The green is chlorhyll, which tubers make when they're exposed to light. Chlorophyll itself is harmless. The concern is that light also raises glycoalkaloids, mainly solanine and chaconine, which potatoes normally carry in small amounts. Light-exposed tubers can hold many times the normal level. The green usually shows as a patch on the side that faced up, or as a green tint under the skin when you peel. Greening doesn't reverse once a potato goes back into the dark.
In the garden, the usual cause is tubers near the surface. Potatoes form along the buried stem, and those close to the top can push up into the light if they weren't hilled or deeply enough, if rain washed soil off the hill, or if the soil cracked. A see-through or -walled container can let light reach tubers too. After harvest, potatoes keep greening wherever they get light: left out in the sun to dry, on a farmers market table, or in a bright kitchen. Even indoor light can do it.
Here's the practical line. A potato with a little green on the surface can be eaten after you peel it and cut away the green flesh and any sprouts or eyes generously, since glycoalkaloids are concentrated in and right under the skin. A potato that's green deep into the flesh, or heavily greened over much of its surface, belongs in the . Taste is a useful warning. If a cooked potato tastes bitter or makes your mouth burn, stop eating it. Cooking doesn't break these compounds down. Large amounts can cause stomach upset, headache, and confusion.
For the potatoes you've already dug, sort them now. Set aside the clean ones, trim the lightly green ones as you use them, and discard any that are green through. Store the rest in complete darkness, in a paper bag, a closed box, or a covered bin, somewhere cool, around 40 to 45°F. Don't store them in the refrigerator, because cold below about 38°F turns starch to sugar.
To prevent greening next season, hill soil or straw up around the stems every couple of weeks as the plants grow, leaving the top 4 to 6 inches of leaves exposed. Cover any tubers you see breaking the surface. In containers, use opaque pots or bags and keep adding mix over the stems. At harvest, get potatoes out of the sun within a few hours and cure them somewhere dark.
- Colorado Potato BeetleStriped orange-yellow beetles and soft orange larvae heavily defoliating potato and eggplant.
- Late BlightLarge water-soaked leaf lesions with white fuzzy growth beneath; plants can collapse within days.
- Corn Earworm / Tomato FruitwormCaterpillars eating corn kernels from the tip; same species bores into tomato and pepper fruit. Often called 'tomato fruitworm' when found on tomato.
- Early BlightDark spots with concentric rings on lower leaves that yellow and drop, working up the plant.
- Black RotV-shaped yellow lesions at brassica leaf margins with blackened veins inside — a bacterial disease that moves through the vascular system.
- Why are some of my potatoes hollow inside when I cut them?Hollow heart in potatoes is caused by rapid, uneven growth — usually after a period of drought followed by heavy watering or rain — and is not a disease.
- How do I store winter squash so it lasts through winter?Cure winter squash at 80–85°F for 10–14 days to harden the skin, then store at 50–60°F in a cool, dry location — not the refrigerator.