Plant Care
Fix PNW Garden Drainage: Glacial Till, Hardpan & Raised Beds
PNW gardens flood every winter because a glacial-till hardpan layer traps water 18-36 inches down. Here's how to diagnose it and fix drainage for good with raised beds, French drains, and rain gardens.
— Image © Plant Reference 2026
Quick Answer
PNW drainage problems stem from a glacial-till hardpan 18-36 inches (45-90 cm) deep that blocks downward flow. Build raised beds 12+ inches (30 cm) tall to lift roots higher.
TL;DR
Most Pacific Northwest drainage failures trace back to glacial till hardpan, a concrete-like compacted layer sitting 18-36 inches (45-90 cm) below the surface that blocks water from draining down. The fix is rarely about your topsoil. Build raised beds as the default, add a French drain on sloped or chronically wet ground, and route roof runoff into a rain garden instead of your vegetable beds. Diagnose first with a simple perc test, then match the solution to the problem.
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Frequently Asked Questions
Heavy clay is dense throughout but you can still dig it, and a perc test will show very slow drainage rather than none. Hardpan is different—it's a distinct, abrupt layer 18-36 inches (45-90 cm) down that feels like hitting pavement with your spade. The clearest tell is a perc test where water still stands after 24 hours combined with that hard layer underfoot. Clay drains slowly; true hardpan barely drains at all.
You can fracture it with a digging fork or broadfork to improve things, and it's worth doing under a new bed. But in the PNW our winter rainfall is relentless, and broken-up hardpan tends to recompact over a few seasons. For reliable, low-maintenance drainage, raised beds are the durable answer. Breaking the hardpan is a helpful supplement, not a standalone fix.
Route it to a spot that genuinely drains—a dry well filled with gravel, a rain garden, or a daylight outlet on a downhill edge of the property. Never pipe it toward your foundation or a neighbor's yard. Check local stormwater rules first; many PNW municipalities have specific requirements for managing runoff on residential lots.
They drain faster, so yes, they need attention during the July-to-September dry gap. The fix is deep, infrequent watering plus 2-3 inches (5-8 cm) of mulch, which can halve your watering needs. Drip irrigation on a timer makes this nearly hands-off and keeps moisture consistent through the dry months.
A properly built rain garden empties within 24-48 hours of a storm, which is far too fast for mosquitoes to complete their breeding cycle. Standing water becomes a problem only if the basin is built over poorly draining soil without the pumice-amended mix. If yours holds water longer than two days, dig down and improve the filtration layer beneath it.
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Written By
M
Mike's been designing residential landscapes in the Seattle area, with a particular focus on Pacific Northwest natives and sustainable practices. He's a Washington State Master Gardener and worked in construction before landscaping, which gives him a practical edge when it comes to building raised beds and hardscaping projects. He runs a small side business propagating and selling unusual perennials at farmers markets on weekends. His property serves as a testing ground for low-maintenance, wildlife-friendly garden designs. Mike's philosophy: if it requires weekly maintenance, it's not sustainable.
