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Identifying Scotch Thistle – Tips For Managing Scotch Thistle Plants

Beautiful but treacherous, the Scotch thistle is the bane of farmers and ranchers everywhere– but it can also make a huge mess in your home garden. Find out what to do about these plants in this article.

Identifying Scotch Thistle

Scotch thistle plants (Onopordum acanthium) boast amazing blooms atop their towering stems, but this invasive species has become a menace to livestock across the country. Its ability to act as a living barbed wire, preventing cows, sheep, and other animals from reaching valuable water sources, has earned the title of noxious weed in most states. Even though it’s not as big of a problem for home gardeners, managing Scotch thistle in your landscape is important in the battle against this troublesome plant.

Although it’s a familiar plant to anyone living in a rural area, Scotch thistle is actually an import from Europe and Asia, used as an ornamental plant in the 19th century. Those early gardeners had no idea the trouble they would unleash with their pretty thistles. The adaptability of this plant is one of its most frightening features. For example, the life cycle of Scotch thistle can change based on the climate, so it may be an annual in one area, but a biennial or short-lived perennial in others.

Positive identification of a Scotch thistle is easy – the sharp-edge, hairy leaves are a dead giveaway. Rosettes of leaves can reach up to 6 feet (2 m.) across and stems may grow from 6 to 8 feet (2 m.) tall. The breathtaking, globe-shaped purple flowers are loved by many, but the seeds they produce can survive in the soil for up to 20 years. Considering that plants produce up to 40,000 seeds, that can create a pretty serious infestation for a long time.

Scotch Thistle Control

As much as Scotch thistle information makes them out to be true monsters of the plant world, they’re surprisingly easy to control in a small scale, which is typically how you’ll find them in the home garden. A few Scotch thistles won’t put up much of a fight, but make sure if you cut them down once they’ve started flowering to burn or bag that flower.

Unlike most plants, Scotch thistle flowers can produce ripe seeds even after being severed from the stem.

The best time to treat Scotch thistle is when it’s still just a rosette on the ground, then a thorough coating of weed killer is all you need. If you’re not ready to break out the herbicide, or your Scotch thistles are in a delicate area, you can hand dig them. Just be sure to wear thick gloves to protect against their sharp thorns.

Note: Chemical control should only be used as a last resort, as organic approaches are safer and much more environmentally friendly.

Common Mullein, Verbascum thapsus

Common mullein.
In midsummer, the tall flower stalks of common mullein, Verbascum thapsus, begin to poke up, making this common weed in the family Scrophulariaceae highly noticeable in the road cuts and waste areas where it thrives. Native to Europe, northern Africa and Asia, it was probably introduced to North America several times as a medicinal herb. In the mid‑1700’s it was used in Virginia as a piscicide (fish poison). It spread rapidly and had become so well established by 1818 that a flora of the East Coast at that time described it as a native. It had reached the Midwest by 1839 and became widely naturalized on the Pacific Coast by 1876. Today common mullein is distributed throughout the U.S. and Canada wherever the growing season is at least 140 days and rainfall is sufficient (50-150 cm), especially on dry sandy soils.
Common mullein is typically found in neglected meadows and pasture lands, along fence rows and roadsides, in vacant lots, wood edges, forest openings and industrial areas.
A dense infestation of common mullein.
This plant, also known as wooly mullein, is an herbaceous biennial or short-lived perennial with a deep tap root. In the first year plants are low-growing rosettes of felt-like leaves. The whorl of leaves emerge from the root crown at the soil surface. The bluish gray-green, oblong to lanceolate leaves are 4-12″ long and 1-5″ wide, and are densely covered in hairs. Vernalization (exposure to cold temperatures) is required to induce flowering the following spring.
A first year rosette (L) and the “wooly” underside of a leaf (R).
In the second year plants produce a flower stalk 5-10 feet tall. The inflorescence is a spike-like raceme, usually singular, but sometimes branched. The alternate leaves on the flower stalk are larger at the base and decrease in size toward the top. The stalk’s growth is indeterminate and the length of the flowering period is related to stalk height, with taller stalks blooming longer. Small yellow, 5-petaled flowers are grouped densely on the leafy spike. White flowers are seen only rarely. They bloom a few at a time throughout the summer, maturing on the stalk from the bottom to the top in successive spirals. Each individual flower opens before dawn and closes by mid-afternoon. The flowers attract a wide variety of insects (bees, flies, butterflies and other insects, only short- and long-tongued bees are effective in cross‑pollination. Flowers are also autogamous, so self-pollination occurs at the end of the day if the flowers were not cross‑pollinated.
The tall inflorescences of common mullein, with the flowers blooming in a spiral up the stalk. Individual flowers have 5 petals.
The fruit is a rounded capsule that splits into two valves at maturity. Each contains dozens of tiny brown seeds. The six-sided seeds are pitted and rough with wavy ridges and deep grooves. Individual plants produce 200-300 seed capsules, each containing 500-800 seeds, so that 100,000‑240,000 seeds are produced per plant. Most seeds fall within a few feet of the parent plant, falling from the capsules when the flower stalk is moved by wind or a large animal. There are no adaptations for long distance dispersal.
The old flower stalks are persistent and conspicuous.
After flowering the entire plant dies – there is no vegetative reproduction. The dead flower stalks are rather persistent, so it easy to detect colonies of this weed at most times of year.
Common mullein is found in many different habitats, occurring primarily in disturbed soils in full sun. The tiny seeds remain viable for decades in the soil (viable seeds have been found in soil samples archaeologically dated from A.D. 1300!), so it is difficult to eradicate completely from an area. Seeds do not germinate well without light, so only those seeds which lie at or near the soil surface will be able to germinate. Populations can reappear quickly after many years when seeds are brought to the surface by soil disturbance. Most seedlings emerge almost entirely on bare sites, such as openings created by animal digging or road construction machinery.
Although common in some areas, it is generally not an aggressively invasive species (except in certain parts of western North America) because its seed requires open ground to germinate. Because individual plants are easy to destroy by hand, it is rarely a problem in gardens and manicured yards but can be quite numerous and conspicuous in other areas at times. It is intolerant of shade, so is easily outcompeted by other plants and agricultural crops. Populations are short-lived on undisturbed sites, “disappearing” into a dormant seed bank within a few years – until the next soil disturbance.
The small seeds of common mullein.
Common mullein is easily managed in smaller areas by manually removing the plants before flowering, preventing soil disturbance and establishing dense vegetative cover that will prevent seed germination. In larger areas, such as nature reserves, single plants and small groups on the edge of the infestation should be targeted first, then working deeper into the infestation. Plants should be pulled, hoed or dug by hand (easiest when the soil is moist) as soon as they are big enough to hold onto, but before they go to seed. Herbicides are generally only used when infestations are very dense but may not be effective because the hairy leaf surface reduces absorption of chemicals. Prescribed burning can also be used during wet weather and when there is snow cover. Mowing is not effective, as the rosettes just increase in size and then bolt once mowing is stopped.
There are a number of beetles that feed specifically on this plant and could be useful for biological control, but only one has been introduced into North America. The curuculionid weevil Gymnaetron tetron was accidentally introduced into Canada before 1937 and has since spread across the continent. The larvae develop in the seed capsules, destroying all the seeds in a seed capsule, but not all the seed capsules on a plant are infested. About 50% of the seeds are destroyed by larval feeding but enough seeds remain that populations are not heavily impacted.
Although thought of primarily as a weed, common mullein has been used as an herbal remedy for coughs and diarrhea, and topical applications against a variety of skin problems. The flowers were also used to make yellow, green or brown dyes, depending on how they were processed.
Common mullein is sometimes grown as an ornamental. Sow seed in late spring to early summer.
– Susan Mahr, University of Wisconsin – Madison

