In 2019 I spent a winter on a line-clearance contract around Rotorua, working the roads between Ngongotahā and the lake edge, and boarded in a rental on a Lynmore section backing onto a stand of redwoods. The owner had spent the previous spring planting rhododendrons, hostas and a griselinia hedge under the redwoods and along a bank that steamed on cold mornings. By the time I moved in the griselinia was dead, half the hostas had never come up, and the rhododendrons were yellow. The soil under the redwoods was bone dry at 100 millimetres. The bank read 38 degrees at 300 millimetres when I put a thermometer in it. Rotorua is a hard place to plant under trees, for three reasons that stack.
Shade under a redwood is not like shade under an oak
A deciduous tree gives you six months of light. A redwood, a Douglas fir or a mature totara gives you none, and the shade is dense from the ground up because the lower branches persist. Very few garden plants tolerate it, and those that do evolved on forest floors: ferns, some shade-tolerant natives, and a short list of exotics.
Shade is the problem people expect. It is not the one that kills most underplanting.
Roots take the water first
A mature redwood pulls several hundred litres of water out of the ground on a warm day, from the top 300 to 600 millimetres of soil where its fine roots are, which is exactly where a new plant is trying to establish. Rotorua’s pumice soils drain fast anyway; under a big conifer they are a desert from November to April. A griselinia with a 150-millimetre root ball competing with a tree that has been there sixty years loses.
The instinct is to dig in compost and build up a bed, and that is where the second layer of damage happens. Piling more than 50 to 100 millimetres of soil over an established tree’s root zone cuts off the oxygen exchange the fine roots depend on; roots that suffocate die back and the tree itself is stressed. Cutting roots larger than about 25 millimetres to make planting holes does the same. A ring of raised beds around a redwood is a slow way to kill it.
Geothermal ground kills from below
The third factor is unique to Rotorua. Across Ohinemutu, Whakarewarewa, parts of Lynmore, Ngapuna and the lake edge, the ground itself is warm. Soil temperatures of 35 to 50 degrees at 300 millimetres are common near active features. Most plant roots stop functioning above about 30 degrees and die above 40, and acidic groundwater around the features makes it worse.
The tell-tales: bare or mossy patches where nothing grows, ground that stays dry after rain, warmth through your boots, and existing trees with dead sides facing the feature. On the Lynmore bank the griselinia died because its roots were cooked, not because of the shade.
What actually grows, and what it costs to find out
Start with a soil thermometer and an afternoon. Probe the planting area at 300 millimetres in a grid and mark anything above 30 degrees as off limits for anything but geothermal-adapted natives such as prostrate kānuka, some sedges and the mosses already living there. In the cooler shade, work with the tree: plant small, plant in the gaps between major roots rather than cutting through them, mulch with bark or chip to 75 millimetres rather than building up soil, and water for the first two summers because the tree will not leave anything for the newcomers.
Species that hold up under Rotorua conifers in my experience: native ferns, renga renga, kawakawa, coprosma, tougher hydrangeas at the edge of the canopy, and hellebores where there is a little morning light.
Where the trees are protected under the Rotorua District Plan, and many of the mature redwoods and natives near the lake are, work within the root zone, including trenching for irrigation, may need consent. Check the schedule before digging. A soil survey and planting plan for a section like the Lynmore one runs roughly NZ$400 to NZ$900 from a Rotorua landscaper who knows the geothermal ground, and planting around NZ$60 to NZ$120 per square metre installed, which is cheaper than replacing NZ$1,500 of dead griselinia twice.
Frequently asked questions
Why do plants die under big trees even when watered? Mostly root competition: a mature conifer takes water from the same top 300 to 600 millimetres of soil a new plant needs, and its shade cuts light to a fraction. In Rotorua warm geothermal ground can also cook roots outright.
Can I build up soil under an established tree to make a garden bed? No more than about 50 to 100 millimetres, and never against the trunk. Deeper fill suffocates the fine roots and can kill the tree over a few years.
How hot is too hot for plant roots in geothermal soil? Most plants stop growing above about 30 degrees at root depth and die above 40. Probe at 300 millimetres before planting and treat anything over 30 degrees as unsuitable for ordinary garden plants.