Pennyroyal

The one mint that is not a kitchen herb. Pennyroyal is sold as a creeping groundcover and flea mint, smells convincingly of peppermint, and carries a liver toxin that has killed people. Tell it by its low creeping habit and its flowers in whorls along the stem.

This one is for identifying, not planting: a prostrate rooting mat with small grey-green softly hairy leaves and lilac flowers in round whorls spaced along the stem - never a spike at the tip. Smell is not a valid test.

Do not plant pennyroyal. What follows is how to recognize it, whether it arrived in a nursery pot, came with a load of soil, or is already established somewhere on your land.

Habit. A prostrate, mat-forming perennial four to eight inches high, its square stems running along the ground and rooting where they touch, with flowering shoots rising to about a foot. Spearmint and peppermint spread underground by rhizomes but stand upright, one to three feet tall.

Leaves. Small — roughly ½ to 1 inch — oval to elliptic, grey-green, softly hairy, with smooth or barely toothed margins. Spearmint's leaves are two to three inches long, bright green, sharply toothed and noticeably crinkled; peppermint's are dark green, often purple-flushed, on purple-tinged stems.

Flowers — the decisive check. From midsummer pennyroyal produces dense, ball-like whorls of small lilac flowers spaced at intervals along the stem, each sitting in the angle of a leaf pair, usually with leaves continuing above the top cluster. Spearmint and peppermint produce a single tapering flower spike at the tip of the stem with nothing above it. This is the difference to teach somebody, because it can be checked from three feet away.

Smell. Strongly of peppermint, slightly harsher and more medicinal. That is not a discriminator. Anyone telling you that you can smell the difference is telling you something that has repeatedly failed in practice.

One relative to know: American pennyroyal (Hedeoma pulegioides) is a different genus — an upright native annual of eastern woods — but it is also pulegone-rich and carries exactly the same rule. Neither plant is a tea.

If you already have it: remove it rather than manage it. Pull whole rooting runners from damp soil, cut before it seeds, bag it as household waste, and keep pennyroyal oil out of the house entirely.

There is no reason to keep pennyroyal, and two reasons to take it out: it is a poison sitting in a bed you gather herbs from, and it is an invasive wetland weed where the ground stays damp. California lists it as an invasive plant — it takes over vernal pools, seasonally wet meadows and creek margins, replacing the native flora those places depend on — and it is a recognized problem weed in Oregon.

Removing it. Work when the soil is damp. Lift the rooting runners whole rather than snapping them off, work outward from the edge of the mat, and go back over the ground two or three times through the season for what regrows. Do not rototill or strim it: every rooting node you chop up becomes a plant. Bag everything as household yard waste; do not compost it, and never leave pulled material where children or livestock could chew it.

If you cannot clear it this season, at least cut it before it flowers and sets seed, and keep it away from ditches, irrigation lines, pond edges and any damp low ground — that is how it leaves a garden and becomes a wetland problem.

The oil is the real hazard. Far more people have been hurt by a bottle of pennyroyal oil than by a plant in a border. Do not buy it, do not keep it, and do not use it on pets or their bedding — dogs and cats have died from flea preparations containing pennyroyal oil, absorbed through the skin as well as swallowed. If a bottle turns up in the house, dispose of it rather than passing it along.

Wear gloves when handling the plant in quantity and wash afterwards. Handling it is not the danger — ingestion and concentrated oil are — but there is no reason to be casual with it.

Which mint is it, what to do if someone drank pennyroyal tea, and what to do about a pet treated with the oil. Any suspected ingestion of pennyroyal oil is an emergency.

Someone drank pennyroyal tea, or took the oil. Treat it as a medical emergency: call poison control or emergency services immediately and do not wait for symptoms. Liver injury can develop over hours to days while a person still feels only nauseated, and early hospital treatment matters — there is an established antidote protocol for this poisoning, and it works best early. Take the bottle or a piece of the plant with you. Do not sit it out at home to see what happens.

Someone took it in pregnancy, or to end a pregnancy. Same answer, without hesitation and without judgement: call poison control or emergency services now. Pennyroyal has killed women taken for this reason. Say plainly what was taken so the medical team can act on it.

“Is this pennyroyal or my spearmint?” Three checks: does it creep and root along the ground rather than standing upright at knee height; are the leaves small, grey-green and softly hairy rather than large, bright green and sharply toothed; and are the flowers in ball-like whorls spaced along the stem rather than a single spike at the tip. Do not decide by smell. If a mint in your garden is unlabelled, do not use it.

A pet was treated with pennyroyal oil, or with a flea product containing it. Call a veterinarian immediately, and wash the product off with soap and water. Dogs and cats have died from dermal exposure as well as from swallowing it.

