What Is Maleic Hydrazide and How Does It Work?

Maleic hydrazide is a man-made chemical that farmers spray on crops not to kill them, but to make them stop growing. Its biggest job is keeping potatoes and onions from sprouting for months after harvest, which is why it can turn up on supermarket produce. It works by pausing the plant’s own cell-making, a little like putting growth on hold rather than poisoning the plant. Safety regulators in the US and Europe rate it as low in toxicity to people, and the chief worry was an impurity that is now capped by law.¹

What Maleic Hydrazide Is

Maleic hydrazide is a plant growth regulator: a chemical that changes how a plant grows rather than killing pests or weeds. A herbicide is meant to kill plants. Maleic hydrazide at normal rates slows or stops growth instead, and it goes on the crop itself, not on weeds around it. Because the plant stays alive, the crop still ripens and can be harvested. It is still regulated as a pesticide product, even though it does not kill pests.

Chemists first made it in a lab in 1895, but nobody knew it affected plants until 1949.² It was first registered for use as a plant growth regulator in the US in 1952.³

Manufacturers make it by reacting hydrazine with maleic anhydride and then neutralizing the result with an alkali.⁴ Traces of leftover hydrazine can therefore end up in the finished product, and that is the point safety rules focus on. It is often sold as a salt, such as the potassium salt, dissolved in water for spraying. Farming guides sometimes shorten it to MH.

How It Stops Plants From Growing

It is sprayed on leaves and soaks in through them. The plant’s own sap transport then carries it down from the leaves into tubers such as potatoes.⁵

A potato’s buds sit below ground, so they cannot be sprayed directly. While the plant is still green and making sugars, it sends those sugars down to fill the tubers, and the chemical rides along in that flow. If the tops have already died, that flow has stopped, so a late spray never reaches the tubers in useful amounts.

Plants grow by making new cells at the tips of shoots, roots and buds. Maleic hydrazide inhibits cell division in those places.⁶ In root tips it disrupts the machinery that pulls chromosomes apart as a cell splits, causing faulty divisions.⁷ Without new cells, a bud cannot grow into a sprout or shoot.

It also shifts the plant’s hormones. Treated onions stored over winter showed less activity of growth-promoting hormones than untreated ones.⁸ In treated peanut seeds, a dormancy hormone roughly tripled while key growth hormones fell by nearly 90% or more.⁹

Mature leaves and stored tissue already exist and keep working, and only new growth that needs fresh cells is held back. At the right rate the effect is a pause; at too high a rate it can harm the crop.

In storage, warmth and time normally wake a potato’s buds, or eyes. With maleic hydrazide in the tuber, the bud’s cells cannot divide, so the eye stays closed and the potato stays firm.

What Crops It Is Used On

Onions treated in the field sprout far less during and after storage.¹⁰ Sprouted potatoes and onions lose weight, firmness and market value, so sprout control lets crops be sold well into the following season. Once moved to room temperature, about 62-68% of untreated onions sprouted versus 10-16% of treated ones.¹¹ When cut open, over half of treated bulbs had no internal sprout, compared with under a fifth of untreated bulbs.¹² An onion with an internal sprout may look fine from outside but is hollowed and soft when cut, which is why the cut-open count matters to buyers.

For potatoes, timing is everything. Growers spray potatoes once in the season, after most tubers reach about 25 mm (1 inch) across.¹³ It must also go on at least 3 weeks before the tops are killed off before harvest.¹⁴ “Tops killed off” refers to a common step before harvest: growers kill the green potato vines so the tuber skins set and the crop digs more easily. Spraying too early can lower yield, and spraying too late gives a shorter dormant period.¹⁵

On tobacco, the target is different. When the flowers are cut off to boost leaf yield, the plant pushes out side shoots called suckers.¹⁶ Growers use maleic hydrazide instead of breaking suckers off by hand, a slow and tedious job.¹⁷ At the right rate it does not kill suckers but stops them growing further.¹⁸ It can also slow small top leaves, so plants are topped to leaves no shorter than 6 inches.¹⁹ Treated tobacco gave higher yield and value per hectare than hand-suckered tobacco.²⁰

It was also the first growth regulator used on turf.²¹ Road crews have used it to slow grass growth and stop seedheads on roadsides, cutting mowing.²²

Home gardeners are unlikely to find it useful. Products are mostly sold for commercial growers and turf managers, and label rates and timing are precise and easy to get wrong.

