When a potato sprouts, it’s waking up from dormancy and trying to grow into a new plant. The tuber begins breaking down its stored starch and protein to fuel those white or purple shoots pushing out from the eyes. This process also ramps up production of toxic compounds called glycoalkaloids, which concentrate in and around the sprouts. A potato with small sprouts is generally still safe to eat after trimming, but the longer sprouting continues, the more the potato deteriorates in both nutrition and safety.
Why Potatoes Start Sprouting
A freshly harvested potato is in a dormant state, held there largely by a plant hormone that suppresses growth. Over time in storage, levels of this dormancy hormone decline while growth-promoting hormones rise. There’s no single threshold that flips the switch. Instead, it’s a gradual shift in the balance between these competing signals, and once the growth hormones win out, the eyes of the tuber activate and sprouts emerge.
Temperature is the biggest external trigger. Potatoes stored at room temperature (65 to 75°F) sprout much faster than those kept cool. Interestingly, extremes in either direction can also break dormancy: prolonged exposure to temperatures below about 35°F or above 86°F can end the resting period, causing sprouts to appear once the potato returns to moderate temperatures. Light, warmth, and humidity all accelerate the process.
What Changes Inside the Potato
Once sprouting begins, the tuber stops being a storage organ and starts acting like a fuel tank for a growing plant. Starch and protein break down into simple sugars and amino acids, which are shuttled from the potato’s interior toward the developing sprouts. This is why a heavily sprouted potato feels softer and more spongy: its reserves are literally being consumed from the inside out. The flesh loses density, moisture, and nutritional value the longer sprouting continues.
Vitamin C content also drops during storage and sprouting, and the overall caloric and nutritional profile of the potato degrades as more of its energy is redirected into growth.
Glycoalkaloids: The Toxic Compounds in Sprouts
Potatoes naturally contain two glycoalkaloids, solanine and chaconine, as part of their defense system against pests. In a healthy, unsprouted potato, these compounds sit mostly in the skin at low levels. Once sprouting begins, concentrations spike dramatically in the sprouts themselves and in the skin and flesh immediately surrounding them. Sprouts and leaves can contain levels exceeding 200 mg per 100 grams of fresh weight, while peeled tuber flesh from the same plant may have undetectable levels.
The toxic dose for humans is estimated at roughly 1 to 3 mg of total glycoalkaloids per kilogram of body weight, with potentially lethal doses in the range of 3 to 6 mg per kilogram. For a 150-pound adult, that means ingesting somewhere around 70 to 200 mg could cause serious symptoms, and higher amounts could be life-threatening. You’d have to eat a significant quantity of sprouts or heavily greened potato to reach those levels, but it’s not impossible, especially for children who weigh less and therefore hit toxic thresholds with smaller amounts.
Greening and Sprouting Are Related but Separate
The green color that appears on light-exposed potatoes is chlorophyll, which is harmless on its own. The problem is that the same wavelengths of light that trigger chlorophyll production, primarily blue and red light, simultaneously activate the genes responsible for glycoalkaloid production. Both pathways respond to the same light-sensing proteins in the potato, so greening and toxin buildup happen in parallel. A green potato isn’t dangerous because of the chlorophyll itself, but the green serves as a visible warning that glycoalkaloid levels have likely increased too.
Sprouting can happen without greening (in the dark), and greening can happen without sprouting (under light before dormancy breaks). But when both are present, the potato has elevated glycoalkaloid levels from two separate triggers at once.
Symptoms of Glycoalkaloid Poisoning
Glycoalkaloid poisoning is uncommon but real. Symptoms are primarily gastrointestinal: stomach pain, nausea, vomiting, and diarrhea. They’re often delayed, typically appearing 8 to 10 hours after eating the potato, which can make it hard to connect the symptoms to the meal. In more severe cases involving larger doses, neurological effects can develop, including headache, confusion, hallucinations, dilated pupils, and changes in heart rate or breathing. Historically documented outbreaks have involved potatoes with total glycoalkaloid levels above 20 mg per 100 grams, well above the typical range for store-bought potatoes.
When a Sprouted Potato Is Still Safe to Eat
A potato with a few small sprouts (under half an inch) that still feels firm is generally fine to eat. Cut out the sprouts and a generous margin of flesh around each one, peel the potato, and remove any green areas completely. The flesh of a firm, sprouted potato that isn’t green typically contains very low glycoalkaloid levels.
A potato you should throw away looks like this: long sprouts (over an inch), significant greening beyond a thin surface layer, or a body that’s soft, shriveled, or wrinkled. That shriveling means the tuber has lost substantial moisture and starch to the growing sprouts. At that point, the potato has degraded in both safety and quality past the point where trimming helps.
Cooking does reduce glycoalkaloid content somewhat. Boiling peeled potatoes is the most effective method, as some of the toxins leach into the cooking water. Frying and baking also reduce levels, but less so. However, glycoalkaloids are heat-stable enough that cooking alone won’t make a badly sprouted or deeply green potato safe. Peeling and trimming before cooking is the more important step.
How to Store Potatoes to Prevent Sprouting
The ideal storage temperature for potatoes is 42 to 50°F, which is cooler than most kitchens but warmer than most refrigerators. A basement, garage, or unheated pantry during cooler months often hits this range. The warmer the temperature, the faster sprouts develop. Darkness is essential since light drives both greening and glycoalkaloid production even before visible sprouts appear.
Keep potatoes in a ventilated container. A paper bag, perforated plastic bag, or open cardboard box all work. Sealed plastic traps moisture and promotes rot. Commercial potato storage uses 90 to 95% relative humidity to prevent shriveling, but most homes are much drier. A perforated bag helps retain some moisture around the potatoes without creating the conditions for mold.
One common storage tip is to keep onions away from potatoes. Onions release ethylene gas, which can trigger dormancy break in potatoes. The relationship is somewhat more complex than the simple advice suggests. Continuous ethylene exposure actually inhibits sprout elongation even as it breaks dormancy, which is why commercial facilities sometimes use ethylene supplementation. But in a home kitchen, the intermittent, uncontrolled ethylene from a bag of onions is more likely to start your potatoes sprouting without providing the consistent levels needed to suppress growth. Keeping them separate is the easier approach.