Riparian planting: what to put along your waterway, and how many
- Travis Currie - Manager
- Aug 7
- 6 min read
Updated: Aug 8
Quick answer: Plant in bands. Sedges and rushes right at the water where it floods, flax and toetoe up the bank, then shrubs and small trees on the drier ground behind. Sedges go in at 1 to 1.5m centres, shrubs and trees at 1.5 to 2m. Environment Southland's costed example works out at 100 plants per 100 metres of bank.
Ten years after the fence goes in, a planted waterway stops looking like a job you did. The water runs dark and cool under the flax. Sedges lean out over the edge where the fish sit. You hear it before you see it, and the paddock either side carries on as though nothing much happened.
That's the version people have in their head when they order the plants.
The one that turns up more often is a line of dead sticks in a strip of thistle, with plant guards still standing where the plants aren't. Same money, same weekend on the spade. What separates the two is which plants went where, how far apart they went, and whether anyone came back in year two.
Why plant it at all, once the fence is up?
Shade and roots, mostly.
A planted margin cools the water. DairyNZ puts the target at 70% shade to hold weed and algae growth down, and notes that New Zealand native fish generally can't handle water above 25°C. Roots hold the bank face together, so the paddock stays in the paddock instead of ending up in the gravels. And the strip filters sediment and nutrient out of run-off before it gets to the water.
The shade part is where people undercook it. DairyNZ's rule of thumb for a north-south flowing stream planted on both banks is that plant height needs to reach at least 80% of the stream width. East-west, one and a half times the width. So a 6m creek needs something taller than a row of sedges to do the job.
What actually goes where?
Three bands, working back from the water.
Band one: the wet edge that floods
Carex virgata, swamp sedge or pukio, and Juncus edgariae, wiwi. Both grow naturally in swamps, drain margins and seepages, and both cope with sitting under water for days at a time. These go closest to the water, where anything woody would drown. The roots bind the bank face and the leaves flop out over the edge, which is the overhanging cover the fish are after.
Band two: the bank itself
Toetoe is a South Island native that grows wild along stream banks and river beds, often right alongside flax, and it handles wind, cold, coastal exposure and dry ground once it's away. Harakeke does a similar job with a heavier root plate and is happy in ground that stays wet, while still shrugging off long dry spells. Both get large, so keep them back off the fence line or you'll be cutting them away from the wires in five years.
Band three: the drier ground behind
Mature manuka takes waterlogging, drought, wind and frost, which is an unusual set to get in one plant, and in the wild it colonises river gravels and wetland margins. Two things to know before you order it: it wants to go in during autumn, and it holds a grudge about root disturbance, so plant it straight out of the pot without teasing the roots.
Cabbage tree grows on most soils and copes with most moisture regimes, though it dislikes a long drought. Its weak point in this particular job is the single trunk, which gives flood water very little to push against. Keep it back on the higher ground rather than down on the flood line.
How many plants do I need?
Between one and one and a half per lineal metre of bank, at standard spacings.
DairyNZ's spacings are 1 to 1.5m for grasses and sedges, and 1.5 to 2m for shrubs and trees. Environment Southland's costed example is simpler again: two alternating rows, everything at 2m, one row of sedge close to the waterline and a row of mixed natives set two metres off the fence. That comes to 100 plants for 100 metres of bank.
So the maths on a real job:
400m of creek, fenced both sides, is 800 lineal metres of bank.
At Environment Southland's density, that's 800 plants.
Tighten the sedge row to 1m centres and leave the mixed row at 2m, and it's 1,200.
Another option is to plant on one bank only to leave one side free for maintenance access, which then divides these numbers in half
Width matters as much as spacing. DairyNZ suggests at least 2 to 5m from the top of the bank on each side, and wider where you can wear it. Leave a strip of grass between the planting and the fence so nothing grows up into the electric wire.
What does riparian planting cost?
The numbers we work on are approximately as follows. Plants at $3.50 each, plant guards at $2.50 each, and $4.50 each for planting (including the plant guard). On top of this a pre-planting spot spray four weeks ahead (often done by the farmer), then two more rounds of release spraying in the following two springs, replacement plants, and labour to pull the guards out at the end.
So for a 100m strip, per bank, plants and guards come to $600. Plus another $450 for planting, plus pre-spraying, release and clean-up. Most riparian budgets I get asked to quote into cover the plants and stop there.
When's the best time to plant a waterway?
Autumn, where you can get on the ground.
DairyNZ recommends winter, June to August, on the grounds that soil moisture is high. That's sound, and winter planting works. Down here I'd go a bit earlier where the ground allows it. Autumn soil still holds some warmth from summer while the air is cooling off, so roots get moving before the hard frosts arrive, and the first light frosts knock the bugs back for you. Then the spring flush drives roots down deep before the ground dries out over summer.
Winter, mid-winter included, is fine. The window I'd avoid is late, heading into summer, when a plant with few roots can go thirsty before it establishes.
One exception to file away: manuka wants autumn specifically, whatever else you're doing.
What kills a riparian planting?
Weeds, in the first three years, ahead of everything else.
Grass and thistle smothering the young stuff. Release twice a year for the first two to three years, by brushcutter or by hand. After three to five years the canopy closes over and the planting starts looking after itself.
No mat or mulch. DairyNZ suggests 30 to 40cm of biodegradable weed mat, mulch or old wool carpet around each plant. Steer clear of plain wood chips, which pull nitrogen out of the soil as they break down and starve the plant you're trying to help.
The wrong plant in the wrong band. A shrub down on the flood line drowns. A sedge up on the dry bank sits there and sulks.
Losing them. Put a stake beside every plant. In year two the grass will be over your head and you'll be glad of it.
Stock. Fence first, plant second, never the other way round.
What if my waterway isn't in Southland?
Same three bands. Change the back row to suit the district.
Out on the Canterbury plains the nor'wester pulls moisture out of new growth faster than young roots can replace it, so the band furthest from the water has to double as shelter, and a watering plan for the first two summers will matter more than any frost rating on the label.
Central Otago flips the problem again. Hot, dry summers and frosts a good deal harder than anything the Southland coast serves up. The sedges still want the wet line, but the dry band behind needs hardy, drought-tolerant choices rather than anything that expects a damp summer.
Common questions
Do I have to fence before I plant?
Yes. Stock should be permanently excluded before a single plant goes in. Cattle will pull out or trample a young riparian planting in an afternoon and you'll be buying the whole lot again.
Can I mix natives and exotics?
You can. Environment Southland costed a single row of Crowsnest poplars at 2m spacings for the same 100 metres at $668, against $1,195 for the double row of natives. The poplars are cheaper and give good summer shade on an east-west waterway, being deciduous, but a single row of trees does a different job from a mixed planting. It won't give you the bank-binding sedge layer or the habitat.
My stream only floods once every few years. Does that change anything?
It moves the line rather than removing it. Ground that floods occasionally still stays waterlogged long enough to kill plants that want free drainage, so treat it as upper bank and choose accordingly.
How long before it looks after itself?
Three to five years to canopy closure, on DairyNZ's reckoning, which matches what I've seen. Budget for release in years one, two and three and you'll get there. Skip year two and you're starting again.
What grade should I be ordering for riparian work?
Smaller grades establish well on damp ground and cost less per plant across a big job. On exposed sites, or where the grass is going to get away on you, a bigger grade buys you a head start.
Get the bands right, fence it properly, and turn up in year two with a brushcutter. Ten years on you'll hear the water before you see it.
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