Asparagus, Celery, Artichokes, and the Odd Vegetables

TL;DR. The stems, shoots, and flower buds: asparagus, celery, globe artichoke, fennel, rhubarb, and sweetcorn. Asparagus is a perennial that takes three years to establish and is the reason a quarter to a half of people notice a distinctive smell in their urine afterwards, an effect that turns out to be about whether you can smell it rather than whether you make it. Celery is not negative-calorie. Rhubarb leaves are genuinely poisonous while the stalks are fine. Sweetcorn is a grain that we eat as a vegetable, and the indigestible bit is the hull, not the whole kernel.

1. What they are

2. Where they come from

Asparagus is native to Eurasia and was cultivated by the Egyptians, Greeks, and Romans, who prized it enough that Emperor Augustus reputedly used "quicker than cooking asparagus" as a phrase for speed.

Celery was grown in antiquity as a medicinal and aromatic herb (wild celery, "smallage") and only bred into the thick mild stalks we eat in seventeenth-century Italy and France.

Globe artichoke is a domesticated thistle, developed in the Mediterranean from the wild cardoon. Both are still grown, one for the flower bud and one for the stems.

Rhubarb came from China and Siberia, imported to Europe originally for its medicinal root, a powerful purgative traded along the Silk Road at great expense. Eating the stalks is a relatively recent, largely British innovation dating to the eighteenth and nineteenth centuries, once sugar became cheap enough to make them palatable.

Sweetcorn is a mutant maize selected for a gene (su, later se and sh2) that slows the conversion of sugar to starch in the kernel. It was developed by Indigenous American farmers and refined enormously in the twentieth century; modern supersweet (sh2) varieties hold their sugar for days rather than hours, which is why the old advice about running from the field to the pot matters less now.

Production: asparagus around 8.5 million tonnes, over 90 percent of it in China, with Peru and Mexico dominating the export trade; celery around 5 million tonnes; artichokes around 1.5 million tonnes led by Italy, Egypt, and Spain; sweetcorn around 10 million tonnes as a vegetable, distinct from the ~1.2 billion tonnes of field maize.

3. How they are grown

Asparagus is a long-term perennial and one of the few vegetables that is a genuine capital investment. Crowns are planted in trenches, and no spears are harvested for the first two to three years while the plant builds its root reserves. After that it produces for 15 to 20 years. The harvest window is short (6 to 8 weeks in spring), after which the spears are allowed to grow into tall ferny foliage that feeds the crown for next year. Cutting for too long exhausts the plant.

White asparagus is the same plant grown under mounded soil or plastic so the spears never see light and never produce chlorophyll. It is milder and more fibrous, and it is a major spring event in Germany, the Netherlands, and France. Purple asparagus is a different variety with anthocyanins and higher sugar.

Celery is a marsh plant and needs consistently wet, rich soil; water stress makes it stringy and bitter. Traditional varieties were blanched by earthing up or wrapping to keep the stalks pale and mild; modern self-blanching green varieties dominate. Celery is one of the more input-intensive vegetables.

Globe artichokes are perennials cropping for 4 to 7 years, needing a mild Mediterranean climate. The bud must be harvested before the flower opens; left alone, it becomes a spectacular purple thistle flower.

Rhubarb is a hardy perennial cropping for a decade or more. Forced rhubarb is the interesting case: crowns are lifted, chilled, and moved into dark heated sheds, where they grow using stored root reserves. The stalks are pale pink, tender, and sweeter, and they grow so fast that you can hear them creaking. The Yorkshire "Rhubarb Triangle" holds a protected designation for it, and it is harvested by candlelight because light stops the process.

Sweetcorn is wind-pollinated, and each silk connects to one kernel: a partly filled cob is a pollination failure. It must therefore be planted in blocks rather than rows so pollen reaches the silks. It respires extremely fast after picking, converting sugar to starch, which is why speed and immediate cooling matter.

4. What is inside them

Per 100 g raw:

Notes:

• Artichoke is the fibre standout, at 5.4 g per 100 g, and much of that is inulin, a fermentable prebiotic. It is also unusually high in polyphenols, principally cynarin and chlorogenic acid.
• Asparagus is a good folate source and contains asparagusic acid, unique to the plant and the cause of the urine effect below. It also contains fructans, making it a genuine prebiotic.
• Celery is genuinely high in sodium for a vegetable, around 80 mg per 100 g, which matters for people on strict sodium restriction and for anyone eating large amounts of celery juice.
• Sweetcorn is a grain, with roughly four times the calories and carbohydrate of the others. It also contains lutein and zeaxanthin at useful levels, and it is one of the better vegetable sources for eye health.
• Rhubarb is a decent calcium source on paper and a poor one in practice, because its oxalate binds the calcium (Chapter 18).

