Tree pits, rain gardens and swales all do two jobs at once: they treat stormwater before it reaches a waterway, and they support the trees and plants growing in them. Built properly they remove contaminants, reduce runoff volume and irrigate the planting. Built badly they block, fail, and kill the tree.
Urban development replaces permeable ground with sealed surfaces, and those surfaces collect the contaminants of ordinary city life before shedding them straight into the stormwater network. A rain garden is an engineered filtration medium — specified materials and plants that catch and hold those contaminants instead. They also soften hard urban space, which is why they turn up in streetscape upgrades as often as in stormwater designs.
Where there is no room for a rain garden, a tree pit does the same filtration work in a smaller footprint, usually around a large specimen tree. What you see at the surface is a small part of the works — most of the construction is below the paving.
Auckland Council and most other authorities set specific requirements for how tree pits are designed, built and maintained. We build to those requirements as part of our streetscape work.
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Hurstmere Road Upgrade
A standard tree pit limits a tree to the soil volume directly beneath the opening, which is why so many urban trees stall at a fraction of their potential size. Strata cells are interlocking modules installed under the paving and filled with structured soil. They carry the load of the surface above while giving roots an uninterrupted volume to grow into.
That volume does two jobs at once. It grows the tree, and it holds water — detaining stormwater beneath the paving, letting it soak and be treated in the soil profile rather than running straight to the network. A cell system sized for a tree is also a detention and bioretention device, which is why it sits within the drainage design rather than alongside it.
The result is a tree that reaches maturity instead of declining after five years, and a catchment that sheds less. Both matter, and the second one is often what gets the system consented.
Some of the strata cell projects Evergreen Landcare has been involved in:
Get that right and a street tree matures into what the design intended: shade, carbon, habitat, cleaner air, and a corridor people want to walk down rather than through. Get it wrong and you have a declining tree in an expensive hole, and replacing it means lifting the paving as well.
We install from 45L grade street trees through to 1000L specimens, including rootball anchoring, engineered media backfill, drainage connection and irrigation. Species selection is matched to the site — the wind, the reflected heat, the salt, the services and the available volume — rather than to a plan drawn before anyone looked at the ground.
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