The Weather Thistle: Nature’s Living Barometer

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Weather thistle (Carlina acaulis). Source: laidbackgardener.blog

So, is it going to rain? Or can you expect a sunny day?

These days, we just pull out our smart phone and check the weather app. Or tune in to the weather channel on our TV. That’s great! But our ancestors had no app, smart phone, weather channel or TV, not even meteorologist to help them. They looked to Mother Nature for signs of what weather was on its way. And some used the weather thistle to predict rain. In fact, if your relatives come from mountainous regions in Europe, where the plant grows wild, this may be a plant they knew well and used daily.

Weather thistle is just one name for this very unusual perennial (Carlina acaulis). It’s also called stemless carline thistle, dwarf carline thistle or silver thistle. It’s an easy-to-grow although rather obscure perennial that acts as a living barometer.

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Flower closed? It’s either raining or going to rain. Photo: MaxPixel.freegreatpicture.com

So how does a thistle predict the weather? Its flower operates exactly like a barometer. When the weather is dry, it opens wide, but if rain is on its way, it closes up tight. It won’t tell you what’s happening overnight, though, as it also closes in the evening to open again the next morning.

20180206D Fritz Geller-Grimm, WC .jpg

This dried weather thistle nailed to a door says both “welcome” and “it’s not going to rain today”. Source: Fritz Geller-Grimm, Wikimedia Commons

If that wasn’t odd enough, the weather thistle continues to predict the weather even after it’s dead, as its dried flower still reacts to changes in air pressure. In fact, in some parts of Europe, you’ll see dried weather thistles tacked onto walls and many people still count more on its power to predict the weather than apps and weather channels. When nailed to a front door, it’s a traditional symbol of welcome.

What Is It?

It must be said from the start that the weather thistle (Carlina acaulis and related species) is an odd plant in so many different ways. Essentially, it’s an ornamental, non-invasive, stemless (that’s what acaulis means: no stem) thistle. However, for all its weirdness, it’s actually a highly attractive perennial and one you can easily grow in your garden.

Due to its stemless nature, the weather thistle is necessarily a very short plant, only about 15 cm (6 inches) high and perhaps 1 foot (30 cm) in diameter at first, although widening over time as the number of rosettes increases.

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Older weather thistles produce several rosettes, each bearing one inflorescence. Source: Walter Culicelli, http://www.campocatinometeo.it

This plant of the Asteraceae family consists of a ground-hugging rosette of dark green, shiny and very prickly leaves bearing a single “flower” (in fact, a compound inflorescence reminiscent of a single flower) smack dab in its center. The inflorescence sits directly on the rosette, quite stemless, but in fact, the plant is not completely acaulescent (stemless): you’ll note there is a short stem under the leaves, especially after the first year of growth.

The 4-inch (10 cm) “flower” (yes, it’s huge!) consists of a cream-colored central disc becoming beige over time surrounded by narrow silvery-white bracts with a papery texture. The effect is striking, like a giant silver daisy!

The bloom lasts a good two months, starting in late July or August, depending on your local climate. It will actually dry on the spot and last through the winter.

The plant initially forms a single rosette, then one to several secondary rosettes, eventually forming a much-widened dome up to 3 feet (1 m) across.

Easy to Grow

The “thistle” side of this plant comes out when you grow it, because it’s as tough as nails.

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In the wild, weather thistle is found on mountain slopes and dry fields. Source: www.natura-2000.eu

Like most thistles, the weather thistle prefers full sun (although partial shade is acceptable) and poor, dry soil. In other words, you can grow under the worst possible conditions, in sand or gravel, and it will thrive! It also tolerates alkaline soils, even chalky ones, although doesn’t complain about slightly acid ones either. You don’t have to mistreat it, though. It does fine in ordinary garden soils, even enriched ones, but although you may feel the need to feed its neighbors, there is no need to add fertilizer or compost for this plant.

What is not negotiable, though is perfect drainage. You can even grow it in clay, but only if any excess moisture drains away quickly. It will not tolerate sitting in soggy soil.

