Nightshade Vegetables: 12 Garden Crops in the Family, Where Solanine Actually Concentrates, and Why Sweet Potatoes Don’t Count

Nightshade vegetables are crops in the Solanaceae family: tomatoes, potatoes, eggplant, peppers and tomatillos, plus lesser-known garden fruits like goji and ground cherries. Sweet potatoes are not on the list. They belong to the morning glory family. The compound behind the scary headlines, solanine, is a real toxin, but in a home garden it concentrates in one place: the skin of a potato that has turned green in the light.

That gap is large. A potato kept in the dark averages 12–20 mg of glycoalkaloids per kilogram. A green one runs 250–280 mg/kg, and its green skin reaches 1,500–2,200 mg/kg. The inflammation claims attached to the whole family are a separate question, and the evidence there is much thinner than the internet suggests.

Which Vegetables Are Nightshades? The 12 Garden Crops

Twelve crops cover almost everything a home gardener grows from the nightshade family, from the supermarket staples to goji and garden huckleberry. I count sweet and hot peppers separately because pungency changes how you handle them, even though both are Capsicum. The one pattern across the table: most of these crops are fine to eat when ripe and carry their highest alkaloid load in leaves, green fruit, sprouts or skin.

The twelve fall into four genera: Solanum (tomato, potato, eggplant, pepino, tamarillo, garden huckleberry), Capsicum (both pepper rows), Physalis (tomatillo, ground cherry, Cape gooseberry) and Lycium (goji). Genus matters when you read a disease table, because a pathogen that attacks one Solanum crop often attacks the next.

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Penn State Extension’s family rotation table also lists tobacco, petunias and million bells as Solanaceae, along with weeds like horsenettle, jimsonweed, buffalobur and wild ground cherry. The weeds matter later, in the rotation section.

What Makes a Plant a Nightshade?

Botanists define Solanaceae by flower structure and chemistry: petals fused into a single five-lobed corolla, usually five stamens, a berry or capsule for fruit, and alkaloids in most species. The University of Hamburg’s family description notes the anthers open through pores or slits and that most members produce poisonous alkaloids.

The flower is the fastest field test. Tomato, potato and eggplant blossoms differ in color (yellow, white to lavender, violet) but share the five-lobed outline with yellow anthers clustered in the middle.

That same shape appears on the wild, dangerous relatives, which is why identification matters. If you are trying to separate a crop volunteer from a toxic wild plant, our nightshade flowers identification guide covers the look-alikes.

Why Sweet Potatoes Are Not Nightshades

Sweet potato (Ipomoea batatas) is in the morning glory family, Convolvulaceae, and NC State Extension calls it only distantly related to potato. What you eat is a swollen root rather than a stem tuber, and the vine’s flowers look like a morning glory’s funnel, not a nightshade’s star.

That has a practical payoff. Virginia Cooperative Extension states that late blight’s hosts are limited to the nightshade family, so sweet potatoes are outside that disease, and our sweet potato growing guide covers the problems they do have.

Solanine and the Other Alkaloids: Where They Concentrate

Solanine and its partner chaconine make up more than 95% of the glycoalkaloids in potatoes, and Oregon State Extension reports that light has the greatest effect on how much builds up. Tomatoes carry a related compound, tomatine, in their green fruit.

The takeaway: a green potato carries roughly 12 to 23 times the glycoalkaloids of a normal one, and its green skin runs about 75 to 180 times higher. The tomato figures are converted from the paper’s mg/100 g (0.03–0.6 ripe, 4–17 green). Tomatine and solanine are different molecules, so do not read the 200 mg/kg potato ceiling as a verdict on green tomatoes.

The potato rules that matter

Oregon State puts the bitterness threshold at 140 mg/kg and a burning sensation in the mouth and throat above 200 mg/kg. Market potatoes should average under 200 mg/kg, and the FAO/WHO considers anything under 100 mg/kg not a concern. Symptoms such as nausea, vomiting and cramps show up 30 minutes to 12 hours after eating, according to Hong Kong’s Centre for Food Safety.

