Preventing Sprouting in Stored Potatoes and Onions: Methods and Impacts

Every year, millions of tonnes of potatoes and onions are lost not in the field, but in storage – and sprouting is one of the biggest culprits. Once a potato or onion begins to sprout, it rapidly loses moisture, shrinks, becomes susceptible to microbial infection, and drops sharply in commercial value. Post-harvest sprouting is considered a prime concern limiting the quality of onions during storage, while for potatoes, uncontrolled sprouting can render entire batches unfit for processing or sale. Managing sprouting effectively – through the right combination of chemical treatments, irradiation, and storage conditions – is one of the most critical skills in post-harvest produce management.

Why sprouting happens and why it matters
Chemical sprout suppressants
Maleic Hydrazide (MH)
CIPC (Chlorpropham)
Ethylene gas for onions
Ionizing radiation for sprouting inhibition
Dose requirements and timing
Regulatory status and public perception
Storage conditions: the foundation of sprout control
Temperature management
Humidity and ventilation
Emerging and alternative approaches

Why sprouting happens and why it matters

Both potatoes and onions are living biological systems. After harvest, they enter a natural dormancy period during which sprouting is temporarily suppressed. Once dormancy ends, tubers begin to sprout, reducing their quality, market value, and suitability for processing. In onions, dormancy passes through three distinct physiological stages – rest, dormancy, and sprout – governed by internal hormone levels such as cytokinins, gibberellins, and auxins.

The consequences of sprouting go well beyond appearance. Sprouting of potatoes during storage results in increased weight loss, increased respiration, decreased nutritional quality, and eventually food waste and economic loss. In onions, the process of sprouting and rooting causes quality losses during long-term storage and reduces market potential. In countries like India, storage losses in ambient conditions can reach as high as 40-50%. In developing countries more broadly, post-harvest losses from production to retail can account for approximately half of total produce losses.

Sprouting is not inevitable, however. A range of proven methods – chemical sprout suppressants, ionizing radiation, and controlled storage conditions – can dramatically reduce these losses.

Chemical sprout suppressants

Chemical inhibitors remain the most widely used tools for commercial sprout control. Two compounds in particular – Maleic Hydrazide (MH) and Chlorpropham (CIPC) – have shaped post-harvest potato and onion storage for decades, each with distinct modes of action and application methods.

Maleic Hydrazide (MH)

Maleic Hydrazide is a plant growth regulator that reduces growth by preventing cell division, but not cell enlargement. It is applied as a foliar spray to standing crops – potatoes, onions, garlic, and carrots – several weeks before harvest, not after. The compound is translocated from the leaves into the developing tubers, where it inhibits cell division in the meristematic tissue, thereby delaying sprout initiation and elongation during storage.

Timing of MH application is critical. If applied too early, it may reduce yield and tuber size, while late application diminishes its effectiveness. When properly applied, MH can delay sprout initiation by about 30 days and suppress sprout elongation for 6-8 months. For onions specifically, MH is generally applied a few weeks before harvest to inhibit sprouting during long-term storage.

MH residues are systemic and provide long periods of reduced sprouting potential, making it particularly effective for long-term storage where an initial treatment reduces the need for repeated post-harvest applications. It is also useful for controlling internal sprouting, a risk that can increase when post-harvest treatments require frequent re-application.

From a regulatory standpoint, MH is actively registered for use in the US for potatoes and onions, with permitted residue levels of 50 ppm in potatoes and 15 ppm in dry bulb onions, though it is banned in EU countries. The US EPA classifies it as a low-toxicity compound, and its genotoxic hazard is considered negligible based on several negative cancer studies, though strict application guidelines must be followed.

CIPC (Chlorpropham)

Chlorpropham (CIPC) has been the most widely used potato sprout inhibitor since the 1950s. It is a low-toxicity carbamate herbicide that acts as a mitotic inhibitor, preventing cell division in developing sprouts. Unlike MH, CIPC is applied post-harvest, after wound healing. It is most commonly applied by thermal fogging through ventilation systems as an aerosol, ensuring uniform distribution throughout the storage facility.

Residue levels of CIPC strongly influence efficacy – higher residues provide longer suppression, often up to 9 months in varieties like Russet Burbank stored at 7°C. The good news for consumers is that cooking significantly reduces CIPC residues: boiling or steaming potatoes results in reduced CIPC residues in cooked tubers due to leaching into the cooking water.

It is important to note that CIPC’s regulatory status has changed significantly. The compound was withdrawn from use in the EU in 2020 following safety concerns, which has led to renewed global interest in MH and other alternatives for potato sprout control.

Ethylene gas for onions

For onion storage specifically, ethylene gas has emerged as an effective and cleaner alternative. Ethylene gas is the only post-harvest application product available for controlling sprout growth on onions, and it does not leave a residue on the produce, allowing an immediately market-ready product. Commercial systems like the Restrain Generator produce low, controlled levels of natural ethylene gas throughout the storage period. Some onion producers use ethylene in combination with MH to extend the storage season, giving growers management flexibility not previously available.

