Felling a tree is the visible part of the job. What happens to the ground afterwards often matters more, especially if the space is going to become a lawn, a garage pad, a vegetable garden or the footprint of a new extension. In Canterbury, where sections range from free-draining stony soils on the plains to silty loess on the hills and soft ground near the coast, the right approach to the stump and roots depends a lot on what sits underneath.
This guide works through the options for dealing with what a removed tree leaves behind, and how to get the ground ready for its next use.
Grinding, digging or leaving the stump
There are three broad choices once a tree is down.
- Stump grinding uses a rotating cutter to chew the stump into chips, typically to between 200 and 400 millimetres below ground level. It is quick, causes little disturbance and suits lawns and garden beds. Costs commonly range from around one hundred and fifty to several hundred NZD per stump, depending on diameter and access.
- Full excavation pulls out the stump and the main structural roots with a digger. It is the better option where foundations, a driveway or services are planned, but it leaves a large hole and disturbs surrounding soil.
- Leaving the stump to rot naturally is cheap and can work in a wild corner of a rural block, though it may take five to fifteen years for hardwoods and can sprout regrowth in species such as willow, poplar and some eucalypts.
Some species, including sycamore and certain willows, will sucker vigorously from roots after felling. Grinding alone may not stop them, and repeated cutting of new shoots or targeted treatment may be needed.
What happens to the roots left behind
Even after grinding, a network of roots remains in the soil. As they decay over the following years, the ground above can settle by a surprising amount, sometimes 100 millimetres or more over a large root plate. On the plains around Christchurch, where gravels sit close to the surface, settlement is usually modest. On the Port Hills loess or on peaty ground around the eastern suburbs, decaying roots can leave voids that collapse unpredictably or channel water underground.
Decaying roots also attract fungi. Most are harmless, but honey fungus can spread from dead roots to nearby living trees and shrubs. If clusters of honey-coloured mushrooms appear around an old stump site in autumn, consider removing more of the root material before replanting susceptible species.
Preparing the ground for its next use
How far to go depends on the plan for the space.
- Lawn. Rake out the grindings, which tie up nitrogen as they break down, and fill with a mix of topsoil and a little compost. Firm it down, water, top up any settlement, and sow in autumn or spring when Canterbury soils are moist but not cold.
- Garden beds. Similar to lawn, but dig a wider area to remove fine roots that compete with new plants. Allow for some settlement in the first year.
- Paths and patios. Excavate below the root zone, remove soft material and lay compacted basecourse. Pavers laid straight over a ground stump almost always sink.
- Structures. For a garage, sleepout or extension, full stump and root removal, then engineered fill or foundations taken down to good ground, is the norm. Building consent conditions may require a geotechnical assessment, particularly on land with earthquake-related ground issues.
Larger sites, or jobs combining stump removal with levelling, drainage and new paths, are where a contractor with experience in Christchurch earthworks makes sense. They can remove root plates, cart away unsuitable soil, bring in compacted fill and shape the ground so water drains away from buildings, which a stump grinder alone cannot do.
Timing and practical considerations
Canterbury’s dry nor’west summers bake exposed soil hard, while winter can leave clay and silt waterlogged and easily damaged by machinery. Late spring or early autumn often offers the best working conditions. Always locate underground services before grinding or digging, since stumps sometimes sit right on top of water, gas or fibre lines. Check with Christchurch City Council or the relevant district council about protected trees before felling, and about consent requirements for any significant earthworks or filling.
Frequently asked questions
Can I plant a new tree in the same spot straight away? It is usually better to shift the new planting a metre or two to one side, where soil is not full of decaying roots and wood chips. If the same spot must be used, remove as much old root as possible and add fresh soil.
Are stump grindings useful as mulch? Yes, on paths or around established shrubs. Avoid digging them into beds or using them for seedlings, as they draw nitrogen from the soil while breaking down.
How long will the ground keep settling? Most noticeable settlement happens within one to three years, but large root plates can continue sinking slowly for longer. Topping up periodically is normal.