“It came back after I pulled it.” Rooted nodes were left behind, or it was chopped by a mower or tiller. Lift whole runners from damp ground, and expect to repeat.

“But it is traditional.” So it is — and the tradition has a body count. See the Medicine section: this is a case where the historical record argues against the practice, not for it.

A long folk-medicine tradition with a well-documented toxicology: pulegone becomes a liver-destroying metabolite, and pennyroyal oil is one of the few herbal preparations with clearly recorded human fatalities. There is no safe home preparation.

This section records how people have traditionally used this plant, and what is known about the risks. It is history and general information — not medical advice, and not a recommendation to treat anything. Plants are not dosed like medicines, some interact with prescriptions, and some are harmful in ways that are not obvious. Talk to a clinician before using any plant medicinally.

Pennyroyal has one of the longest medicinal histories of any European herb and one of the clearest modern verdicts against it.

What it was used for: a digestive and cold remedy, an insect repellent, a flavouring in old English cookery (the name “pudding grass” comes from a stuffing) and, above all, as an emmenagogue and abortifacient — the use that appears again and again in herbals from Pliny onward.

What the chemistry does: pennyroyal oil is dominated by pulegone. The liver converts pulegone into menthofuran and related reactive metabolites, which consume the liver's glutathione defences and then attack liver tissue directly — a mechanism closely comparable to a paracetamol overdose. The result can be fulminant liver failure, and in severe poisonings kidney injury, seizures and death. Documented fatalities in the medical literature involve small volumes of the concentrated oil, and serious poisonings have also followed strong leaf infusions.

The abortifacient point deserves stating plainly, because the folklore has not died: pennyroyal is not a reliable way to end a pregnancy. The amount that harms the person taking it arrives before, and independently of, any effect on a pregnancy. The recorded outcome of this tradition is dead and injured women, not a working remedy.

Treatment exists and works better early. Poison centres manage pennyroyal ingestion much as they manage a paracetamol overdose, with N-acetylcysteine to restore the liver's glutathione, alongside supportive care — which is exactly why the instruction is to call immediately rather than to wait and see.

The conclusion is simple and not a matter of opinion: there is no safe home preparation of pennyroyal — no tea, no tincture, no oil, no dose, and nothing at all in pregnancy or while nursing. If you want a mint for a teapot, grow spearmint or peppermint. Educational information, not medical advice.

A Roman strewing herb, a medieval flea remedy and a stuffing flavouring, whose folk reputation as an abortifacient outlived every warning attached to it - and still turns up as a nursery groundcover.

Pennyroyal's Latin name records its oldest job: pulegium comes from pulex, the flea. Romans strewed it in bedding and burned it to drive fleas and mosquitoes out of a room, and medieval Europe carried the practice forward — which is why it still turns up in garden centres today labelled flea mint, and in the pet aisle as a “natural” flea oil, an idea that has killed dogs and cats.

It was also a kitchen plant. The English name pudding grass comes from its use in stuffings and black puddings, in an era when the leaf was used sparingly as a seasoning rather than steeped by the mugful.

The strand of its history that matters most is medical. From Pliny onward, pennyroyal appears in herbal after herbal as a plant to bring on menstruation and end pregnancy, and that reputation travelled to North America and survived into the twentieth century in home-remedy books and, later, on the internet. Every recorded fatality attached to this plant sits in that tradition. The colonial-era common name that pairs it with Indigenous women is best left in the historical record it belongs to; it is not a name to use, and the practice it refers to was not a safe one for anybody.

The plant itself, meanwhile, did what escaped mints do: it naturalized. In California it now occupies vernal pools and seasonally wet meadows in numbers that have earned it a place on the state's invasive plant list — a second, quieter reason not to put one in the ground.

Hardy roughly zones 6-9 and happiest on damp ground - which is precisely the habitat it damages. It is at its most invasive in California and the Pacific Northwest, where seasonal wetlands give it exactly what it wants.

Pennyroyal is hardy to roughly zones 6-9, dying back in cold winters and resprouting from rooted stems, and evergreen-ish in mild ones. What it really wants is moisture: damp meadows, pond and ditch margins, seasonally flooded low ground, irrigated turf edges. It tolerates full sun where the ground stays wet, and part shade elsewhere.

That preference is the whole invasion story. In California it has colonized vernal pools and seasonally wet meadows — habitats with a highly specialized native flora and very little tolerance for a mat-forming competitor — and it is listed as an invasive plant there; it is a recognized weed in Oregon and elsewhere in the Pacific Northwest for the same reason. Across the interior and the East it naturalizes more locally, in wet ditches and pasture.

In a hot, dry garden with no irrigation it struggles, and in the coldest zones it is unreliable. Neither is a reason to plant it. The places where it grows easily are exactly the places where it does harm, and the plant is dangerous to have around a kitchen anywhere.