Why is my Portuguese laurel hedge dying in patches?

Quick answer: Nearly always phytophthora root rot, brought on by wet ground rather than cold. Portuguese laurel handles frost down to about -15C, so cold is rarely the culprit. Look for plants failing in scattered ones and twos along the row, leaves yellowing then bronzing, and soil that stays damp. The fix is drainage, not fertiliser.

A row of Prunus lusitanica going in along a new build is one of the safer bets in landscaping. It clips tight, it holds its leaf, it takes a hard cut and comes back, and it shrugs off frost that turns other hedges to charcoal. So when one starts dying, people rule out the obvious things fast and then get stuck.

I came across an example of this in Queenstown. A laurel row on a narrow strip between a dark board-formed concrete wall and a big concrete pad, plants failing in no particular order down the line. A couple fully bronzed off, a couple more going yellow, and either side of them plants that looked perfect.

That scattered pattern is the whole diagnosis. It's what tells you the trouble is in the ground.

What does the pattern tell you?

Which plants have died, and in what order, narrows it down before you dig anything.

• Scattered singles and pairs, healthy plants between them. Root rot. The wet doesn't sit evenly under a row, so the plants sitting in the worst of it go first.

• The whole row failing evenly, from one end to the other. Something applied to all of them at once. Spray drift, a fertiliser mistake, or a planting job done wrong every time.

• One clean run at one end. Usually a site condition that changes along the row. A downpipe outlet, a low corner, or the point where the topsoil ran out and the fill started.

• Brown crisp tips and margins, the middle of the leaf still green, the whole hedge alike. That's moisture leaving the leaf faster than the roots can replace it. Drought or wind burn, not disease.

Why is wet ground the usual cause?

Because phytophthora is a water mould. It needs saturated soil to move, and it gets it in ground that stays wet for days at a time.

Once it's into the roots, the plant loses its ability to draw water up. The symptoms then look like thirst. Leaves droop and yellow, stems discolour, and the plant slowly collapses over a few months. The Plant Company describe seeing it start several weeks after a long wet spell, and they make the point that matters most here: the gardener sees drooping leaves, assumes the plant is dry, and waters it. Which makes it worse.

So the test is simple. Push a finger, or better a spade, into the soil beside a sick plant. If it's wet down there and the plant is still wilting, you have your answer. Wet soil plus a wilting plant is root rot until proven otherwise.

Two more things you'll find if you dig one out. The roots will be dark brown or black instead of pale and firm, and the ground around them often smells sour, the way a forgotten vase of flowers does.

What if the site is a strip between concrete?

Then the drainage failure was built in before a single plant arrived, and this is the version I see most.

Work out what the strip is actually catching. Concrete sheds close to every drop of rain that lands on it. If you've got a 600mm planting strip taking runoff from a 6m pad on one side and a wall on the other, that strip is fielding roughly ten times the rainfall it would get standing on its own in a paddock. Southland gets that. Central Otago gets it in bursts.

Now work out where the water goes next. Usually nowhere. The subgrade under a concrete pad is compacted hard before the pour, and that compaction doesn't stop neatly at the edge of the formwork. The strip is a trough with sealed sides and a packed floor. Water arrives fast and leaves slowly, and the plants sit in it.

The strip looks like a garden bed and behaves like a pot with no hole in the bottom.

Two look-alikes to rule out on the same kind of site, because they bronze laurel off in a similar way:

• The rain shadow under an eave. A strip tucked hard against a wall under a wide soffit can miss most of the rain that falls, while the strip four metres away drowns. Same hedge, opposite problem. Check where the drip line actually lands.

• Reflected heat off a dark wall. A dark board-formed or stained wall in full afternoon sun pushes heat back into the foliage on that face. You'll see the burn on the wall side of each plant and green foliage on the outer face. Root rot doesn't pick a side like that.

Is it the frost?

Almost never, and this is where people lose a season. Portuguese laurel is hardy to around -15C. Alexandra, Ranfurly and Clyde can hand out a harder frost than most of the country and the plant will still be standing.

What frost does do is burn soft new growth in late spring, which browns the tips and looks alarming for a month. The plant grows out of it. A frost-burnt laurel has damage on the new shoots and sound older leaves underneath. A laurel with root rot is failing from the whole plant at once, and the neighbours a metre away are fine.

