Silver thistle is a fascinating beauty for full sunny locations, which is surprisingly easy to care for. Here you will find how to grow it in your garden.
Profile of silver thistle:
Scientific name: Carlina acaulis
Plant family: daisy family (Asteraceae)
Other names: stemless carline thistle, dwarf carline thistle, smooth carlina
Sowing time: autumn
Planting time: spring or autumn
Flowering period: July to September
Soil quality: sandy to loamy, calcipholous, nutrient-poor to moderately nutrient-rich, low in humus
These information are for temperate climate!
Use in: group planting, dry stone walls, rock garden,
Winter hardiness: hardy, USDA Plant Hardiness Zones: 4 (-32 °C / -25 °F)
Bee and insect friendly: Yes
Plant characteristics and classification of silver thistle
Plant order, origin and occurrence of silver thistle
The silver thistle (Carlina acaulis) is a widespread species in Europe, which occurs mainly in mountainous areas. There it can be found from the valleys to altitudes of 2,800 meters (9,200 ft) on grazed meagre unimproved grassland, common pasturages and sunny forest edges. In some countries, the perennial plant, which belongs to the daisy family (Asteraceae), is an endangered species and must not be taken from the wild. While the name silver thistle is derived from its inner, silvery-white, shiny bracts, its name carline thistle is reminiscent of its earlier use in the treatment of pig diseases.
Characteristics of silver thistle
The silver thistle forms a basic, star-shaped spreading leaf rosette, on which the flower sits directly or from which a short, sturdy flower stalk emerges, which carries the flower. The taproot of Carlina acaulis grows up to one meter (40 in) deep into the soil and anchors the 15 centimeter (6 in) high perennial safely in the ground.
The thorny serrated leaves of the silver thistle impressively prove why it belongs to the large group of thistles worthy of being planted in gardens. The individual leaves are serrated up to the central nerve and grow up to 8 cm wide. As a leaf rosette, they lie in a star shape over the ground.
Instead of ligulate florets as in many other composite plants, the center of the flower is surrounded by silvery-white bracts like a halo of rays. The flower center itself is formed by hundreds of densely packed creamy-white to pink tubular flowers. The approximately one centimeter (0.4 in) long, narrow corolla can only be pollinated by long-nosed insects such as bees, bumblebees and butterflies. The silvery bracts have an important function, because they close over the flower basket before rain falls, thus protecting the flowers and later especially the fruits from rain. This closing is triggered by an increase in humidity: the underside of the bracts then absorbs more water than the upper side of the leaf and the leaf bends upwards. Carlina acaulis is thus considered a weather messenger, as the perennial announces expected rain. The diameter of a silver thistle flower, measured with the inner bracts, reaches 5 to 11 cm (2 to 4.4 in). Its main flowering time is in July, August and September.
The seeds of the silver thistle have a fine pappus, which enables them to reach a greater range when spread by the wind. Furthermore, the seeds are spread by passing animals, in whose fur the thorny bracts get stuck.
Silver thistle – cultivation and care
The silver thistle prefers sunny locations where it is warm and comparatively protected. Due to its maximum height of 30 centimeters (12 in), it does not require too much space upwards. However, if you cultivate a near-natural garden, you should allow the Carlina acaulis a little more space, because it likes to propagate itself and can close gaps relatively quickly. A planting place in a rock garden or near a wall is ideal. Light shade is also well suited, but the thistle should not be planted in complete darkness.
The silver thistle forms a proportionally seen quite long deep or tap root. It therefore requires sufficient loose soil to be able to spread and anchor itself in it. Apart from this, the substrate for the silver thistle should be loose, well water-permeable and poor, and should have a basic pH value. These requirements are met by dry, sandy soil, which is prepared with lime.
The silver thistle can be planted or sown directly into the bed. Since it is relatively resistant to cold, it can be planted outdoors as early as April. For a strong and fast growth, however, pregermination indoors is recommended. This is done as follows:
- fill a tall planter with soil and moisten it well. The substrate should be moist throughout, but no longer be dripping.
- place the seeds on the substrate and press them slightly. A thin covering with sieved substrate is also possible, but should really only be very fine.
- place the prepared planters in a light, slightly warm place. The temperature should be around 20 °C / 68 °F.
- until germination, the silver thistles are watered at small intervals and in sips, so that the substrate is not very moist throughout.