Is It Safe for People

Maleic hydrazide has low toxicity whether swallowed, absorbed through skin or breathed in, placing it in the US EPA’s lowest toxicity category.²³ It does not irritate skin or eyes or trigger skin allergies.²⁴ Animal studies showed no harm to fertility, unborn young or the nervous system.²⁵

That category describes a single exposure, such as a spill or accidental swallowing. Long-term eating of small residues is a different question, which is why the long-term animal studies and cancer reviews are reported separately.

On cancer, the US EPA classified maleic hydrazide itself as showing evidence of not causing cancer in humans.²⁶ European reviewers judged it unlikely to damage genes or cause cancer, provided the hydrazine contaminant is kept low.²⁷ Some lab-dish tests at high concentrations did show gene effects.²⁸ Those effects were likely caused by the very salty test conditions, and tests in live animals were negative.²⁹ Lab-dish results do not always match what happens in a body, so regulators weigh live-animal evidence more heavily.

The highest daily dose with no harm in long animal studies was 25 mg per kg of body weight.³⁰ For a 70 kg adult, that animal no-harm level would work out to 1,750 mg a day. Safe intake limits for people are set well below it, with safety margins, and residues on food are many times lower than such amounts.

How Much Ends Up in Food

Each food has a maximum residue level (MRL), the most of a chemical legally allowed to remain on it. The international limit for garlic, for example, is 15 mg per kg.³¹ Labs can detect it down to about 0.33 mg per kg, well below that limit.³²

Limits come from residue levels measured in field trials where the product was used as directed, then set a bit above what those trials found. A legal residue limit is therefore a ceiling for farming practice, not the point at which food becomes unsafe. Food above it can be rejected at the border or in the market, even if the amount would not be dangerous.

A worst-case European estimate that assumed every food carried the full legal limit came out above the safe daily intake.³³ Using typical residues actually found in trials, the highest estimate was about 30% of the safe daily intake, for Dutch toddlers.³⁴

Washing and peeling help less than you might hope. The chemical moves inside the tuber or bulb, so it is not only on the surface, and washing removes dirt but not what is held inside. Peeling may cut some residue, but no figure exists to rely on. Organic produce rules do not permit synthetic sprout suppressants like it, so organic potatoes and onions should not have been treated with it.

Treated tobacco leaf can carry residues too. Some trials found lower residues and nitrosamine levels when it was used for chemical topping.³⁵ Any residue is minor compared with the known harms of smoking itself.

The Hydrazine Contaminant

Factory-grade maleic hydrazide contains hydrazine as an impurity.³⁶ Hydrazine is a reactive chemical left over from making the product, and it has been linked to tumours.³⁷ So in 1982 the EPA capped hydrazine at 15 ppm in maleic hydrazide products, a level judged not to raise lifetime cancer concern.³⁸ The limit applies to the product itself, before it is diluted for spraying, so the amount reaching a field is smaller still.

Lab testing found hydrazine levels of 0.03 to 0.26 ppm in technical product and formulations.³⁹ Those samples held far less than the 15 ppm legal cap.

Effects on Wildlife and Soil

Maleic hydrazide is practically nontoxic to birds, fish, water invertebrates and honey bees.⁴⁰ That is a regulatory label meaning very high doses were needed to harm test animals, far above anything they would meet in a field. Expected levels in water also fall below any level shown to harm fish, water insects or water plants.⁴¹

The real risk is to other plants. It does exceed levels of concern for plants on land and in wet margins near treated fields.⁴² Spray drift onto gardens, hedges or wild plants is therefore the main environmental worry.

Affected plants tend to stall at the shoot tips rather than brown and die. New leaves stay small or fail to appear, and flowering and seed heads may be reduced, while older growth usually looks normal. Anyone near treated fields or roadsides who sees stunted plants can ask about spraying dates.

In ordinary living soil, nearly half broke down within 20 days.⁴³ In sterilised soil hardly any broke down, showing soil microbes drive the breakdown.⁴⁴

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