Westringia 'Grey Box': A low-maintenance hedge for Kiwi gardens

There's a certain look a low clipped hedge gives a place. A soft grey-green line running along a path edge, or squaring off a courtyard, holding its shape right through the year while everything around it comes and goes. No fuss, no drama. Just structure, sitting there doing its job.

That's the picture Westringia 'Grey Box' sells. And to be fair to it, it delivers on that picture more often than not. It has become the first thing a lot of people reach for when they want the Buxus look without the buxus headaches.

But it is not a plant for everywhere. The advice floating around about it contradicts itself in one important place, and that contradiction has cost people hedges. So before you order a few hundred for a job, worth getting straight on what it will and won't do.

What is Westringia 'Grey Box'?

A compact, tight-growing shrub with fine grey-green foliage and small white flowers through summer.

Botanically it's a selected form of Westringia fruticosa, sometimes sold as coastal rosemary, which tells you plenty about where it comes from. The Australian coastline, not a New Zealand hillside. So it's an exotic, whatever "native box" wording you might see on a label.

Left alone it makes a tidy ball, somewhere around 40 to 60cm high and about as wide. Trimmed, it will hold as low as 30cm. That size range is exactly what makes it useful. It stays where you put it without you having to fight it every season.

Why do people pick it over Buxus?

Three reasons, mostly.

It doesn't get box blight. That alone has shifted a lot of jobs off buxus in the last few years – and it's the main reason people switch.

It fills in faster. Buxus is a slow customer, and a low buxus hedge is a long-term project. Westringia gets to a usable line noticeably quicker.

It handles dry and wind well. Once it's away it copes with drought and exposure better than most fine-leaved things of that size, which counts for a lot on a Canterbury job where the summer wind pulls the moisture out of soft new growth before you've finished unrolling the hose.

And it flowers, which buxus doesn't. Small white flowers over the grey through summer. Not the reason you'd plant it, but you'll take it.

So will it survive a hard frost?

This is the part you need to be careful with, and it's why a fair few Grey Box hedges have disappointed people.

Read the New Zealand nursery listings and you'll see "frost hardy" repeated almost everywhere. Read the Australian and overseas horticultural sources for the same species and you get a very different account: Westringia fruticosa rated around USDA zone 9, comfortable to roughly -1C for short spells, with shelter recommended anywhere properly cold. Those two descriptions can't both be true in the way people read them.

My read is that the truth sits in the gap. "Frost hardy" in a New Zealand catalogue usually means it copes with the sort of light frost most of the country gets, and on a milder coastal site that's a fair description. It does not mean it will shrug off a Central Otago winter, or a hard clear night out around Lumsden with a first-year plant sitting there with barely any roots down.

So the practical version. On the coast, and through the milder parts of Canterbury, it's a reasonable bet. Inland Central Otago, high country, or any place that goes hard and clear on winter nights, I'd think twice. And everywhere, a first-winter plant is far more exposed than an established one.

What actually kills it?

Wet ground, and it isn't close.

Westringia wants free-draining soil and it wants it badly. Sat in heavy, cold, sodden ground it goes into a slow fade and often finishes up with phytophthora root rot. Clay lines. Low corners where the water still sits three days after rain. Anywhere the ground stays greasy from May to September. Those are the spots that will cost you the hedge, and they'll do it quietly enough that you won't spot it until the plants start going.

That matters more through the low country and down our way than most places, because our winter trouble isn't really the temperature reading. It's that the ground never gets a chance to dry out between one rain and the next. Weeks of it, then months.

If the line you're planting into is at all doubtful, the fix isn't a better plant. It's better ground. Raise the bed, break the pan, get the water moving somewhere. Same as any building job, get the base wrong and everything sitting on top of it is on borrowed time.

How far apart should it go, and when?

For a low hedge, 25 to 30cm centre to centre is what most growers recommend. That's tighter than you'd space nearly anything else, and it's the trade-off for a small plant: more per metre, but the line closes up quickly. 25cm if you want it filled in sooner, 30cm if you're patient and watching the plant count.

Full sun is best. It will take part shade, but it thins out and gets leggy in anything deeper, and a leggy Westringia never really comes back to tight.

Autumn planting suits it as well as it suits most things. The soil is still carrying summer warmth, the air's cooling off, and the plant gets its roots working before it has to carry any real top growth. One caveat though, given the frost question above. A late autumn planting on a cold inland site is asking a young plant to face its hardest winter with the least to fall back on. On those sites I'd lean towards spring and give it a full season to get established first.

How do you trim it?

Little and often, and never hard back into bare wood.

Westringia doesn't reliably shoot again from old bare stems the way some hedging will. Cut back past the leaves and you can be left with a hole that just sits there. Keep the shears inside the green, two or three light trims through the growing season, and it stays dense on its own.

Feed lightly in spring if the ground's poor. It isn't a hungry plant, and pushing it into soft growth heading towards a cold snap does it no favours at all.

What if my site is too cold or too wet for it?

Then don't force it. There are hardier things that give you a similar low, grey, well-behaved line.

Brachyglottis monroii

The closest match in feel. Dense and low, around a metre if you leave it alone, with leathery olive-grey foliage and fuzzy undersides that catch the light. It's rated considerably hardier than Westringia and takes wind, coastal exposure and dry in its stride. A light prune after flowering keeps it tight instead of woody. Same drainage requirement, mind. It solves your frost problem, not your wet one.

Podocarpus nivalis (snow totara)

The one for genuinely cold ground. It's an alpine plant, so alpine conditions are its home patch: frost, wind, exposure, poor soil. Deep green to blue-green needle foliage that often bronzes off in the cold, which some people love and some don't. Naturally low and spreading, and it clips into a tidy low hedge with a light going-over once or twice a year. Slow, so budget for time rather than money.

Ozothamnus vauvilliersii (mountain cottonwood)

If you want silver rather than grey, and a bit more height. Small leaves with silver hair underneath, frost and wind tolerant, free-draining sites.

None of these is a like-for-like swap for Grey Box. But on a site where Westringia would spend its whole life on the edge of what it can handle, a plant that's at home there is worth more than one that looked right in the catalogue.

Where does that leave it?

Westringia 'Grey Box' is a good plant that gets recommended too broadly. Get the site right, full sun, free-draining ground, no brutal frosts, and it will hold a clean low line for years with almost nothing asked of you. Get the site wrong and no amount of watering, feeding or good intentions will save it.

If you're weighing it up for a job and you're not certain the site suits, tell us where it's going and what the ground does in winter. We'll give you a straight answer, including the times when the answer is something else.

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