Jul

Ballota nigra – July 14th 2025 – Black Horehound

Ballota nigra is a perennial plant, believed to have been introduced to Britain from Europe during the Iron Age and therefore classed as an ‘archaeophyte’. The name ‘Ballota’ comes from the Greek ‘ballo’ (to reject), referring to its offensive smell and the observation that cattle reject it. Two alternative common names in English reflect its bad smell: ‘Black Stinking Horehound’ or ‘Stinking Roger’. It belongs to the botanical Family of mint and dead-nettles, the Lamiaceae, with its square stems, leaves in opposite pairs, abundant hairs and bilaterally-symmetrical flowers, recalling such common wild plants as red dead-nettle Lamium purpureum, and hedge woundwort Stachys sylvatica (the latter is also a bit stinky).

Ballota nigra at Arthur’s Seat, Holyrood Park, Edinburgh. Image: Chris Jeffree.

We saw Ballota nigra a few weeks ago at Holyrood Park (Edinburgh) growing on a cliff on the west side of the volcanic plug known as Arthur’s Seat, just above Queen’s Drive. It seemed a strange habitat for this plant. The BSBI say it is ‘a herb of hedgerows, field-borders, walls, waysides and waste ground, often on disturbed nutrient-rich soils near habitations’. That’s more-or-less how I remember it during my childhood in south England, where the plant was much more common than here in Scotland. So what’s it doing on a Scottish cliff?

Ballota nigra at Prestonpans, East Lothian. Image: Chris Jeffree.

Well, it is only one of several rare-in-Scotland species known to grow in this precise location. At Arthur’s Seat we also saw the extremely rare fern, Forked Spleenwort Asplenium septentrionale and the quite rare Maiden Pink Dianthus deltoides, but the rarest of all the plants is a rock-dwelling moss, Grimmia anodon, which is found nowhere else in the UK. However, this time we couldn’t complete our survey as a rainstorm drenched us and we retreated down the hill towards shelter and lunch.

Ballota nigra, flowering stem, 40-100 cm tall. Leaves stay green in winter, flowering is June to October. Image: Chris Jeffree.

Later, I looked in the BSBI’s database and saw that Ballota nigra has been recorded at several Midlothian locations, including Edinburgh castle, Craigmillar castle, and (bizarrely) near the tramline at Edinburgh airport. The only thing that may link these sites is the chemistry of the rocks. At Arthur’s Seat the rocks are basalt, formed 300 million years ago from the rapid cooling of volcanic larva. Basalt is often used in construction and may have been used for the tramline near the airport. It contains moderately high concentrations of calcium and magnesium, and so the soil formed from its weathering may favour plants that grow in alkaline and calcium-rich soils (such plants are called ‘calcicoles’). The limestones and clays of the southern Britain may have something in common with the thin soils on the cliffs of Arthur’s Seat. On the other hand, Ballota nigra is said to be ‘nitrophilous’ i.e. nitrogen-loving but these soils on basalt are unlikely to be high in nitrogen. I trawled through Google Scholar to find research that helps us to understand the sparse distribution of Ballota in Scotland.

Detail of the inflorescence. Please note: hairy all over, flowers in whorls, Calyx about 1 cm, tinted purple, funnel shaped with five pointed teeth. Images: Chris Jeffree.

However, I found no relevant ecological research on the soil preferences and general habitat requirements of the species except for the listing in Hill et al. (2004): these authors say the plant requires an above-average nitrogen supply and a high pH. The habitat classes are given as ‘boundary and linear features, eg hedges, roadsides and walls’. No cliffs. I wonder if our Scottish plants are genetically different from the rest of the UK population. Ecotypic variation has not been investigated, although various subspecies are recognised from other parts of Europe. In Britain we have subspecies meridionalis . Here are the eight listed in Plants of the World Online:

Stepping back to the year 1597, we find two relevant quotes in Gerard’s Herbal. He knew the plant: “it grows in the corn fields about Montpellier” and also “it is found in gardens amongst pot herbs, and oftentimes amongst stones and rubbish in dry soils”. He didn’t visit Scotland as far as we know.

Ballota nigra, British and Irish distribution according to the BSBI database.

The lack of ecological research is made up for by a wealth of detail on medicinal properties. This is not surprising, as it was highly recommended by the ancient herbalists and has been used in traditional medicine for over two thousand years. There are many research publications about the chemical compounds of this plant in relation to possible pharmaceutical use. As we often find, knowledge from Ancient Greece comes first. The Greek physician-botanist and father of herbal medicine was Pedanius Dioscorides 40-90 AD. He declared Ballota nigra to be an antidote for the bite of a mad dog. Gerard (1597) agreed. A modern pocketbook on medicinal plants (Hensel 2008) says: ‘it has been used as a remedy for spasmodic coughs, cramps, nausea and hysteria, and, externally, in the treatment of gout’. Its reputation as a herbal medicine may explain why it is sometimes found growing in the wild in those far-away regions that have been settled by Europeans. They would have taken their favourite herbal plants with them on the boat.

Ballota nigra, global distribution of records according to GBIF.

References consulted

Gerard J (1597). https://www.exclassics.com/herbal/herbalv1.pdf

Hensel W (2008) Medicinal plants of Britain and Europe. London: A & C Black.

Hill MO, Preston CD and Roy DB (2004) PLANTATT-attributes of British and Irish plants: status, size, life history, geography and habitats. Centre for Ecology & Hydrology, 2004.

Przerwa F, Kukowka A and Uzar I (2020). Ballota nigra L.–an overview of pharmacological effects and traditional uses. Herba Polonica 66, 56-65.