How to Grow Milk Thistle
Silybum marianum
Milk thistle is a bold, architectural cool-season biennial grown for its silymarin-rich seeds and its dramatic white-marbled leaves. Sow direct or in modules in spring or late summer, 1 cm deep, in full sun and well-drained soil. It needs almost no feeding and tolerates dry conditions well. Deadhead after flowering to prevent prolific self-seeding, and harvest seeds when the fluffy pappus appears. Wear gloves — the spines are genuinely sharp.
When to Start
Sow indoors 6 weeks before last frost, or direct sow outdoors after frost risk has passed and soil reaches 8°C.
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Growth Stages
Approximate days from start
Care Essentials
- Milk thistle is a lean feeder — on reasonably fertile soil no feeding is needed at all. On very poor soil, a single light balanced dressing at transplanting is sufficient.
- Milk thistle self-seeds prolifically if seed heads are left to ripen — deadhead after flowering unless you want it to naturalise. Young leaves (spines trimmed) are edible raw or cooked. Harvest seeds when the fluffy pappus begins to emerge from the seed head. Wear gloves when handling — the spines are sharp.
Growing Conditions
- Min. soil temperature
- 8°C+
- Germination temperature
- 10–20°C
- Direct sow
- —
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Spacing & Planting
| Plant spacing | 60 cm |
| Row spacing | 75 cm |
| Sow depth | 1 cm |
| Mature height | 120 cm |
| Mature spread | 60 cm |
Pests & Diseases
Pest Aphids
Identification: Colonies of greenfly on soft new shoot tips, especially in warm weather.
- Knock off with a jet of water.
- Encourage ladybirds and lacewings.
- Insecticidal soap spray for heavy infestations.
Pest Leaf Miners
Identification: Pale winding tunnels inside the leaf tissue.
- Remove and destroy affected leaves.
- Avoid overhead irrigation which stresses plants.
- Systemic insecticide as a last resort.
Disease Powdery Mildew Erysiphe spp.
Symptoms: White powdery coating on leaves in hot dry conditions with poor airflow.
Treatment: Improve spacing, avoid overhead watering, remove affected leaves promptly.
Prevention: Good airflow and correct plant spacing from the start.
Growing Conditions
| Min. soil temperature | 8°C |
| Germination temperature | 10–20°C |
| Base growth temperature | 5°C |
| Photoperiod sensitivity | day neutral |
Frequently Asked Questions
- How far apart should I plant Milk Thistle?
- Plant Milk Thistle 60cm apart with 75cm between rows.
- Is Milk Thistle frost hardy?
- Milk Thistle is classified as Half Hardy.
