Many species of flowering plants belonging to the Centaurea genus are called bluets. This group of plants includes both annuals and perennials, and all of the species are herbaceous, meaning the parts of the plant that are above ground, like stems and leaves, die off every year. Most species in the Centaurea genus are thistle-like plants, with feathery flowers shaped like cups or baskets that are usually various shades of blue, but can also be red, white or yellow in color. Bluet, sometimes spelled blewitt, can also refer to other types of plants, including plants in the genus Hedyotis, Houstonia, or Oldenlandiopsis that all belong to the Rubiaceae plant family. These plants are usually low growing or creeping, with small flowers that are blue or white in color.
Bluets belonging to the Centaurea family are related to daisies and thistles, and various species are common in the Northern hemisphere both as garden plants and wild flowers. Various types of these bluets are known by different names, for example bachelor buttons, knapweeds, basketflowers, cornflowers, and starthistles. In British English, bluets are sometimes also called loggerheads. In the wild, these plants are strong growers, and can even become invasive if the growing conditions are favorable for them. Centaurea plants prefer full sun and often produce a lot of nectar, attracting many types of insects, including bees and butterflies.
Many species of Centaurea are easy to grow as garden plants, and their seeds are commonly available in stores. The seeds can often be sown directly in the garden in late spring or early summer, and should be covered thinly and kept moist in order to germinate successfully. Generally, the plants prefer full sun, are relatively drought-resistant, and will tolerate various kinds of soil. Centaurea bluets usually grow 18-36 inches (45-90 cm) tall, and the flowers can be used as cut flowers, or dried.
The other kind of bluets, belonging to the Rubiaceae plant family, include Oldenlandiopsis, more commonly known as creeping-bluet, and the Hadyotis genus of plants. These plants are most commonly found in warm, tropical areas like Hawaii, Mexico and Central America. The closely related plant genus Houstonia includes many species called bluets, and these flowers are common in many parts of North America both as wild flowers and garden plants. It includes species like Houstonia caerulea, also known as azure bluet, and Houstonia pusilla, or tiny bluet. Most Houstonia species usually grow no higher than 3 inches (7.5 cm) tall and often make excellent plants for rock gardens.