Invasive thistles such as Canadian thistle and Scotch thistle can be a real headache for gardeners and landowners. While both are aggressive and can overrun native plant species, understanding the key differences between these two thistle species is crucial for effective management.

This article explores the 10 best differences between Canadian thistle and Scotch thistle, offering insights into their physical characteristics, growth habits, habitat preferences, control methods, and more.

Whether you’re trying to eradicate them from your land or simply wish to understand the distinctions, this guide is packed with valuable information.

Related: Best Herbicide for Scotch Thistle

What Is The Difference Between Canadian Thistle And Scotch Thistle? 10 Best

1. Scientific Classification

The first difference to note is their scientific classification. Both species belong to the same family—Asteraceae—but they are distinct species with different botanical names. Canadian thistle, scientifically known as Cirsium arvense, and Scotch thistle, known as Onopordum acanthium, have different characteristics at the genetic level. Understanding their classification helps in differentiating the two and identifying the most effective control methods.

• Canadian Thistle: Cirsium arvense
• Scotch Thistle: Onopordum acanthium

2. Physical Appearance

Both thistles are known for their spiny leaves and prickly stems, but they vary in size, shape, and flower structure.

• Canadian Thistle: Typically grows between 1 to 3 feet tall, featuring narrow, lance-shaped leaves with a spiny edge. Its flowers are purple and occur in clusters.
• Scotch Thistle: Can grow much taller, reaching up to 8 feet in height. It has larger, broader leaves with silvery-white hairs, and its flowers are large, pink to purple, and appear singly or in small groups.

3. Leaf Structure and Color

The leaves of these two thistles are another area of difference.

• Canadian Thistle: The leaves are typically dark green with pale veins, and they are usually smaller with spiny edges.
• Scotch Thistle: Its leaves are wider, larger, and have a characteristic silvery-white coating due to fine hairs, giving it a more distinct appearance.

4. Root System

One of the most important distinctions between Canadian and Scotch thistle lies in their root systems.

• Canadian Thistle: It has a deep, creeping root system, allowing it to spread aggressively. The plant reproduces both by seeds and by sending out underground runners, making it very difficult to control once established.
• Scotch Thistle: It generally has a taproot, which is less invasive than the spreading roots of Canadian thistle. While Scotch thistle can form new plants from seeds, its root system is not as resilient in spreading.

5. Flowering Season

Both thistles bloom during the summer, but their flowering seasons vary slightly.

• Canadian Thistle: Typically blooms between June and August, producing small, purple, or pinkish flowers in clusters.
• Scotch Thistle: Flowers appear a bit later, generally from July to September, with larger, purple flowers that attract bees and other pollinators.

6. Habitat and Growing Conditions

Both thistles are adaptable and thrive in disturbed soils, but their preferred habitats differ somewhat.

• Canadian Thistle: Prefers well-drained soils and can be found in fields, pastures, roadsides, and waste areas. It can tolerate a range of soil types and climates.
• Scotch Thistle: It thrives in dry, rocky, and often nutrient-poor soils. It’s more likely to be found in open fields, along roadsides, and in coastal areas.

7. Toxicity to Livestock

Both thistles can be problematic for livestock, but Canadian thistle is generally more toxic.

• Canadian Thistle: Its leaves and stems are toxic to livestock when consumed in large quantities, especially to cattle and horses.
• Scotch Thistle: While it can cause issues if ingested, Scotch thistle is generally less toxic to livestock compared to Canadian thistle.

8. Reproduction and Seed Dispersal

The ways in which these thistles reproduce also set them apart.

• Canadian Thistle: It reproduces by both seeds and vegetatively through its creeping root system, making it a persistent and aggressive invader.
• Scotch Thistle: This species primarily reproduces through its seeds, which are dispersed by wind. While it can spread rapidly, it doesn’t have the vegetative propagation that makes Canadian thistle harder to control.

9. Control and Management

Managing both thistles requires a different approach due to their distinct biological features.

• Canadian Thistle: Due to its creeping root system, it requires continuous control methods, including mowing, herbicide application, and digging out roots.
• Scotch Thistle: Although it can spread rapidly through seeds, it is easier to control by cutting back the plants before they can seed. Herbicides and regular mowing are also effective.

10. Invasive Nature and Impact

Both species are considered noxious weeds, but their invasiveness varies.

• Canadian Thistle: More invasive, it is known for outcompeting native plants and can be very difficult to eradicate once it has established itself.
• Scotch Thistle: Though invasive, it’s less aggressive in terms of root spread. However, it still competes with native species and reduces pasture quality.

What Is The Difference Between Canadian Thistle And Scotch Thistle FAQs

1. What’s the main difference between Canadian and Scotch thistle? The main difference is that Canadian thistle has creeping roots, while Scotch thistle has a taproot.

2. Are both Canadian and Scotch thistle harmful to livestock? Yes, both can be harmful, but Canadian thistle is more toxic to livestock than Scotch thistle.

3. How do Canadian thistles spread? Canadian thistles spread both through seeds and by creeping roots that form new plants.

4. Can Scotch thistle be controlled naturally? Yes, Scotch thistle can be controlled by regular mowing and by preventing the plants from flowering and seeding.

5. Are Scotch thistles taller than Canadian thistles? Yes, Scotch thistles can grow up to 8 feet tall, much taller than the 3-foot height of Canadian thistles.

6. What type of soil does Canadian thistle prefer? Canadian thistle thrives in well-drained soils but can adapt to a variety of soil types.

7. How can I prevent both types of thistle from spreading in my garden? Regular weeding, proper lawn care, and using targeted herbicides can help prevent both species from taking over.

Conclusion

In summary, while both Canadian thistle and Scotch thistle are invasive species, they differ significantly in their physical characteristics, reproductive strategies, and management techniques. Canadian thistle’s aggressive spreading and deep root system make it more challenging to control compared to Scotch thistle.

Understanding these differences is crucial for effective weed control and maintaining healthy landscapes. If you are battling thistles on your property, it’s essential to choose the appropriate management method tailored to the species. Take proactive measures now to protect your land from these persistent invaders!

Call to Action If you are struggling with Canadian or Scotch thistle on your property, don’t wait for the infestation to worsen. Contact a local pest control expert or land management professional today for advice on effective control methods. Let’s work together to restore your garden and preserve your local ecosystem