Silybum Species, Milk Thistle, St. Mary's Thistle
Silybummarianum

Synonym:Silybum intermedium

Synonym:Silybum leucanthum

Synonym:Silybum mariae

Category
Biennials

Water Requirements
Average Water Needs; Water regularly; do not overwater

Sun Exposure
Full Sun

Foliage
Grown for foliage
Deciduous
Good Fall Color

Foliage Color

Height
4-6 ft. (1.2-1.8 m)

Spacing
24-36 in. (60-90 cm)

Hardiness
USDA Zone 6a: to -23.3 °C (-10 °F)
USDA Zone 6b: to -20.5 °C (-5 °F)
USDA Zone 7a: to -17.7 °C (0 °F)
USDA Zone 7b: to -14.9 °C (5 °F)
USDA Zone 8a: to -12.2 °C (10 °F)
USDA Zone 8b: to -9.4 °C (15 °F)
USDA Zone 9a: to -6.6 °C (20 °F)
USDA Zone 9b: to -3.8 °C (25 °F)

Where to Grow

Danger
Plant has spines or sharp edges; use extreme caution when handling

Bloom Color
Medium Purple

Bloom Characteristics

Bloom Size

Bloom Time
Mid Summer
Late Summer/Early Fall

Other details
May be a noxious weed or invasive

Soil pH requirements
6.6 to 7.5 (neutral)
7.6 to 7.8 (mildly alkaline)
7.9 to 8.5 (alkaline)

Patent Information
Non-patented

Propagation Methods
From seed; direct sow outdoors in fall
From seed; winter sow in vented containers, coldframe or unheated greenhouse
From seed; sow indoors before last frost
From seed; direct sow after last frost

Seed Collecting
Allow seedheads to dry on plants; remove and collect seeds
Wear gloves to protect hands when handling seeds
Properly cleaned, seed can be successfully stored