It’s not surprising that this plant is very cold hardy (zone 3), giving its alpine origins. However, it is also very heat resistant and doesn’t fear long, hot summers either.

The weather thistle is difficult to move, at least after its first year of life, because of its long carrot-shaped taproot. It’s best to plant it from the start in a permanent spot rather than trying to transplant a mature specimen. For the same reason, it’s not an easy plant to divide, even when multiple stems are present.

Normally, weather thistle is always multiplied by seed, a simple enough process that will give a blooming plant the second year. I suggest starting new plants every 4 or 5 years, as it is not necessarily a very long-lived plant. Don’t count on it maintaining itself by self-sowing, either. It rarely does so in home gardens.

For dried specimens (they make a rather unique ornamental wall decoration, for example), cut off the rosette in late summer, before seeds form. Note that the plant doesn’t always grow back when you cut off its head!

The weather thistle is a good choice for rock gardens and wall plantings. It can also be used to edge well-drained flower beds. Just don’t place it where you tend to walk barefoot: those prickly leaves are quite nasty!

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Here, seeds are starting to form and this usually attracts goldfinches. Source: MaxPixel.freegreatpicture.com

The flowers attract bees and butterflies during the summer, seed-eating birds in the fall and winter.

A Royal Name

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The genus Carlina honors French emperor Charlemagne. Source: Romartus, Uncyclomedia Commons

The name Carlina honors Charlemagne (Carolus Magnus in Latin), because legend has it that an angel showed him this plant saying it could cure the bubonic plague that was devastating his troops. Its root, called carlinae radix, is still used in traditional medicine as an antibiotic, antispasmodic, carminative, diaphoretic, digestive, diuretic and emetic. Be careful, though, as it is purgative at large doses.

The large flower bud can be harvested when immature, cooked and eaten like an artichoke, a use which earned it the nickname hunter’s bread.

Other Carline Thistles

There are about 30 species of in the genus Carlina, all generally referred to as carline thistles or weather thistles. Most are biennials or annuals, but there are also other perennials and even a few shrubs. Only a few are cultivated, though. The weather thistle  described above (C. acaulis) is referred to a stemless carline thistle or stemless weather thistle when comparing it to other species and is probably the most widely grown species.

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Carlina acaulis simplex ’Bronze’ differs from other types by its more upright habit and its bronze leaves.  peaganum Flickr

There is a taller form of weather thistle, C. acaulis simplex, often called short-stemmed stemless thistle (I kid you not!). As the common name suggests, the inflorescence of this species is not stemless (acaulescent) but raised above the leaves on a short stem. It can reach 1 foot (30 cm) in height, twice that of its stemless brother. This subspecies tends to be monocarpic and is (or ought to be) sold as a biennial. The cultivar ’Bronze’ with dark purple foliage, is popular and a little more perennial than the species, flowering more than once.

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Golden carline thistle (Carlina acanthifolia). Source: FGZ, http://www.viajesapie.com

The golden carline thistle (C. acanthifolia), is very similar to C. acaulis and just as ground-hugging, but its flowers golden rather than silver. It’s a perennial. Zone 4.

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With its upright habit, smaller leaves and multiple blooms, common carline thistle (Carlina vulgaris) is the most thistlelike of the commonly grown species. Source: http://www.inaturalist.org

Finally, common carline thistle (C. vulgaris) is a biennial species of greater height (20 inches/50 cm) with a distinctly upright habit. The first year, it produces the typical carline ground-hugging rosette, but the second year, up pops an erect stem bearing several smaller flower heads. Zone 4.

All these carline thistles have barometer flowers.

Where to Find One

Here’s the bad news. Carline thistles of any species are hard to find in garden centers. Some alpine nurseries do offer them, but most of the time, this is one perennial you’ll just have to grow from seed. Below are a few catalogs (there are many others) that offer seeds of Carlina acaulis and other carline thistles.

Enthoplants
Gardenseedsmarket
Richters Herbs
Sand Mountain Herbs20180206A Carlina acaulis HC

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