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Onopordum acanthium – Scotch Thistle

Scotch thistle, like foxglove, is a biennial. The rosette is formed the first year, and it flowers during the second growing season. It dies after flowering. Scotch thistle is the most well-armed plant you will find outside of the deserts of the American southwest. Speaking of the Southwest, it is considered an invasive in the Grand Canyon. Onopordum acanthium is classified a noxious weed in at least 14 states, with each plant capable of dispersing thousands of seeds into the wind.

Scotch thistle, Onopordum acanthium, has devastating thorns and beautiful flowers.

Most of us first experience scotch thistle from the safe confines of an automobile. What are those beautiful purple flowers popping in and out of view as we trundle down the highway? Scotch thistle is just one of many announcements by nature that summer has arrived.

I’ve always been a little fascinated by this ornery plant with the spectacular flowers. The first time I remember seeing thistle was in Scotland, where it is the national flower.

People have been getting pricked by Scotch thistle for a very long time.

Scotch thistle is actually somewhat rare in Scotland. Many people confuse it with spear thistle, Cirsium vulgare. There is debate as to whether Scotch thistle or spear thistle is the actual symbol of Scotland. Thistle became the national emblem of Scotland in the 1200’s, and the Order of the Thistle, the highest honor in Scotland, was founded in 1540. Scotch thistle is definitively one of the symbols of Lorraine, a region in Northeastern France.

I love the Scots, an undoubtedly ornery and fierce bunch for sure. Only they would select such a pugnacious plant as their national flower. The Scots may love thistle, and I have an affinity for it as well. Ranchers of Australia and the western United States take a much dimmer view.

As one can imagine with any plant that has lots of thorns and spreads easily, Scotch thistle is a serious problem in areas where livestock needs to forage. It was introduced as an ornamental in the 1800’s, no doubt by a Scotsman bringing part of his homeland with him.

It is so aggressive, Scotch thistle is capable of making land unusable for livestock.

Widely distributed all over the United States but especially rampant out west, it seems like virtually every university in the region is conducting research on how to eradicate scotch thistle. The answer may be grazing thistle with sheep and goats, who apparently will eat just about anything. Perennial grasses seem to be able to out-compete thistle in many ecosystems.

Scotch thistle has beautiful flowers and sharp spines. It can spread aggressively.

Native to Europe and western Asia, Onopordum acanthium grows four to eight feet tall and is hardy zones 4-8. It likes full sun and is extremely adaptable regarding soil type and water requirements. Scotch thistle is one of those plants we see along the roadsides or in the fields. After flowering, its appearance rapidly declines.

I’ve never seen it purposefully grown in a garden or offered for sale. After writing the preceding sentence I decided to have a look around the internet, and perhaps unsurprisingly you can find it offered for sale. Outside of Scotland perhaps, I cannot imagine purposefully introducing Scotch thistle into any ecosystem. I admittedly find thistle compelling in the landscape, but that doesn’t mean we should endanger ecosystems or our ranchers’ livelihoods.

Onopordum acanthium is a prolific seeder.

Scotch thistle spreads only via seeds and completes its life cycle in two years. I’ve read a number of reputable sources that have indicated as many as 14000, 20000, and 40000 seeds may be produced from a single plant. I have not counted the number of my seeds myself, but it is clear that even with the different statistics presented Scotch thistle is a highly prolific seed producer. The seeds remain viable for up to 20 years, nowhere near as long as the 1300 years of sacred lotus seeds, but impressive nonetheless.

The fact Onopordum acanthium does not reproduce by rhizomes is important as it makes it potentially easier to control. Plants that spread via rhizomes can be a nightmare to eradicate from gardens and ecosystems. Cutting back the plants before they bloom will prevent Scotch thistle from reproducing. Cutting it back below ground level will kill it outright.

Most resources indicate flowering to occur between July and October, but in the Deep South, Scotch thistle is in full bloom by mid-June.

Genus/speciesOnopordum acanthium
Common name(s)scotch thistle, cotton thistle, woolly thistle
Of notebienniel – flowers second year – native to Europe and Asia – considered invasive in many areas
Fall colorinsignificant
Water requirementsdrought tolerant, average
Soil qualityaverage
Hardiness zone(s)5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b
Typeherbaceous perennial
Deer resistantyes
Deciduous or evergreendeciduous
Flower colorpink, purple/violet
Bloom periodsummer
Exposurefull sun

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