The chemical that finishes New Zealand's potatoes

I keep my potatoes the way you're told to — paper bag, dark cupboard. The organic ones I buy start to sprout in three or four weeks, pale shoots pushing out of the eyes. It's completely normal. A potato is a living thing, and what it wants to do is become more potatoes.

So here's the small question that started a long one. Why doesn't the supermarket bag do that? Why can a conventional spud sit for months and stay hard and blind, while mine wake up by the end of the month?

The answer is that a commercial potato is sprayed at three separate stages on its way to you — while it grows, at harvest, and in storage — and at each stage the lead chemical is one the European Union has banned. None of it appears on the bag.

The crop hiding in plain sight

The potato is New Zealand's largest vegetable crop: around 419,000 tonnes a year off 8,400 hectares, worth close to a billion dollars. But only about 40 percent reaches the table as a recognisable spud. Nearly 56 percent goes to processing — chips, fries, crisps. More than half the crop is grown not to be a potato but to be turned into something else, to a specification, in a factory. "Can New Zealand Feed Itself?" That changes how heavily it's worked. A potato gets three chances to be sprayed; let's follow them in order.

Stage one: growing

This is the heaviest stage, and the one no one pictures. The reason is a single disease — late blight, Phytophthora infestans, the water mould that rotted Ireland's crop in the 1840s. It still moves through a damp field in days, and the only conventional defence is to keep the foliage coated in fungicide, often weekly: eight or ten sprays in a wet season.

The workhorse has long been mancozeb — and it's the first of our three. The EU banned mancozeb for agriculture from January 2022, the regulator calling it an endocrine disruptor, classing it as toxic to reproduction, and flagging its breakdown product ETU as a probable carcinogen and water contaminant. In New Zealand it remains registered and routinely used, and the national Total Diet Survey has duly found dithiocarbamate fungicide residues on potatoes. It has company — newer late-blight fungicides like azoxystrobin (Amistar) and Zampro in rotation, insecticides against the tomato–potato psyllid, and, late in the season, maleic hydrazide, a sprout suppressant sprayed on the leaves to do a storage job months ahead of storage. The growing potato is simply the most-sprayed potato — treated more often, with more compounds, than at either stage people actually worry about.

Stage two: harvest

Potatoes don't ripen to a schedule. At some point the grower needs the green tops gone — to set the skins, cap tuber size, and let the harvester work. This is desiccation: kill the top to lift the bottom. Mostly it's done with a herbicide, and the one that finishes a New Zealand crop is diquat, our second EU-banned chemical.

The European Commission withdrew diquat in 2018 over risks to operators, bystanders and wildlife; the UK and China followed. The US, Canada and Australia kept it — so the EU is the cautious outlier, not New Zealand the reckless one. But it's the outlier worth noticing. Here, diquat is registered for exactly this use: "potato haulm destruction," on the label. Tellingly, New Zealand did restrict diquat's close cousin paraquat in 2019 — it looked at one member of the family and acted, and left the one finishing the potato crop alone.

In concentrated form diquat is a serious poison — kidney damage, no antidote — but those harms come from swallowing it, not from food. Little reaches the flesh, because diquat is a contact herbicide that isn't carried through the plant: it's sprayed on the tops, and the tuber sits underground behind its skin. Newer research shows it damages the gut and its bacteria so reliably that scientists use it to induce gut damage in the lab — but only at high doses; no one has studied what a food-level dose does to the human microbiome, and the safety assessments never looked. (Glyphosate, for the record, is a red herring on potatoes — pre-harvest desiccation with it isn't even approved here.

Stage three: storage

Left alone, a potato breaks dormancy and sprouts — making more glycoalkaloids as it goes, which is why you're told to cut sprouts and green skin out. For a crop that must last between harvests, the industry overrides that two ways. The first we've met: maleic hydrazide, sprayed on the leaves and drawn into the tubers. Its toxicity is low, but it's systemic, so it genuinely ends up in the flesh.

The second is the third of our EU-banned trio: CIPC (chlorpropham), the dominant storage suppressant for half a century, fogged through sheds to keep spuds blind for the better part of a year. The EU banned it from 2020 over dietary risks from its breakdown product, 3-chloroaniline. New Zealand keeps it legal — chlorpropham still has a potato residue limit on the books — though I could find no current local product, and our staggered year-round harvest means we need long chemical storage far less than single-harvest Europe. The likely picture is maleic hydrazide in the field, cool storage, and CIPC kept on the books more than in the shed. But no one publishes it.

The pattern, and who's watching

Stand back and the shape is hard to miss: at each stage the lead chemical is one the EU banned and New Zealand kept — mancozeb growing, diquat at harvest, chlorpropham in store. The EU worked through them one by one; we kept all three. You can argue the EU is precautionary to a fault. But it's a striking thing to notice on something as ordinary as a potato.

So what keeps any of it in check? Mostly an assurance scheme, NZGAP, under which certified growers keep audited spray diaries recording every application. So each spray is recorded — just not for you. The same system can trace the bag in your kitchen back to the production line; the information chain runs all the way from paddock to bench and stops one link short of the eater. Government testing is thin — a few hundred samples a year, built around a standard screen that doesn't even capture diquat, which needs a dedicated test of its own; there's no sign that test is run on potatoes. And Greenpeace notes the regulator hasn't prosecuted a single residue breach in a decade, even when it found a banned pesticide at many times the legal limit. The cheerful front-of-house, meanwhile — the recipes-and-nutrition world of vegetables.co.nz — never raises any of this. Not from secrecy. It simply isn't anyone's job to tell you.

The alternative

There's exactly one label that covers all three stages, and that's certified organic: no mancozeb, no diquat, no chlorpropham. Which is why my organic potatoes sprout — that's not a failure of freshness, it's the absence of the chemistry that holds the others still. The catch is how little there is: well under one percent of the crop, almost entirely table potatoes, so organic fries essentially don't exist — organic stays a rare exception here, as it is in other aisles. You'll find it fresh at the likes of Commonsense Organics and Farro, through a veg-box or CSA scheme, at farmers' markets where you can ask the grower directly, or in your own garden — a potato is almost aggressively willing to grow.

The bag tells you the variety, the region, maybe the grower. It doesn't tell you what it was sprayed with while it grew, how the plant was killed, or what kept it asleep on the way to the shelf. The only label that answers all three is the organic one. It costs more, and it sprouts in your cupboard. Now you know why on both counts.