Ionizing radiation for sprouting inhibition

Gamma irradiation is one of the most effective physical methods available for long-term sprout control in both potatoes and onions. It is a cold process that works by damaging the meristematic (growing) tissue of the produce. Irradiation results in sprout inhibition when applied after harvesting, as the produce is in an active metabolic state and more sensitive to irradiation, which disrupts nucleic acid, nucleotide, and hormonal synthesizing systems and therefore prevents sprouting.

Dose requirements and timing

The effectiveness of irradiation depends critically on dose and timing. For potatoes, the best results are obtained at doses of 10-12 krad (100-120 Gy), while higher doses of 15-20 krad can cause physical damage to the tubers. For onions, doses of 6-9 krad gave the best results when stored at room temperature, with no sprouting detected over a 10-month period.

Timing is just as important as dose. A dose of 60 Gy gave maximum sprout inhibition in onions when applied a fortnight after harvest, when the bulbs were in the deepest period of dormancy. Delaying irradiation beyond this window reduces its effectiveness – and may actually stimulate sprouting at lower doses.

Compared to chemical treatment, irradiation offers some distinct advantages for potato quality. Irradiation caused complete sprouting inhibition for up to 10 months, while chemical treatment controlled sprouting for only 7-8 months. Weight loss was also higher in chemically treated samples, and irradiation did not negatively affect Vitamin C or niacin content compared with chemical treatment.

Regulatory status and public perception

Food irradiation for sprouting inhibition is approved in more than 40 countries. In India, the Bhabha Atomic Research Centre (BARC) in Mumbai established a commercial onion irradiator near Nasik specifically to reduce post-harvest losses. The FDA permits low doses of gamma radiation for fresh produce preservation, and it is recognized internationally as a safe and effective post-harvest technology. Despite scientific consensus on its safety, consumer acceptance remains a challenge in some markets.

Storage conditions: the foundation of sprout control

Chemical and physical treatments are most effective when supported by proper storage conditions. Temperature and humidity are the two primary variables that determine how quickly sprouting begins after dormancy breaks.

Temperature management

For potatoes, the most prevalent long-term storage method worldwide is at 8-12°C with 85-90% relative humidity, typically combined with CIPC or another sprout suppressant. However, there is a critical trade-off: stored potatoes will sprout at temperatures above 5°C, but at temperatures below 7°C, starch converts to sugar – creating an unacceptably dark colour in cooked products like chips and crisps. This narrow temperature window is why chemical intervention is considered virtually essential for commercial potato storage.

For onions, the recommended storage temperature is 0-5°C at 65-70% relative humidity for a potential storage duration of 6-8 months. Onions stored at intermediate temperatures – roughly 10-18°C – are most prone to sprouting, which is why both very cold and very warm storage can suppress it through different physiological mechanisms.

Humidity and ventilation

Sprouting in onion bulbs generally occurs when the dormancy breaks at an ideal temperature of 18-25°C combined with relative humidity above 85%, which elevates endogenous ethylene levels and accelerates metabolic activity. High humidity in onion storage therefore directly accelerates sprouting. Adequate ventilation is equally important: for bulk storage of onions or garlic, ventilation systems should provide airflow from the bottom of the storage room at a rate of 2 cubic feet per minute per cubic foot of produce. Uneven loading of potatoes in bulk storage inhibits air movement and leads to storage losses from inadequate ventilation.

Emerging and alternative approaches

Growing concerns about chemical residues and the withdrawal of CIPC from EU markets have accelerated research into greener alternatives. Natural compounds derived from essential oils – particularly spearmint, clove oil, and S-carvone – have shown real promise. Clove oil is an approved sprout inhibitor under organic food programs, and while its effects are temporary (lasting approximately two to three weeks), it can eliminate visible sprouts rapidly and is suited for organic production.

S-carvone is a natural volatile that leaves little or no residue and is used commercially in organic potato stores in countries like the Netherlands and Switzerland, where it is marketed under the trade name Talent™. Research published in Scientific Reports has also identified simple maleic acid and l-tartaric acid as cost-effective alternatives capable of hindering sprouting for up to 6 and 4 weeks respectively, without affecting moisture content or tuber quality – a useful finding particularly for developing countries where commercial suppressants may be prohibitively expensive.

The direction of travel is clear: integrated sprout management – combining pre-harvest chemical treatment, well-timed post-harvest application or irradiation, and precise control of storage temperature and humidity – consistently outperforms any single approach used in isolation.

What do you think? Given the trade-offs between chemical residues, consumer acceptance, and storage effectiveness, which sprouting inhibition method do you think is most suitable for small-scale farmers in tropical climates – and should food irradiation receive more mainstream adoption as a post-harvest technology?

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Table of Contents

Why sprouting happens and why it matters 🔗

Chemical sprout suppressants 🔗

Maleic Hydrazide (MH) 🔗

CIPC (Chlorpropham) 🔗

Ethylene gas for onions 🔗

Ionizing radiation for sprouting inhibition 🔗

Dose requirements and timing 🔗

Regulatory status and public perception 🔗

Storage conditions: the foundation of sprout control 🔗

Temperature management 🔗

Humidity and ventilation 🔗

Emerging and alternative approaches 🔗