weeds.

A rain garden in New Brunswick costs $500 to $2,000 depending on size, soil amendment needs, and plant choices. Direct a downspout into the garden using a buried 4-inch corrugated pipe or a surface-level stone channel. Mulch with 2–3 inches of shredded hardwood bark (not stone, which reduces infiltration). Maintenance is minimal — weed occasionally during the first two years until plants fill in, and remove accumulated sediment from the inlet area annually each spring.

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