If the diagnosis lands on frost and you replant on that basis, you'll put the new plants back into the same wet hole and lose them too.

Can you save the plants you've got?

Partly, and it depends how far along they are.

Plants already fully bronzed off are gone. Pull them. Plants only just yellowing on the lower leaves sometimes come back if you fix the water first, in that order and not the other way round.

What actually works:

1. Stop watering it. If there's irrigation on the row, turn it off entirely and check the ground with a spade before you turn it back on.

2. Give the water somewhere to go. On a strip between hard surfaces that means a saw cut and an outlet through the concrete, or a slotted pipe taken out to daylight at the low end. That is a concrete saw and a spade, so price it as a builder's job. Nothing else on this list fixes it permanently.

3. Lift the root zone. Where an outlet isn't possible, raising the bed 150 to 200mm above the surrounding level buys the roots air. Free-draining mix, not topsoil, and not a layer of scoria in the bottom of the hole. A coarse layer under fine soil holds water above it rather than letting it through.

4. Then, if you want, treat. A phosphorous-acid drench or a trichoderma product can help a plant that's still got roots to work with. Neither will beat a wet hole.

5. Replace the gaps in autumn. Don't drop new plants into the same holes over summer with the drainage untouched.

Drainage is the only one of those five with a long life. The other four buy time.

What should you plant instead, if the strip can't be drained?

Sometimes the concrete isn't coming up and the outlet isn't going in. In that case stop trying to make laurel work there.

Two natives we grow that take ground staying wet, both of which will sit in that strip without complaint:

• Coprosma propinqua (mingimingi). Grows naturally on stream banks and swampy ground, so it takes anything from wet to free-draining, and it's high on frost, wind and salt tolerance. Reaches 1 to 3m, wiry and fine-leaved, and clips into a reasonable informal hedge. It won't give you laurel's big glossy leaf and it never will.

• Carex secta (purei). If the strip is low, narrow and permanently damp and you want it to look deliberate rather than apologetic, a block of purei looks better than a hedge with gaps in it. It thrives in wet and swampy ground.

If it's the formal clipped look you're after and the ground can't be fixed, the choice is a raised bed or a different plant. Not a different brand of laurel.

What about the berries and the stock?

Two things to know if this hedge is on a farm boundary or near a bush edge, since you're likely replanting anyway.

Portuguese laurel is cyanogenic, like the rest of the cherry laurel group. Chewed, crushed, wilted or frozen tissue releases hydrogen cyanide, and regrowth after cutting carries the glycosides strongly. The Irish Veterinary Journal has a case report of 36 dairy-cross weanlings dying over twelve days beside a laurel hedge. Portuguese laurel is the safer of the two laurels to have about the place. It isn't safe. Keep the prunings out of the paddock.

And DOC's 2024 list of environmental weeds in New Zealand names both Prunus lusitanica and Prunus laurocerasus. The seed is bird-spread. Trim after flowering so berries never set, and don't specify it against a bush margin, a gully or a reserve boundary.

Common questions

How long does it take for a laurel to die of root rot?

Months, not days. Usually a slow yellowing over one season and a bronze-off the next. That long fuse is what gets it blamed on whatever the weather did most recently.

If one plant has it, will the whole hedge get it?

Not necessarily. The pathogen is already in most soils. What spreads it is the wet, so the plants that go next are the ones sitting in the wettest ground, not the ones nearest the last casualty.

Can I replant a laurel in the same hole?

Only once the water problem is sorted. If nothing about the drainage has changed, you're buying the same plant twice.

Does adding compost fix a wet strip?

No, and it can make it worse in a boxed-in bed. Organic matter holds water. What a trapped strip needs is an outlet and coarse material, not richer soil.

Is there a laurel that copes with wet feet?

Not one we'd put our name to. All the laurel group want moist but free-draining ground. If the site holds water, change the plant rather than hunting for a tougher laurel.

The short of it

Wet ground kills more Portuguese laurel in New Zealand than frost ever has. The plant is tough enough for anywhere in the South Island, so when one dies the frost gets the blame by default. Dig a hole beside a sick one before you buy a single replacement.

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