5. What they do in your body

Asparagus and urine

Asparagus contains asparagusic acid, found in no other food. Your body metabolises it within 15 to 30 minutes into several volatile sulphur compounds, principally methanethiol and dimethyl sulphide, which are excreted in urine and produce a distinctive smell.

The interesting part is the variation. Early research argued that some people produce the compounds and others do not. Better-designed studies established that almost everyone produces them, and only some people can smell them. The ability to detect the odour is associated with genetic variation in a cluster of olfactory receptor genes, identified in a genome-wide association study of over 6,000 people. Estimates of the proportion who cannot smell it range from around 40 to 60 percent depending on population and method. So if you have never noticed, the most likely explanation is anosmia to a specific molecule, not a different metabolism.

Artichoke, bile, and cholesterol

Artichoke leaf extract stimulates bile flow (a choleretic effect), a traditional use with reasonable supporting evidence. Meta-analyses of artichoke leaf extract find modest reductions in total and LDL cholesterol, in the range of 0.2 to 0.4 mmol/L, and some benefit for functional dyspepsia symptoms. The trials are small and mostly use standardised extracts rather than the vegetable.

Cynarin is also responsible for one of the strangest effects in food: artichokes make subsequent food and drink, particularly water, taste sweet. Cynarin appears to temporarily suppress sweet receptors, so when it is rinsed away the receptors rebound and register sweetness. This is why artichokes are notoriously difficult to pair with wine.

Celery

Apigenin, phthalides (the compounds giving celery its smell, particularly 3-n-butylphthalide), and nitrate. Small trials of celery seed extract have shown modest blood pressure reductions, plausibly through the phthalides.

Celery is not negative-calorie. The idea that digesting it costs more energy than it provides is wrong: the thermic effect of food is roughly 5 to 30 percent depending on macronutrient (Chapter 12), never over 100 percent. A stick of celery provides around 6 kcal and costs perhaps 1 kcal to process. It is very low calorie, which is a real and sufficient virtue.

Celery juice, promoted heavily in the late 2010s for a long list of conditions, has no clinical evidence for any of them. It also concentrates sodium and oxalate and removes the fibre.

Rhubarb and oxalate

Rhubarb leaves contain 0.5 to 1 percent oxalic acid and are genuinely poisonous: large quantities cause vomiting, kidney damage, and, in documented historical cases, death. Poisonings occurred in Britain during the First World War when the leaves were promoted as a food.

The stalks contain far less (around 0.2 to 0.6 percent) and are safe in normal quantities, though relevant to people who form calcium oxalate stones. The traditional practice of cooking rhubarb with plenty of sugar and eating it with custard or cream is, incidentally, good oxalate management: dairy calcium binds oxalate in the gut before it can be absorbed.

Okra and sweetcorn

Okra's slime is mucilage, a soluble fibre. Small trials suggest modest effects on post-meal glucose, plausibly through the same viscosity mechanism as other soluble fibres. Acid (tomato, lemon) and high dry heat reduce the sliminess; low, wet cooking increases it.

Sweetcorn "passing through whole" is a partial truth. The outer hull (pericarp) is cellulose, which you cannot digest, so it passes intact and visible. The starchy interior is digested normally. Chewing well breaks more hulls. This is also why sweetcorn is a useful marker for measuring your own gut transit time (Chapter 10).

6. Who should be careful

• IBS. Asparagus, artichoke, and fennel bulb are high-FODMAP (fructans and, for artichoke, inulin). Celery and rhubarb are moderate. Sweetcorn is moderate. Artichoke is one of the more reliable triggers.
• Kidney stones. Rhubarb is among the highest-oxalate foods there is. Celery is moderate.
• Warfarin. Asparagus, celery, and rhubarb are all moderate to high in vitamin K. Consistency, not avoidance.
• Celery allergy is a named allergen requiring declaration in the EU, is more common in central and northern Europe than elsewhere, cross-reacts with birch and mugwort pollen, and can cause anaphylaxis. It is one of the more serious vegetable allergies and it survives cooking.
• Phytophotodermatitis. Celery, parsnip, and fennel contain furanocoumarins; handling the sap and then going into sunlight causes blistering burns. Celery infected with pink rot fungus produces far more, which is an occupational issue for harvesters.
• Kidney disease. Artichoke and fennel are high in potassium.
• Asparagus and gout. Asparagus is moderately high in purines. The evidence that vegetable purines raise gout risk is weak, unlike meat and seafood purines, so this restriction is largely outdated.

