A new driveway changes the look of a Dunedin section overnight, and the strip of bare ground alongside it is an obvious place to plant. Many homes in Mornington, Kaikorai Valley, Mosgiel and the hill suburbs sit on heavy clay soils, often on slopes, with cold southerlies and frosty winters to contend with. Plant the wrong thing too close and the roots may lift the edge of the concrete in ten years. Plant nothing and the clay bakes, cracks and washes onto the drive every time it rains.
Why clay changes the rules
Clay soils hold water and nutrients well but drain slowly and compact easily. In winter they stay wet and cold; in a dry summer they shrink and crack. That shrink-and-swell cycle matters near concrete. Large trees with extensive root systems can draw enough moisture from clay to cause it to shrink beneath a slab edge, contributing to settlement and cracking, while roots themselves can lift thinner sections.
Construction makes things harder still. Excavation and compaction for the driveway base often leave the adjoining strip dense, poorly drained and full of rubble. Before planting, it is worth digging a test hole: if water sits in it for a day after filling, drainage needs attention first.
Preparing the strip
Good preparation is the difference between plants that struggle for years and plants that establish in one season:
- Remove construction debris, basecourse spill and any large stones from the top 300 millimetres.
- Loosen compacted clay with a fork or spade rather than a rotary hoe, which can smear clay into a hard pan.
- Work in generous amounts of compost and, where helpful, gypsum, which can improve the structure of some clays.
- Consider mounding the bed slightly so plant crowns sit above the wettest soil.
- Finish with a layer of coarse mulch to protect the surface from crusting.
Avoid raising soil against the driveway edge or covering any drainage outlets, and keep the finished soil level just below the concrete so water drains away from it rather than across it.
Choosing plants that suit the spot
The safest options beside a driveway are shrubs, grasses and small trees with modest, fibrous root systems. Species that cope with Dunedin’s clay, frost and wind include:
- Hebe varieties, compact and hardy, with flowers that draw bees.
- Carex and Chionochloa tussocks, which hold soil and tolerate cold.
- Corokia and Coprosma, tough, wind-tolerant shrubs easily kept low.
- Pittosporum dwarf forms, useful for a clipped edge.
- KÅwhai or small ornamental trees, planted at least three to four metres from the concrete.
Avoid willows, poplars, large eucalypts and big conifers anywhere near a driveway on clay. Their thirsty, wide-ranging roots are the most likely to cause trouble.
Working with the driveway builder
The best time to think about planting is before the concrete is poured. Experienced Dunedin concrete driveway builders can allow for thickened edges, reinforcing and a root barrier along the side where trees are planned, and can set falls so stormwater runs to a drain rather than into a garden bed. A short conversation at the planning stage costs little compared with cutting out and relaying cracked panels later.
Root barriers are a sensible option where a small tree is planned within a few metres of the edge. They are installed vertically to a depth of around 600 millimetres, deflecting roots downward. They work best with modest trees, not as a licence to plant large species close to concrete.
Planting is best done in autumn or early spring, when the soil is moist but not saturated. In Dunedin, mid-winter planting into cold, waterlogged clay often leads to losses. Water new plants deeply but infrequently through their first summer, since clay that dries out on top can still be damp lower down, and overwatering is as common a killer as drought.
Frequently asked questions
How far from a driveway should a tree be planted in clay soil? Small trees are generally best placed at least three to four metres away, and large trees considerably further. Shrubs and grasses can go much closer.
Does gypsum really help Dunedin clay? It improves the structure of some clays but not all. Adding plenty of organic matter is the more reliable long-term improvement.
Can a root barrier be added to an existing driveway? Yes, by trenching along the edge, though care is needed to avoid damaging the slab or major roots of established trees nearby.