Scotch thistle

Scotch thistle: how do you recognise it?

Habit

Biennial. A large leaf rosette in the first year; in the second a branched, upright stem about 2 m tall and 1 m wide.

Foliage

Large, deeply cut, spiny-toothed, grey-green to silver-grey because of woolly hairs.

Stem

Winged with spiny edges, white-hairy; sturdy and branched.

Flower

Purple thistle heads of 4–5 cm with a spiny, globular involucre. July and August.

Roots

A deep taproot; the plant dislikes transplanting, so sow in place.

Flowering: July–August · Sowing: April–May and September

Why grow it?

• A striking, silver-grey structural plant of about 2 m, with purple thistle flowers in its second year in July and August.
• Bees and butterflies visit the flowers; later birds such as goldfinches eat the seeds.
• Very drought-tolerant and suited to sunny, dry, poor soil.
• The silver-grey foliage catches the eye even in winter and early spring.
• Little work: no watering, no fertiliser, no pruning.

Scotch thistle: where does it grow best?

• Full sun for good growth, silver colour and flowering.
• Well-drained sandy, loamy or clay soil, neutral to chalky; poor soil is fine.
• Dry; it dislikes wet feet and is very drought-tolerant.
• Allow at least 1 m per plant and place it at the back, where nobody brushes against the spines.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Neutral or chalky

Moisture

Dry

Size

2 m tall, 1 m wide

Hardiness

Hardy (to -20 °C)

Goes well with

• Sea holly: Another silver-grey, spiny structural plant for dry soil.
• Ornamental grasses: Soft stems that soften the spiny structure.
• Viper’s bugloss: Blue flowers in the same dry, sunny corner.
• Evening primrose: Yellow flowers that open in the evening and attract moths.
• Wild marjoram: Low and aromatic; fills in the base and attracts bees and butterflies.

Scotch thistle: when and how to plant it?

1. Sow in April–May, or in September, directly where it is to grow; avoid transplanting seedlings and rosettes.
2. Rake the soil fine and sow on the surface, about 1 cm deep; firm it down.
3. Keep the soil moist until the seed germinates after 2–3 weeks.
4. Thin to at least 1 m between plants; the rosettes grow nearly 1 m wide.
5. In the first year it forms only a rosette; the stem and flowers follow the year after.

Care

Water

Only water seedlings at the start. After that it looks after itself, even in dry summers.

Feeding

Not needed; rich soil gives floppy, over-tall plants.

Support

Usually unnecessary; in a windy spot a strong stake can help.

Deadheading

Cut off spent heads before the seed ripens if you do not want self-seeding.

Clearing

Remove the dead stems after flowering or in winter. Wear thick gloves.

Tip: An impressive silver-grey thistle of 2 m for a dry, sunny spot. Sow in place and give it room; cut off spent flowers if you do not want seedlings.

Scotch thistle: when to prune it?

When: It is not pruned; you remove spent flower heads and afterwards the stem, wearing thick gloves.

How:

• Cut off spent flower heads before the seed ripens if you do not want seedlings.
• Leave a few if you want to feed birds and accept self-sown plants.
• Cut the stem at the base after flowering or in winter.
• Pull up unwanted seedlings while young; it is easiest then.

Caution: The whole plant is spiny: wear thick gloves and long sleeves. It self-seeds freely and can quickly take over a corner or a natural meadow.

Pruning calendar and basic rules

Propagation

Sowing

In April–May or in September in place, about 1 cm deep. Germination after 2–3 weeks.

Self-sowing

Let a few flower heads ripen; seedlings come up on their own and form a rosette the year after.

Scotch thistle: how does it get through the winter?

Hardy to −20 °C.

• Fully hardy, down to about −20 °C: no protection needed in Belgian winters, even as a rosette.
• Leave the stems with seed heads standing: they feed birds.
• In a pot it is too big and too deep-rooted; sow it in open ground instead.

Diseases and pests

• Excessive self-seeding: It self-seeds a lot. Cut off flower heads before they ripen or pull up seedlings.
• Powdery mildew: White powder on the leaves in warm, humid periods. Give plants space and remove affected leaves.
• Aphids: Sometimes on young shoots and buds. Rinse them off with water or let ladybirds do the work.
• Slugs: Young seedlings and rosettes are sometimes nibbled. Search for them in the evening or lay a ring of coarse sand.

Through the year

• February: Remove last year’s dead stems.
• April: Sow in the final position.
• May: Sow. Thin seedlings to 1 m.
• June: In the second year the stems grow strongly. Support if needed.
• July: Second-year plants flower.
• August: Flowering. Cut spent heads to limit self-seeding.
• September: Sow or collect seed. Leave heads for the birds.
• October: Rosettes stay in place. Remove stems.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Birds

Berries, seeds or insects as food, or a place to nest.

Did you know?

• The English name Scotch thistle refers to Scotland, where a thistle is the national emblem; which thistle species it originally was is still disputed.
• In its first year the plant forms only a rosette, often more than half a metre wide; it flowers in the second year only.
• The seeds feed seed-eating birds such as goldfinches.