Marsh gentian

How to recognise it

Habit

Perennial plant, 15–40 cm tall, with one or a few upright, usually unbranched stems from a short rhizome.

Leaf

Narrowly linear to lance-shaped, 2–4 cm, opposite, with one distinct vein; dark green and hairless.

Flower

From late July to September, one to several trumpet-shaped flowers 2.5–5 cm long at the top and in the upper leaf axils; deep azure blue, dotted greenish inside, with five greenish stripes outside. They close in dull weather and at night.

Fruit/Seed

An elongated capsule with countless tiny seeds that ripen in autumn.

Roots

A short rhizome with fairly thick roots.

Look-alikes

Harebell has smaller, nodding, pale blue bells and grows on drier soil. Cross gentian has broad leaves and smaller flowers in clusters, on chalky soil. Garden gentians such as Gentiana sino-ornata are not native and often have trailing stems.

Flowering: July–September · Pruning: October–November

Why grow it?

• Food plant of the Alcon blue: the butterfly lays its white eggs on the buds, the caterpillars first feed inside the flower and are then taken into the nest of Myrmica ants and fed there.
• Deep blue flowers in late summer, when the heather is in full bloom or starting to fade.
• Bumblebees crawl right into the flowers to reach the nectar and pollinate them as they do.
• A flagship species of wet heath: where it grows, the heathland is still in good condition.
• A rewarding challenge for anyone with a heather or bog garden on acid soil.

Marsh gentian: where does it grow best?

• In the wild: wet and damp heath with cross-leaved heath, heathy grassland, pool margins and purple moor-grass meadows, on poor, acid sandy or peaty soil.
• It needs open, low vegetation and plenty of light; in tall grass or under shrubs it disappears.
• In the garden: full sun to light partial shade, on moist to wet, acid, nutrient-poor soil, for example sand mixed with acid leaf mould.
• Suitable for a heather garden, the edge of a pond with acid soil, or a large pot of acid, sandy compost that never dries out.

Goes well with

• Cross-leaved heath: The typical shrub of wet heath, with pink bells on the same acid, wet soil.
• Heather: Flowers purple at the same time, in slightly drier spots of the heather garden.
• Devil’s-bit scabious: Blue pompons in August–September and a top plant for butterflies.
• Common cottongrass: White tufts in spring on wet, acid soil.
• Tormentil: Small yellow flowers all summer, on the same poor, acid soil.

Marsh gentian: when and how to plant it?

1. Never pick or dig up plants in the wild: the species is rare and threatened. Buy plants or seed of native origin from a specialist nursery.
2. Sow in November–January on the surface of acid, sandy seed compost and do not cover: the seed needs light and a cold period to germinate. Stand the pot outside and keep it moist.
3. Seedlings grow slowly; prick them out only when they have a few pairs of leaves and grow them on in small pots for another year.
4. Plant in April–May, 20–30 cm apart, preferably in a small group, in a spot that never dries out.
5. Use rainwater, not tap water: tap water is often hard and makes the soil less acid.
6. Keep the surroundings low and open so the young plants get enough light.

Care

Watering

The soil must always stay moist, even in summer. Water with rainwater in dry spells.

Feeding

Never: fertiliser and lime are fatal. Poor, acid soil keeps it healthy.

Mowing

Mow the surroundings once a year in October–November, after seed set, and remove the cuttings.

Competition

Pull out birch seedlings, brambles and overly dense purple moor-grass so the plant is not shaded.

Self-seeding

Let the capsules ripen and split open: new seedlings then appear around the plant on bare, moist soil.

Tip: Always water with rainwater: it keeps the soil acid enough, and that is the main secret of a healthy marsh gentian.

Pruning and mowing

Pruning months: October–November.

When: In October–November, after seed set.

How:

• Leave the stems after flowering until the capsules are brown and the seed has dispersed.
• Then cut or mow the dry stems short and remove them.
• Now and then, scrape open a small patch of soil next to the plant: new seedlings can germinate there.

Caution: Never mow in summer: you would lose buds, flowers and the eggs of the Alcon blue.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Seed

Collect ripe seed from your own plants in autumn and sow on the surface of acid seed compost in November–January; leave outside. It germinates in spring; flowering after 2–3 years.

Division

Old plants with several stems can be carefully divided in April. They establish poorly, so sowing is better.

Marsh gentian: how does it get through the winter?

Hardy to −30 °C.

• The stems die back in autumn; the plant overwinters as buds on the rhizome, just below the surface.
• Fully hardy, even in severe frost.
• Meanwhile the caterpillars of the Alcon blue spend the winter in the nests of Myrmica ants.

Diseases and pests

• Drying out: In a dry spot the plant withers in summer. Choose a moist spot or stand the pot in a saucer of rainwater.
• Slugs: Young shoots and seedlings are vulnerable. Protect them in spring.
• Crowding out: Purple moor-grass, birch and bramble outgrow it. Mow every year and pull out woody seedlings.
• Poor growth: Yellow leaves and weak growth point to soil that is too limy or too rich. Use rainwater and poor, acid soil.

Through the year

• January: Sow on the surface; leave the pot outside.
• April: New shoots appear. Keep an eye on seedlings. Plant.
• May: Plant. Protect against slugs.
• June: The stems are growing. Water in dry spells.
• July: First buds and flowers. Look for the white eggs of the Alcon blue.
• August: Full flower. Bumblebees crawl into the flowers.
• September: Last flowers and ripening seed.
• October: Collect seed. Mow after seed set.
• November: Remove the cuttings. Sowing is possible.
• December: Resting below ground. Sowing is possible.

Did you know?

• The genus Gentiana is said to be named after Gentius, a king of Illyria in the 2nd century BC, who according to Pliny was the first to use gentian.
• ‘Pneumonanthe’ means ‘lung flower’: the plant was once associated with the lungs.
• The Alcon blue can only survive where marsh gentian and the right Myrmica ants occur together: the ants feed the caterpillar in their nest until it pupates there.