• Green is a warning flag, not the poison. Greening and glycoalkaloid formation are two independent light-driven reactions, so a green tuber signals light exposure and high alkaloids, but you cannot judge safety by color alone.
• Peeling helps, cooking doesn’t. Hong Kong’s Centre for Food Safety says 30–80% of the glycoalkaloids sit in the peel. Oregon State says glycoalkaloids are heat-stable: boiling has a negligible effect, and frying at 210°C (410°F) removed only about 40%.
• Cutting makes it worse. Slicing triggers a burst of synthesis. In Oregon State’s test, peeled Russet Burbank slices under fluorescent light went from 2 to 74 mg/kg in 48 hours, so keep tubers whole until you cook them.
• Store them dark and cool. Oregon State recommends 5–8°C (41–46°F) with humidity above 90%. In the garden, hilling soil over the developing tubers blocks the light that drives this, which is my inference from the mechanism rather than a tested recommendation in these sources.

Tomato, ground cherry and huckleberry: ripeness is the rule

Tomatine drops sharply as the fruit ripens. Choi et al. (2010) tracked 11 ripening stages and found dehydrotomatine fell from 48.2 mg/100 g in the greenest fruit to undetectable at the red stages. Ground cherries and Cape gooseberries follow the same logic: NC State lists solanine and related alkaloids in every part except the ripe fruit. Garden huckleberry is the strictest, and the Arizona-Sonora Desert Museum says to harvest about two weeks after the berries turn black, when the skin dulls and softens, and usually to cook them.

The nicotine question

Yes, nightshade vegetables contain nicotine, because it is another alkaloid the family makes. Siegmund and colleagues (1999) found 2–7 micrograms per kilogram in fresh tomatoes, potatoes, aubergines and peppers, falling as tomatoes ripen. Their estimate of average dietary intake was about 1.4 micrograms a day, roughly a millionth of a gram.

Do Nightshades Cause Inflammation?

No good evidence shows that nightshade vegetables cause inflammation or arthritis flares in people generally. Harvard Health’s Dr. Robert Shmerling wrote in May 2026 that there is no convincing evidence that potatoes or tomatoes are bad for rheumatoid arthritis and that Mediterranean-style eating patterns have the stronger support.

The claim has a lab basis, though. A 2002 study in Inflammatory Bowel Diseases exposed cultured gut cells and mouse intestine to solanine and chaconine. The barrier disruption appeared in mice genetically prone to IBD but not in control animals, and feeding those predisposed mice physiologic doses aggravated colon injury. A 2013 cell study found the opposite signal: chaconine and solanidine lowered inflammatory messengers (IL-2, IL-8) and solanine lowered nitric oxide production in stimulated immune cells.

The honest summary: the evidence is thin in both directions, and nothing here tests healthy adults eating normal servings. Dr. Shmerling’s stance is practical: follow whatever diet you feel helps, as long as it is safe. If you suspect a trigger, an elimination trial supervised by your doctor is the sensible test. For gardeners, the one exposure you can control is the green-potato one.

Planting Nightshades Together: Shared Diseases and Pests

Treat the whole family as one crop in your rotation, because the same diseases and caterpillars move between tomato, potato, pepper and eggplant. Penn State Extension says plants in a family share pests and diseases that build up if the family is planted repeatedly, and wild relatives like horsenettle and jimsonweed can carry them over. Our jimson weed identification guide shows one to pull on sight.

The longest fixed interval in UConn’s table is four years, for eggplant verticillium, and it lists tomato verticillium as “several years; as long as possible.” Planning at least three years between nightshade crops in a bed, and four after a verticillium problem, is my working rule; extension tables vary by disease, so treat it as a planning heuristic. Our vegetable crop rotation guide and the 4×8 raised bed rotation plan show how to lay it out.

