Plumeless Thistle Identification & Management

Common Name: Plumeless Thistle, Welted Thistle, Bristly Thistle, Broad-winged Thistle 
Scientific Name: Carduus acanthoides  
Legal Status: Prohibited-Eradicate 

Plumeless Thistle was added to the Maryland Noxious Weed Regulations in 1984. It is one of four thistle species on the noxious weed list. The law prohibits the import and transport of noxious weeds throughout the state and requires that infested lands be managed for their eradication or control. 

What is Plumeless Thistle?

Plumeless thistle is an aggressive biennial in the sunflower family and closely related to bull thistle and musk thistle. It is native to Europe and Asia and was first introduced to North America in the late 1800s as an ornamental.

Plumeless thistle reproduces solely by seed and germinates in the spring, but continues emerging through the fall. During the first year, it produces a rosette of leaves and a fleshy taproot. In the second year, the plant bolts and flowers. Blooming in Maryland occurs from June through September, with peak flowering often occurring in July. One to three weeks after flowering, the seeds are dispersed by wind, water, and animals. Individual flower heads generally produce ~ 50 to 80 seeds and a single mature plant can produce upwards of 9,000 to 10,000 seeds per season. While the majority of seeds germinate within the first one to three years, deeply buried seeds may survive for over 10 years. 

Plumeless thistle prefers growing in open, sunny areas and is most commonly found in disturbed sites. It typically inhabits pastures and fields, stream valleys, roadsides, rights-of-way, woodland edges, agricultural land and waste areas. Infestations can reduce productivity of pastures and rangeland preventing livestock from grazing plants which can also reduces native plant bio-diversity.

Appearance

Plumeless thistle forms a low-growing basal rosette (cluster of radiating basal leaves) that can reach up to 2 feet in diameter. In its second year, a tall central stem emerges and grows upward, typically reaches 3 to 5 feet in height by mid-summer, and sometimes growing as tall as 6 feet. As it develops, several heavily spined stalks branch out giving it a bushiness and candelabrum-like appearance. Stems are narrow and covered with spiny, leafy wings that extend all the way up to the base of the flower head. They may also be covered in fine, bristly hairs. Leaves are deeply lobed, narrow, spiny and wavy with yellow spines along with white-colored leaf margins and a woolly or hairy underside.

Solitaty or clustered flower heads (2 to 5 per cluster) grow at the ends of the highly branched, winged stems. These small, globular or bulbous shaped flower heads are reddish-purple to pink in color and measure 0.5 inch to 1 inch in diameter. The flower base is encased in numerous, narrow, needle-like, spiny bracts. The small seeds (5 mm long and 1 mm in diameter) are oblong, smooth, slightly curved, grey to light brown in color and often with tuft of white, fine, and simple bristles (pappi). The bristly pappi appear more fluffy than feathery and act as a parachute that allow the seeds to be easily dispersed by wind within 7 to 10 days of flowering. The bristles often break off and fall away separately. The name "plumeless" refers specifically to this lack of feathery (plumose) hairs on the seed-like fruit. Its large, fleshy taproot acts as a nutrient reserve, allowing it to re-sprout if not fully removed.

Key characteristics

These characteristics distinguish Plumeless thistle from bull thistle, Canada thistle and musk thistle:

• Flowers — Small purple pink flowers appear solitary or clustered on the stem with a spiny globular flower base. 
• Leaves — Deeply lobed and spiny with yellow spines and white hairy underside.
• Stems — Heavily spined stalks branch out giving more of a bushy appearance.
• Roots — Large taproot.

Prevention

• Scout frequently in the early growing season and act early, as seeds can remain dormant in the soil for up to 10 years.
• Keep lawns and pastures thick and healthy, plumeless thistle thrives in disturbed/bare soil. Re-seed soil with a certified weed-free seed to help crowd out the plumeless thistle. 
• Keep equipment clean. Equipment is the most common method for moving seeds.  
• When harvesting crops, it’s best do the non-infested fields first and do the infested fields last. This prevents plumeless thistle seeds from spreading. 

Management

Managing plumeless thistle requires a multi-year, integrated control strategy focused on preventing seed production and reducing root reserves. Combining these control methods can enhance the best long-term control for plumeless thistle. 

• Chemical — Apply a post-emergent broadleaf herbicide when the plant is actively growing. The most effective timing is to apply herbicide during the rosette stage in late fall (October - November) as it is actively transporting nutrients to its roots, and kills the plant before winter. Mid spring, before bolting, is the second best time to apply herbicides. For bolted plants, applying herbicide to the cut surface can also be effective. Read labels before use. Use a surfactant to increase absorption of the herbicide by the leaves. Do not apply when the soil is dry or the plant is stressed.
• Mechanical — Mowing close to the ground prior to seed dispersal, this will limit the amount of seed available for germination. This method will need to repeated several times throughout the growing season (minimum three times). Wash equipment thoroughly to prevent seeds from spreading to new areas. Cultivating/tilling 2-4 inches below the soil surface to destroy their root system. Digging/hand pulling prior to flowering is an effective method for controlling small infestations, this removes the entire taproot (use protective gloves).
• Biological — Livestock (cows, goats, and sheep) can help prevent infestations in rangelands and hay meadows, most effective in the early spring during the seeding and rosette stage. But will be difficult to manage on large infestations.
• Cultural — Planting competitive cover/smother crops can help suppress seedling development. Apply mulch/synthetic tarps to soil surface to exclude sunlight and prevent seed germination. Crop rotation interrupts weed life cycles by planting different crops in sequence.

Note: Land under the Conservation Reserve Program should consult with USDA Farm Service Agency on weed control and management.

Resources

For specific herbicide recommendations as well as organic options, contact University of Maryland Extension Weed Management Specialist, Kurt Vollmer: [email protected] or Ben Beale: [email protected]

MDA has entered into an agreement with many counties in the State to provide technical assistance to landowners. Many of the programs provide herbicide application to landowners on a fee-for-service basis. To learn whether your County has a Weed Control Program, visit plants and pests. 

For weed identification contact your local Extension office or your regions MDA Agricultural Inspector.