- If they do not germinate after 3-4 weeks, the sowing must be exposed to cold (-4 / 25 °F and 4° C / 39 °F) for 2-4 weeks.
- as soon as young plants are visible, the intervals between watering can be gradually increased.
- when the plants have reached a height of about 10 centimeters (4 in), they may be placed or planted outdoors.
Apart from the time it takes to grow or germinate, the silver thistle is extremely drought-resistant. It therefore rarely requires additional watering. Watering can therefore be limited to longer periods of drought or cultivation in a tub. Watering is done with soft or hard water, lime is no problem for the plant. Between waterings, the substrate should be allowed to dry out on the surface; the thumb test is a good indication of this.
Since the silver thistle thrives mainly on lime-lean meadows or on stony, lean substrates, less is more when fertilizing. Basically, it can be completely abstained from. However, there is nothing to be said against occasionally providing an extra boost of nutrients with organic fertilizers. Suitable for this purpose is untreated pond water, plant liquid swill and compost. However, only in very small doses and at large intervals. Alternatively, some fresh soil can also be applied. Spring and summer are ideal times.
The frugal silver thistle thrives very well in the right location even without additional water and nutrients. However, when cultivating it in a pot, one should take care that the substrate does not dry out completely if the drought persists and that the plant is placed in a frost-free area over the winter.
Pruning is neither necessary nor useful with the silver thistle. However, it is possible and advisable to remove damaged parts of the plant.
A division should be avoided with the silver thistle, because its long taproot would be damaged.
Silver thistles easily and willingly seed themselves. If you discover young plants, you can dig them up and replant them in the desired location. If you want to collect the seeds specifically, you must not miss the ripening stage in the following spring. A fleece bag placed over the flower prevents the seeds from being spread immediately by the wind.
Alternatively, one can also obtain offspring of Carlina acaulis by root cuttings. In winter, when the soil is not frozen, the root ball of the silver thistle is dug out. Remove the soil adhering to the root as best as possible and cut out root pieces about 5 cm long with a sharp knife. Put them one by one into pots with growing soil and pot the mother plant as well. Place in a cool place over the winter and make sure that the soil does not dry out. Only when the cuttings sprout, put the pots in a lighter place, from spring to the outside. In late summer, the young plants will be large enough to be planted in the desired location. The wintered mother plant can of course be planted outside in the bed again in spring.
Diseases and pests
At a suitable location, the silver thistle is usually spared from pests and plant diseases. If small larvae are found in the bottom of the blossom (receptacle), they are probably those of a thistle weevil (Larinus turbinatus), which is specialized in Carlina acaulis and other thistle species. However, the adult weevil also does positive things by pollinating the flowers.
The silver thistle is frost hardy by itself and usually survives winters without further protection. However, the requirement for this is that it is placed in a suitable planting location. As already mentioned, a sunny and warm location is favorable. Additional protection against wind, such as a wall or house wall, is favorable. In very hard winters, the soil can still be insulated with mulch, straw or a layer of foil or fleece. However, Carlina acaulis should not be left unprotected outdoors in a tub, as it lacks the protection of sufficient soil volume. With small pots, the simplest way to spend the winter is to bring it inside. Here the silver thistle should be kept frost-free at a temperature of up to 10° C / 50 °F. A complete drying out of the soil is to be avoided urgently. It is sufficient to check the substrate once a week and water it in sips if necessary.
Use in the garden
The silver thistle is perfectly suited for rock and gravel gardens and near-natural designs. When choosing its neighboring plants, one should rely on partners that give it enough space and effect. The gentian, yarrow, gray-leaved pinks and drought-tolerant grasses harmonize beautifully with it. Carlina acaulis is also a valuable ingredient when planting and designing trays or boxes with an Alpine look. The dried flowers are often used in dry arrangements.
Silver thistle as medicinal plant
Almost forgotten is the use of the silver thistle as a medicinal plant. In the past, its taproot was used mainly against some diseases in humans and animals as well as in magic. Charlemagne is said to have dreamed of the silver thistle as a remedy against the plague. However, while its effectiveness against the plague has not been proven, its ingredients, which include essential oils and resins, bitter substances, carlina oxide, enzymes, flavonoids, tannins and inulin, are considered laxative, antibiotic, diuretic, antispasmodic and diaphoretic. Preparations containing extracts of the silver thistle root are used as laxatives, for colds or skin diseases.
However, one should refrain from using one’s own preparations, as the carlina oxide contained in the roots is toxic.