7. How to eat them

• Asparagus: snap rather than cut; the spear breaks naturally where the woody part ends. The woody ends make good stock. Roast, griddle, or steam briefly; boiling loses folate and vitamin C. Buy it in season (April to June in the northern hemisphere) and understand that airfreighted Peruvian asparagus in December is one of the higher-carbon items in a supermarket (Chapter 9).
• Artichoke: trim the stem (it is edible and tastes like the heart, so peel rather than discard it), cut off the top third, remove the tough outer leaves, and boil or steam with lemon. Scrape the flesh from the base of each leaf with your teeth, remove the hairy "choke" (the immature flower), and eat the heart. Small young artichokes have no choke and can be eaten whole. Cut artichoke oxidises fast; acidulated water prevents it.
• Celery: the leaves are the most flavourful part and are usually thrown away. Use them like a herb. Celery is one third of the mirepoix and sofrito base that underlies most of European cooking for a reason.
• Fennel: raw and shaved it is crisp and aniseedy; roasted or braised it becomes sweet and mild, because the anethole responsible for the aniseed note is volatile. The fronds are a herb.
• Rhubarb: cook with sugar; it is far too sour otherwise. Do not eat the leaves. Forced rhubarb is more tender and needs less sugar.
• Sweetcorn: cook it as soon as possible after buying, especially non-supersweet types. Do not salt the cooking water, which toughens the kernels. Griddling or roasting develops much more flavour than boiling.
• Okra: to reduce slime, cook fast and hot, or add acid, or cook whole rather than sliced. To use it as a thickener, as in gumbo, do the opposite.

8. Choosing and storing

• Asparagus: tight closed tips, firm straight spears, no woody or dried ends. Store upright in a jar with a couple of centimetres of water in the fridge, or wrapped in damp paper. It deteriorates fast, losing sugar within a day or two.
• Celery: firm, snapping stalks with fresh leaves. Store wrapped in foil in the fridge, which works substantially better than plastic because it allows ethylene to escape while retaining moisture. Revives in cold water.
• Artichoke: heavy, tight, squeaking when squeezed. Fresh cut stem. Two weeks in the fridge.
• Fennel: firm, white, unblemished bulb with fresh green fronds. A week in the fridge.
• Rhubarb: firm, crisp stalks. Remove and discard the leaves immediately. Keeps a week wrapped in the fridge; freezes very well chopped.
• Sweetcorn: bright green tight husks, moist golden silks, and plump kernels all the way to the tip. Refrigerate in the husk and eat within a day or two.

9. Myths and confusions

Don't be confused: rhubarb leaves are toxic and rhubarb stalks are not. The difference is roughly a factor of two to five in oxalate concentration, plus the fact that nobody eats a kilogram of stalks. Compost the leaves; do not add them to salad.

• "Celery has negative calories." No food does.
• "Celery juice heals chronic illness." No clinical evidence for any of the claims.
• "Sweetcorn is indigestible." Only the hull. The starch inside is digested normally.
• "Only some people's bodies make the asparagus urine smell." Almost everyone makes it; only some can smell it.
• "White asparagus is a different vegetable." Same plant, grown in the dark.
• "Artichokes are difficult and not worth it." They take five minutes to prepare and are among the highest-fibre, highest-polyphenol vegetables available.

10. The bottom line

• Asparagus is a 15-year perennial with a six-week season, best eaten local and in spring; the urine smell is universal and the ability to detect it is not.
• Artichoke is the fibre and polyphenol standout of this group, has genuine modest evidence for bile flow and cholesterol, and makes everything you eat afterwards taste sweet.
• Celery is very low calorie, not negative-calorie, unusually high in sodium for a vegetable, a significant allergen in Europe, and its leaves are the best part.
• Rhubarb stalks are food, rhubarb leaves are poison, and cooking rhubarb with dairy is accidental good oxalate management.
• Sweetcorn is a grain eaten as a vegetable; the visible whole kernels in your stool are hulls, and they make a convenient gut transit marker.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Asparagus perennial establishment, white asparagus blanching, and the harvest window follow European and US extension guidance. Celery blanching and its history as a medicinal herb follow Vaughan and Geissler, The New Oxford Book of Food Plants. Forced rhubarb and the Yorkshire Rhubarb Triangle protected designation follow Defra and PDO documentation. Sweetcorn su, se, and sh2 mutations and post-harvest sugar conversion follow Tracy's sweetcorn breeding reviews. Asparagusic acid metabolism and the genetics of detecting the odour follow Markt et al., BMJ, 2016, a genome-wide association study of over 6,000 people identifying olfactory receptor variants. Artichoke leaf extract for cholesterol and dyspepsia follows Cochrane reviews by Wider et al. and Holtmann's dyspepsia trial. Cynarin's sweet-taste effect follows Bartoshuk's sensory work. Celery seed extract and blood pressure follows small trials on 3-n-butylphthalide. Rhubarb leaf oxalate poisoning follows the First World War British record and toxicology references. Okra mucilage and glycaemic effects follow small trials. Celery as a named EU allergen follows Regulation 1169/2011.

Open questions. Celery juice claims that circulated widely in the late 2010s have never been tested in any controlled trial, so there is nothing to evaluate rather than negative evidence. Artichoke extract trials are small and mostly use standardised extracts rather than the vegetable.

👉 Next: mushrooms, which are not plants at all.