My sister’s section in Churton Park sits on ground heavier than mine, and when she had the back lawn re-laid she went straight from bare dirt to turf on the same weekend, keen to get it done before a family gathering. That lawn was a bog by the following spring and a cracked, hard patch by the following summer, and neither problem had anything to do with the grass itself. It had everything to do with the ground underneath never having anywhere for water to go. I have watched enough lawns on heavy Wellington soil go the same way, including an early attempt of my own, to say the seed or turf is close to the last decision that matters, not the first.
Why a heavy-soil lawn needs drainage sorted before anything green goes down
Fine clay particles pack together tightly enough that water struggles to move through them, so rain that would soak away within hours on sandy ground can sit at or near the surface for days on heavy clay. A lawn grown on top of that behaves exactly like the ground underneath it: waterlogged and soft after rain, then baked hard once summer dries the same clay out again. Fixing that pattern after the lawn is already in is far harder than fixing it before, because by then you are digging through established turf to get drains in rather than working with open ground.
How much topsoil actually matters
A shallow skim of topsoil laid straight over compacted clay gives grass roots almost nowhere to go once they reach the boundary between the two layers, and that boundary becomes exactly where water collects. Most lawns on heavy ground do better with 150 to 200 millimetres of reasonable topsoil, and the transition into the subsoil below it matters more than people expect — a sharp, compacted interface between topsoil and clay traps water at that line just as effectively as the surface would without any topsoil at all.
The sand mistake and what subsoiling actually fixes
Tipping a modest layer of sand into heavy clay and rotary-hoeing it through is one of the more common attempts at improving heavy soil, and on its own it is often close to useless, because a small proportion of sand mixed through clay-dominant soil does not open up drainage paths — it can behave more like adding aggregate to concrete, filling gaps rather than creating them, and in some cases leaves the ground worse structured than before. What tends to help far more is genuine subsoiling or ripping to break up compaction at depth, combined with proper falls across the lawn area so surface water has somewhere to travel rather than pooling wherever the ground happens to be lowest.
Subsoil drains and picking the right season
On a lawn that holds water badly even with reasonable falls and decent topsoil, subsoil drains laid at a typical depth of around 400 to 600 millimetres, spaced roughly four to eight metres apart depending on how heavy the clay is, do the job that gravity and topsoil alone cannot — the same sizing logic that applies to drains and soakpits anywhere else on a suburban section. Timing the actual seeding or turfing around the Wellington calendar matters too: autumn, once the worst of the summer heat has passed but before the coldest, wettest months arrive, gives grass the longest run to establish roots before it has to cope with either extreme, and NIWA’s climate data for the region is a reasonable way to check how a given year’s rainfall pattern is shaping up before committing to a date.
Frequently asked questions
Why did my new lawn on heavy soil turn into a bog the first winter? It almost always means the ground had nowhere for water to go before the grass went in, either from poor falls, no subsoil drainage, or too little topsoil over compacted clay.
Does adding sand to clay soil improve a lawn? Not reliably on its own; a modest amount of sand mixed through heavy clay can fill gaps rather than open drainage paths, and genuine subsoiling combined with proper falls usually does far more.
When is the best time to seed or lay turf on heavy Wellington soil? Autumn is generally the safer window, giving roots time to establish before winter waterlogging and before the following summer’s heat, rather than rushing turf down right before an event.