Every winter the line-clearance crew I climb with takes out a few dozen trees along the rural roads west of Hornby where old shelterbelts have grown into the 11 kV lines. The stumps stay, and about a third of the time I get a call six months later asking why a dead poplar has fifty shoots coming out of the ground. Last year it was a lifestyle block on Marshs Road: nine hybrid poplars felled in June, stumps left at knee height, and by December every stump had a two-metre thicket and suckers coming up in the lawn eight metres away. Whether a stump comes back depends on the species and on what you do in the first fortnight.
Which species come back, and how
Most conifers are done once cut. Radiata pine, macrocarpa, Lawson and Leyland cypress do not resprout; the stump slowly rots and that is the end of it. The trees that do come back split into two groups.
Stump sprouters regrow from buds around the cut: willow, eucalyptus, elm, oak, ash, sycamore, wattle and most fruit trees. Cut one at knee height and you get a ring of shoots two metres tall within a year. Root suckerers are the real problem: poplar, robinia, tree of heaven, elm again and some cherries send up new stems anywhere within the root spread, which for a mature poplar is 15 to 20 metres from the trunk. You cannot grind your way out of a root suckerer because the roots that sprout are nowhere near the stump.
Grinding: what it does and does not do
A stump grinder chews the stump down to 200 to 300 millimetres below ground and leaves a hole full of chip and soil. That is enough to lay turf, run a mower or build a path over. It is not enough to pour a slab on, because the remaining roots rot and the ground settles for years. Grinding removes the buds a stump sprouter needs, so on a willow or a gum it is permanent. On a poplar or robinia it removes the stump and does nothing about roots that still hold a season of stored starch.
Prices in Canterbury run about NZ$120 to NZ$250 for a stump under 300 millimetres across, NZ$300 to NZ$600 for a 600-millimetre macrocarpa, and NZ$800 to NZ$1,500 for the big 1.2-metre shelterbelt stumps that need a tracked machine. Access matters: a grinder that fits through a 900-millimetre gate is a small grinder, and a small grinder on a big stump is a full day.
Poisoning: timing is everything
Herbicide only works if it reaches living tissue, and the living tissue is the thin cambium ring under the bark, not the heartwood in the centre. It has to go on within minutes of the cut, before the surface dries and seals, and on the ring rather than the middle. Concentrated glyphosate painted onto the outer 30 to 50 millimetres of a fresh cut is the usual approach; metsulfuron is often added for poplar and willow because glyphosate alone is unreliable on them. Autumn cuts, when the tree is drawing starch down into the roots, carry herbicide further into the root system than spring cuts.
Done properly, poisoning kills the roots as well as the stump, which is why it is the answer for root suckerers. It leaves the stump standing, so you look at it for another five to ten years unless you also grind it. A pack of stump herbicide is NZ$40 to NZ$90 and the work takes minutes per stump at felling time. Done a week later, it mostly does not work at all.
What I recommend on a Canterbury shelterbelt
- Conifers — grind or leave to rot. No treatment needed.
- Willow, gum, oak, fruit trees — grind, or cut low and treat the ring at felling. Either works.
- Poplar, robinia, tree of heaven — treat the ring the moment the tree is cut, then grind the following year. Expect to pull suckers for two seasons regardless.
- Anywhere a structure is going — grind deep and dig out the main roots, or accept settlement.
On the Marshs Road poplars we came back, cut every sucker, treated the fresh stubs and ground the nine stumps for about NZ$2,900. Treated the day they were felled, it would have been under NZ$400. The New Zealand Arboricultural Association has guidance on stump treatment and herbicide use at nzarb.org.nz, and for what a full removal involves when the stump is the least of the problem there is a worked example in the redwood removal at a Rotorua lakefront house.
Frequently asked questions
Will a stump grow back after grinding? Stump sprouters such as willow, gum and oak will not. Root suckerers such as poplar and robinia keep sending up shoots from roots the grinder never touched unless the stump was treated with herbicide at felling.
How much does stump grinding cost in Christchurch? About NZ$120 to NZ$250 for a small stump, NZ$300 to NZ$600 for a 600-millimetre stump, and NZ$800 to NZ$1,500 for large shelterbelt stumps needing a tracked grinder.
When should stump poison be applied? Within minutes of the cut, onto the outer cambium ring rather than the centre. Autumn felling moves the herbicide further into the roots than spring felling.