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Canada Thistle

Biological Control
Biological control is the deliberate introduction of insects, mammals, or other organisms which adversely affect the target weed species. Biological control is most effective when used in conjunction with other control techniques.

  • Urophora cardui, a stem gall fly, is the only Canada thistle biocontrol insect approved for use in both Washington and Oregon. Females lay eggs on developing shoots, while larvae burrow into shoots. Their feeding triggers the formation of galls (abnormal swellings) that stress the plant. Growth and flowering may be reduced, but this agent alone does not kill plants or prevent spread.
  • Rhinocyllus conicus, a seed head weevil and Ceutorhynchus litura, a crown/root weevil are agents approved for use in Oregon.
  • Mowing is not recommended in conjunction with biocontrols.

Herbicide Control

  • Only apply herbicides at proper rates and for the site conditions or land usage specified on the label. Follow all label directions and wear recommended personal protective equipment (PPE).
  • For control of large infestations, herbicide use may be effective, either alone or in combination with mowing. Treated areas should not be mowed until after the herbicide has taken effect and weeds are brown and dead.
  • Monitor treated areas for missed and newly germinated plants.
  • Selective herbicides are preferred over non-selective herbicides when applying in a grassy area.
  • Minimize impacts to bees and other native pollinators by controlling weeds before they flower. If possible, make herbicide applications in the morning or evening when bees are least active. Avoid spraying pollinators directly.

Specific Herbicide Information
Herbicides are described here by the active ingredient. Many commercial formulations are available containing specific active ingredients. References to product names are for example only. Directions for use may vary between brands.

  • Effective control can be achieved by using herbicides.
  • A foliar spot application of glyphosate (Roundup) applied in the spring when plants are 6-10 inches tall is an effective treatment. (Note: glyphosate is non-selective and should not be used where grass is desired.)
  • Products containing triclopyr, 2,4-D amine, dicamba, a combination of 2,4-D and dicamba (e.g. Weedmaster), clopyralid, or aminopyralid (Milestone) are effective when applied during the growing season.
  • Cutting at bloom (usually near summer solstice) and applying Milestone or clopyralid to the regrowth late summer or early fall is very effective.
  • Annual treatments for 2-3 years will be necessary.

This BMP does not constitute a formal recommendation. When using herbicides, always consult the label. Please refer to the Pacific Northwest Weed Management Handbook or contact your local weed authority.

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