Goji is the odd one out because it is a perennial. Utah State Extension says plants start fruiting at 2 years and reach maximum production at 3 to 5, so it stays put while your annual nightshades rotate. Site it well away from the pepper and eggplant beds, since those are alternate hosts for the goji gall mite.

Specific problems have their own pages: tomato late blight, hornworm versus aphid triage and Colorado potato beetle control.

Nightshade Vegetables FAQ

Are tomatoes, peppers and eggplant nightshades?

Yes. All three are Solanaceae, as are potatoes and tomatillos, according to Penn State Extension’s family list. Our guides to tomatoes and peppers cover growing each one.

Are sweet potatoes nightshades?

No. Sweet potatoes are in the morning glory family (Convolvulaceae) and only distantly related to potatoes, per NC State Extension.

Does cooking remove solanine from potatoes?

Barely. Oregon State Extension says glycoalkaloids are heat-stable, boiling has a negligible effect and frying at 210°C removed about 40%. Peeling and discarding green or sprouted parts does more.

Can I eat green tomatoes?

Green tomatoes carry far more tomatine than ripe ones (4–17 mg/100 g versus 0.03–0.6 mg/100 g). The sources I reviewed give no safe serving size, so treat them as an occasional food and pick fully ripe fruit when you have the choice.

Should I stop eating nightshades if I have arthritis?

Harvard Health finds no convincing evidence that avoiding them helps rheumatoid arthritis. If you want to test it, do an elimination trial with your doctor rather than cutting the whole family permanently.

Sources

1. Oregon State University Extension Service. Glycoalkaloids in Potato Tubers (EM 9407).
2. Centre for Food Safety, Hong Kong. Toxic Glycoalkaloids in Potatoes. Food Safety Focus.
3. Friedman M, Levin CE, McDonald GM. alpha-Tomatine determination in tomatoes by HPLC using pulsed amperometric detection. Journal of Agricultural and Food Chemistry, 1994.
4. Choi SH et al. Changes in free amino acid, phenolic, chlorophyll, carotenoid, and glycoalkaloid contents in tomatoes during 11 stages of growth. Journal of Agricultural and Food Chemistry 58:7547–7556, 2010.
5. Siegmund B, Leitner E, Pfannhauser W. Determination of the nicotine content of various edible nightshades (Solanaceae) and their products. Journal of Agricultural and Food Chemistry 47:3113–3120, 1999.
6. Shmerling R. Do nightshade vegetables worsen rheumatoid arthritis? Harvard Health Publishing, May 2026.
7. Patel B et al. Potato glycoalkaloids adversely affect intestinal permeability and aggravate inflammatory bowel disease. Inflammatory Bowel Diseases, 2002.
8. Kenny OM et al. Anti-inflammatory properties of potato glycoalkaloids in stimulated Jurkat and Raw 264.7 mouse macrophages. Life Sciences, 2013.
9. NC State Extension. Ipomoea batatas (Sweetpotato), Extension Gardener Plant Toolbox.
10. NC State Extension. Physalis (Ground Cherry, Cape Gooseberry), Extension Gardener Plant Toolbox.
11. Penn State Extension. Plant Rotation in the Garden Based on Plant Families.
12. University of Connecticut IPM. Having problems controlling vegetable crop diseases? Try rotation.
13. Virginia Cooperative Extension. Late Blight of Tomato and Potato (ANR-6), 2024.
14. Utah State University Extension. Tomato and Tobacco Hornworms.
15. Utah State University Extension. Goji in the Garden.
16. Royal Horticultural Society. Solanum betaceum (tamarillo).
17. University of Hamburg. Solanaceae, The Families of Flowering Plants (DELTA).
18. Arizona-Sonora Desert Museum. Garden Huckleberry or Chichiquelite (Solanum melanocerasum). Plant care information.
19. Prohens J, Ruiz JJ, Nuez F. The pepino (Solanum muricatum, Solanaceae): a “new” crop with a history. Economic Botany 50:355–368, 1996.
20. Purdue University NewCROP. Capsicum Pepper.