Introduction and Description

Scotch thistle is a noxious weed found in 40 of Idaho’s 44 counties. It is a biennial that occasionally may be a short-lived perennial, which was introduced from the Mediterranean region. Scotch thistle can form dense stands in fertile soil, which are virtually impenetrable.

The plant has robust spiny leaves with white woolly hairs that give them a grey cast. There are more hairs on the lower leaf surface than on the upper leaf surface. The stem has spiny “wings” running down the stem. The leaf outline is egg shaped to narrow spatula shaped, but the general outline is interrupted by irregular spiny teeth along the leaf margin. Leaf length varies between 4 and 20 inches long, and the leaves are arranged in an alternate pattern along the stem. Flowers are arranged in heads, generally with one head per branch, but can range from 2 to 7 heads, with flower color ranging from white to purple. The receptacle (lower, green part of the flower head) is fleshy and deeply pitted, but without bristles. The stems and flower heads can persist through the winter making infestations detectable in the subsequent year. The seeds are spatula-like in shape with 4 to 5 angles and without hairs. Seeds are a mottled brown to nearly black.

Seeds may germinate in the fall after rains have begun. The rosette (clump of leaves at ground level) remains through the entire next year, achieving a diameter up to 4 feet, and producing a thick taproot. The plant will bolt after it’s a full year as a rosette.

Background

Scotch thistle was introduced to the United States in the 1800’s. It is often found in disturbed areas and favors sites with frequent moisture and dominated by annual weeds such as downy brome. Seedlings usually do not compete well with perennial grasses so Scotch thistle is usually found in recently disturbed sites or sites dominated by annual plants. It is not uncommon to find this weed in rangelands and along river corridors.

Scotch thistle competes with, and decreases, desirable forage. The sharp spines deter livestock and wildlife from grazing. In fact, if the stand is dense enough it will create a natural barrier that prevents animals from moving between grazing sites.

Seeds are typically dispersed long distances by water, wind, livestock, and human activity. Seed production of a head appears independent of plant density and each flower can produce between 110 to 140 seeds. Depending on the number of flowers, a plant can produce between 8,400 to 40,000 seeds. Less than 20% of the seeds produced are ready to germinate and the remainder lies dormant in the soil as a reserve for future population establishments.

Scotch thistle seeds contain a water-soluble germination inhibitor that requires moisture before they can break dormancy. Light can also inhibit germination; therefore, seeds need to be buried in the soil or covered by plant residue in order to sprout, otherwise they may remain dormant for up to 20 years.

Management and Control

Scotch thistle only reproduces by seeds. Therefore, preventing seed production and spread should be the main focus of any management strategy. Planting certified seed, cleaning equipment, using screens in waterways, etc. would help prevent seed movement to new sites. Unless the seeds are physically moved, natural dispersal is not far from the parent plant. Therefore, controlling small isolated populations should be the first priority to prevent further spread and establishment.

Mechanical Control

Since Scotch thistle reproduces by seeds only, severing the root below the soil surface will kill the plant. Mowing works partially well if the seed heads are destroyed. More than one mowing will be needed since Scotch thistle does not mature uniformly. Plant should be mowed after plants bolt and before more than a few plants flower since seed from cut flowers can mature. As with any mechanical control strategy, enhancing or establishing perennial competitive plants is necessary to reduce future germination and establishment of Scotch thistle.

Grazing

Proper grazing management can maintain perennial grass health and allow grasses to compete with Scotch thistle. However, continuous stocking of animals in an area tends to reduce grass health. Goats will graze Scotch thistle and have been used successfully as part of a weed management plan. Sheep and cattle will not graze Scotch thistle. In years where grass stand health is reduced, an herbicide may be used to maintain low densities of Scotch thistle, allowing recovery of grasses. Grass stand recovery should include delayed grazing in the next growing season.

Chemical

Several herbicides used in range, pasture and non-crop areas control Scotch thistle. Most herbicide applications are made to growing rosettes. Guidelines are provided below but are not a substitute for the herbicide label. The manufacturer’s label must be followed.

Herbicide	Rate	Timing	Remarks
2,4-D	1.5 to 2 lb ae/A	Spring or Fall	Apply to rosettes in fall or prior to bolting in the spring.
Dicamba (Banvel, Clarity)	0.5 to 1 quart/A Banvel or Clarity	Fall or Spring	Apply to rosettes in fall or prior to bolting in spring.
Picloram (Tordon 22K)	0.5 quarts/A Tordon 22K	Fall	Residual control at 0.25 lb ae/A (0.5 quarts/A Tordon 22K)may extend for 1 year. Most vegetable crops are susceptible do not apply in diversified agricultural production area.
Chlorsulfuron (Telar)	0 1 oz of Telar	Apply to rosettes	Do not apply to dry soils that are sources of dust because of potential for off-site movement of the herbicide.
Metsulfuron (Escort)	1 oz of Escort/A	Apply to rosettes	Use a silicone surfactant or methylated seed oil/ silicone surfactant blend to increase effectiveness
Clopyralid+2,4-D amine (Curtail)	1 to 5 quarts/A of Curtail	Apply to rosettes	In CRP applications wait 20 days after application before mowing or cultivating to allow movement of herbicide through the plants.
Clopyralid (Stinger or Transline)	0.125 to 0.5 quarts/A of Stinger or Transline	Apply to rosettes or prior to flowering
Triclopyr + clopyralid (Redeem R&P)	0.75 to 1 quart/A of Redeem R&P	Apply to rosette or early bolting	Use at least 10 gal/A water and add surfactant according to label.

Summary

A control strategy should include, reducing Scotch thistle populations using the tools listed above, altering management to reduce recurrence of a problem, and to reseed if competitive perennials are absent. Since seeds can remain in the soil for up to 20 years, yearly surveys should become part of the management program in order to maintain low populations of Scotch thistle. Competitive plants are key to long-term control of Scotch thistle because its seedlings are less competitive against established perennials.