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Key identifying traits
Very large thistle growing up to 8 feet tall
Stems have broad spiny wings
Leaves are spiny and covered with fine dense hair
Hairy leaf surface causes a grayish appearance
Flower heads numerous, 1 to 2 inches in diameter
Flowers violet to reddish; bracts taper to a spine
Upper leaves alternate and coarsely lobed
Basal leaves up to 2 feet long and 1 foot wide
Biology and ecology
- Tap-rooted biennial that reproduces by seed
- Invades roadsides, range land and disturbed areas
- Thrives in sunny sites, but can tolerate shade
- Up to 50,000 seeds per plant; seeds viable to 6-15
years
- Repeated branching may result in plants 5 feet wide
- Dense stands can create barriers that restrict
livestock
- Drought tolerant but can flourish along open streams
Control
Prevention- Learn
to identify the plant; know your property; beware of fill dirt, hay,
manure and seed from outside your area
Biological – No known
biological effect as yet
Cultural – Competitive
vegetation helps reduce open spaces for invasion
Mechanical – Mowing can stop
seed production in short term, but plants can regrow; digging cutting and
cultivation are effective; monitor for new growth season long; seed bank
will be present for years
Chemical – Several effective
at label rates; best timing is in early spring when plants are in rosette
stage and again in fall when more rosettes germinate;
refer to the
PNW Weed Management Handbook for specific chemical recommendations |
giant bush & pickup (blue tint
is from dye in the herbicide mix)
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