Plant taxonomy classifies this type of perennial bachelor button as Centaurea montana 'Amethyst Dream.' Descriptive of the flower color, 'Amethyst Dream' is the cultivar name.
Centaurea montana is an herbaceous perennial.
While 'Amethyst Dream' does exhibit a gray-green foliage that some find handsome, many value the plant mainly for its purple flowers. But the floral value goes beyond mere color: the flowers are unusually lacy, giving them an exquisitely delicate appearance. The centers are reddish.
The unopened flower buds are interesting in their own right, reminiscent of little pineapples. In zone 5, blooming begins in May and continues through July. Approximate plant dimensions at maturity of this clump-forming perennial are 20 inches tall x 27 inches wide.
No doubt, the outstanding feature of this plant is its delicate flowers. They display a fine texture that can either be appreciated in its own right or set off against flowers of different colors or coarser texture to create a striking contrast.
Cousin Cultivar With a White Flower
Centaurea montana 'Amethyst in Snow' also has bi-colored, lacy flowers. Fringe-like, white petals surround a purplish center. But this cultivar is noteworthy for more than just being the white-flowered version of 'Amethyst Dream.' Spreading via stolons under optimal conditions (especially in the North), it can "fill in" an area nicely and crowd out weeds, making it an effective ground cover.
Perennial bachelor buttons are indigenous to Europe. In North America they grow best in planting zones 3 through 9.
Sun and Soil Requirements
Plant in full sun for optimal flowering. They need a well-drained ground in which to grow. Once established, they will be drought-tolerant perennials; take care not to over-water. But young plants will need adequate water in order to become established. In terms of soil fertility, this is one of those plants that actually seems to dislike rich soil.
Deadhead after flowering to encourage reblooming in late summer, although the second blooming typically won't be as vigorous as the first one. Since the 'Amethyst in Snow' cultivar spreads via stolons, the plants can become crowded and so can profit from division every few years.
Likewise, because of the proclivity of some types of perennial bachelor buttons to spread vigorously in some regions (mainly in parts of the North), a portion of your care for this plant may entail checking its advance if you don't want more than what you already have.
The plant is prone to powdery mildew. The preventive care recommended to combat this disease is to provide good air circulation. You can achieve this by planning so as to avoid overcrowding in your flower border and by dividing.
Uses in Landscaping
In regions where perennial bachelor buttons tend to become naturalized plants, they can be grown with various types of wildflowers to form a wildflower garden. Because of their drought-resistance, they are useful in large rock gardens. They've traditionally been used in cottage gardens and, more generally, make good edging plants.
'Amethyst Dream' does well enough to attract butterflies to warrant inclusion in butterfly gardens.
Centaurea montana 'Amethyst Dream' is just the tip of the iceberg when it comes to types of Centaurea. By the way, if that genus name makes you think of Greek mythology, you're on the right track for its derivation. It is said that the most famous centaur, Chiron, who was much skilled in the medicinal uses of plants, applied Centaurea to wounds to heal them. And there you have the origin of the genus name—as hard as it is, admittedly to relate such a rugged, hulking beast as a centaur to a plant with such a delicate flower. The specific epithet, montana refers to the plant's mountain habitat in its native Europe.
For examples of some of the different plants that belong to this genus, let's start with the yellow Centaurea called Centaurea macrocephala. The species plant, Centaurea montana has blue flowers—thus an alternate common name, "mountain bluets." But using that common name can lead to confusion, since there is also a little wildflower named "bluets."
The common name, "perennial bachelor buttons," originates from the fact that bachelors traditionally inserted these flowers in their buttonholes when calling upon their lady friends.
Centaurea montana and Centaurea macrocephala are perennials, but there is an annual bachelor button (Centaurea cyanus). This annual (and sometimes the perennial, too) can be referred to as "cornflower," reflecting the fact that the plants are a common sight in "corn" (i.e., grain) fields in southern Europe.
Finally, even a notorious weed named "spotted knapweed" belongs to the Centaurea genus. We could go on, but you get the idea: this genus is nothing if not diverse.