Black horehound

How to recognise it

Growth form

Upright, branched perennial, 50–100 cm; the whole plant is softly hairy and grey-green to dark green.

Leaf

Opposite, stalked, oval to heart-shaped, 2–5 cm, with a scalloped edge and a wrinkled surface. Bruised, it smells unpleasantly musty.

Flower

Purplish-pink, sometimes white, two-lipped flower of 12–15 mm, with a hairy upper lip and a three-lobed lower lip, in dense whorls in the leaf axils. Flowers in June–September.

Fruit/Seed

Four small, dark nutlets in the calyx. The funnel-shaped calyces with five broad, sharply pointed teeth remain on the plant long after flowering.

Stem

Square, hairy, often flushed reddish.

Confusion with

Red dead-nettle is smaller, flowers mainly in spring and does not smell. Hedge woundwort also smells unpleasant, but has dark purplish-red flowers in long spikes, larger, long-stalked leaves and grows in woodland. You recognise black horehound by its funnel-shaped calyx with five broad teeth.

Flowering: June–September · Pruning: March

Why grow it?

• An excellent nectar plant for bumblebees and long-tongued bees, which can easily reach into the deep flowers.
• Flowers for a long time, from June into September, in rough places where little else flowers in summer.
• Hoverflies and butterflies also visit the flowers.
• A tough plant for a difficult, dry spot at the foot of a wall or under a hedge.
• A piece of village history: it has grown for centuries near farms, old walls and churchyards.
• The sturdy winter stems with their empty calyces give shelter to small insects.

Black horehound: where does it grow best?

• In the wild at the foot of walls, along hedges, near farms, in farmyards, on verges and on waste ground.
• Full sun to half shade.
• Fertile, dry to moist soil, often somewhat limy; also on rubble and stony ground.
• In the garden: in a rough border, at the foot of a wall or shed, along a hedge or in a wild corner of the nature garden.

Goes well with

• Wild teasel: Tall and sturdy in the same rough, sunny corner.
• Wild marjoram: Pink flower clusters for butterflies in a dry spot by a wall.
• Garlic mustard: White flowers in spring at the foot of a hedge, before the black horehound flowers.
• Common mallow: Pink flowers on the same fertile verge, all summer long.

Black horehound: when and how to plant it?

1. Buy a plant or seed from a nursery specialising in native plants. Do not dig up plants in the wild.
2. Sow in September–October, in pots or directly where it is to grow on bare soil, and cover the seed with a thin layer of soil.
3. Leave the pots outside over winter; the seed germinates in spring. Sowing in March–April is also possible.
4. Plant in March–April or in October, 40–50 cm apart.
5. Water after planting; after that, extra water is rarely needed.

Care

Water

Only after planting and in long droughts. It copes well with dry soil.

Feeding

Not needed: on fertile soil it grows well enough by itself.

Mowing

On a verge or in a rough corner, mow once a year in March; or in late autumn, leaving part of it standing for winter.

Spreading

It self-sows but is not invasive. Pull up surplus seedlings in spring.

Letting it seed

Leave some of the stems with seed: that way it self-sows quietly and stays in the same place for years.

Tip: Plant black horehound at the foot of a sunny wall or shed, a few metres from the patio: you will have bumblebees on the flowers all summer without being bothered by the smell.

Pruning and mowing

Pruning months: March.

When: In March, before the new shoots start to grow.

How:

• Leave the dead stems over winter: small insects overwinter among the calyces.
• In March, cut them back to 5–10 cm above the ground.
• If it flops over on rich soil, cut the stems back by a third at the end of May: it branches and stays sturdier.
• Put the cuttings on a brush pile or in a rough corner.

Caution: Wear gloves if you do not want the smell on your hands: bruised leaves leave a musty odour.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Seed

In September collect the ripe nutlets from the calyces and sow them in autumn. It also self-sows.

Division

Split older clumps with a spade in March–April and replant at once.

Cuttings

In May, root young, non-flowering shoots of 8–10 cm in moist cutting compost.

Black horehound: how does it get through the winter?

Hardy to −25 °C.

• The leaves mostly die back; a few green leaves often remain at the base.
• The dead stems with their sturdy, funnel-shaped calyces stand for a long time and look beautiful with frost.
• Fully hardy.

Diseases and pests

• Smell: Bruised, the plant smells musty and unpleasant. Do not plant it next to a seating area or along a narrow path where you keep brushing past it.
• Self-sowing: On open ground many seedlings can appear. Pull them up in spring or cut back some of the stems before the seed ripens.
• Flopping: On soil that is too rich or too shady the stems become weak. Give it a sunnier spot or cut it back at the end of May.
• Powdery mildew: White powder on the leaves in dry summers; usually harmless. Leave it or cut away the worst stems.

Through the year

• March: Cut back the dead stems. Pull up unwanted seedlings.
• April: Plant or divide clumps.
• May: Rapid growth. Cut stems back by a third if needed. Take cuttings.
• June: Flowering begins: bumblebees on the flowers.
• July: Full flower.
• August: Flowering; bees and hoverflies. Water in long droughts.
• September: Last flowers. Collect seed.
• October: Sow or plant. Leave the stems standing.
• December: Dead stems and calyces provide winter shelter.

Did you know?

• Grazing animals leave black horehound alone because of its smell and bitter taste. That is why it often survives in farmyards and pastures where everything around it has been grazed.
• The name Ballota comes from the Greek ballote, a plant name already found in the work of the Greek physician Dioscorides. Its English name black horehound refers to its likeness to the related white horehound.
• The empty, funnel-shaped calyces stay on the stem for months after flowering, like rows of tiny cups.