Centaurea

( sen - taw - REE - uh )

At-A-Glance Information

This genus of flowering plants includes perennials, annuals, and biennials and need full sun and moderate water. They are commonly called Centaury, Starthistles, Knapweeds, Loggerheads, Basket Flowers, or Bluets. These interesting blooms will attract a variety of bees, butterflies, and hummingbirds to your garden. Annuals are grown for use in cut flower arrangements, and all types are relatively easy to grow and care for. Add Centaurea flowers to mixed beds and borders, cut flower gardens, or in pollinator or drought-tolerant landscapes.

Centaurea Species and Varietals

The genus Centaurea is a large genus of over 700 species that is a member of the plant family Asteraceae. The genus is named to reference Chiron, the centaur in Greek mythology who discovered the medicinal uses of the plant. Species hybridize readily, so there are many crosses and varietals to choose from in the group. The best growing zones for Centaurea vary depending on the species. For more information, see the descriptions below. To find your zone, click here: Zone Map.

Centaurea acaulis (Stemless Star Thistle)
This species is native to Algeria through Tunisia, and produces flowers at ground level from a basal rosette of leaves. These plants don't exceed more than a few inches tall, and produce yellow flowers that have no stem or a very short stem. Foliage is grey-green.

Centaurea alba (Centaurea montana var. alba) (Pale Knapweed)
Native to central Spain and Portugal, this species produces flowers from April through October in its natural habitat. Individual plants reach up to 18 inches tall when mature and in bloom, and the species produces mauve-pink thistle-type flowers.

Centaurea americana (Basket Flower)
This annual grows best in zones 1-24, H1 and H2 and is native to central and southwestern United States and northern Mexico. These plants grow to 5-6 feet tall and 3 feet wide and have rough green leaves. Pale pink flowers bloom in summertime and are four inches wide. Great for fresh or dried arrangements.

Centaurea cineraria (Centaurea gymnocarpa) (Dusty Miller)
This perennial species of Centaurea grows best in zones 8-24, H1 and H2, and is grown as an annual in all other zones. Native to Italy, this plant has a compact growth to 1-3 feet tall and 1 foot wide. Leaves are white and velvety. Flowers are purple, though in some varieties they are white. Plants should be trimmed back after bloom, and are very drought tolerant but can also take regular water.

​Centaurea cyanus (Cornflower, Bachelor's Button)
This annual grows best in zones 1-24, H1 and H2, and is native to northern temperate regions. Gets 1-1.5 feet tall and less than one foot wide, with narrow grey-green leaves and spring to midsummer flowers that come in a variety of colors: blue, pink, rose, wine-red, and white. Great for boutonnieres.

​Centaurea hypoleuca 'John Coutts'
This perennial flowering plant grows in zones 2-9 and 14-24. Native to Asia Minor, this species forms a clump of green leaves 1.5-2 feet tall and just as wide. Flowers are pink.

Centaurea macrocephala
Best in zones 2-9 and 14-24, this perennial native to the Caucasus is an attractive plant with coarse foliage that reaches 4 feet tall and 2 feet wide; summer blooms are yellow and have brown bracts, resembling Thistle. Great for fresh or dried arrangements.

​Centaurea montana
Native to central European mountains, this perennial does best in zones 2-9 and 14-24. Leaves are grey-green and form a clump around 2 feet tall and wide. Cornflower-like blooms top stems in late spring through midsummer. Variety 'Black Sprite' (pictured) has spidery, dark flowers. This species should be given protection from snails, and divided every other year. Water regularly.

​Centaurea moschata (Amberboa moschata) (Sweet Sultan)
This annual flowering plant is native to Asia Minor and does best in zones 1-24. Has an erect habit, and branches out at the base. Gets to 10 inches wide and 2-3 feet tall. Flowers are fluffy and bloom from spring to fall in shades of lilac and pink, though sometimes in white or yellow in certain varieties. Great as a cut flower. Needs a lot of heat for best performance, and does not do well with overhead water.

​Centaurea rothrockii (Basketflower)
Can be an annual or biennial, and grows best in zones 1-24. Native to Southwestern United States, this plat gets to 3-6 feet tall and 2 feet wide. Flowers have an exotic look, are about 5 inches wide, and bloom from the middle of summer through fall. Blooms are cream or yellow in the centers, and pink around the edges. Seeds for this species should be started inside in frost-prone areas, and then planted outdoors in spring after the last chance of frost has passed. If cutting for arrangements, cut just before the flowers open. 

Caring for Centaurea

Centaurea, or Knapweed, is best planted in wildflower gardens or massed together in borders, where they can grow a bit wild. Both annual and perennial varieties are typically started from seed, though some nurseries may carry starter plants. Install Knapweed in springtime, when temperatures have reached above 65 degrees Fahrenheit. Seeds can also be started indoors about a month before the last frost date of your zone. In places with mild wintertime temperatures and weather patterns, these plants can be installed in the middle of autumn for an early springtime bloom. These flowers are considered easy to care for. They prefer a spot in the garden that receives full sun to partial shade. They will bloom and grow best in a spot that receives between 6 and 8 hours of direct sun each day, but in the warmest climates they appreciate some afternoon shade. In locations without enough sun, these plants will get leggy and require a bit more pruning to maintain a bushy appearance. Knapweed needs to be planted in soil that is well-draining with a pH of around 7.0 (or a bit above). If local soils are particularly poor in nutrients or do not drain well, make sure to amend the planting area with plenty of organic compost before installing.

After planting, water regularly as they establish a strong root system. At maturity, these plants are drought-tolerant and can get by on rainwater alone for most of the year, with supplemental waterings during extended periods of dry weather. Take care to not overwater Centaurea flowers, as too much moisture causes weak stem growth and droopy flowers. These plants do not need much in the way of fertilizers; in springtime, simply blanket the planting area with a couple inches of organic compost and work it into the soil. Before applying, pull back any mulch, and reapply the mulch afterwards. This will provide these flowers with enough nutrients to bloom and grow well in their most active seasons. Pruning is simple for these flowers; deadhead as needed to neaten the appearance of plants and to encourage more blooms to pop. In the middle of summer, many Centaurea species can look a bit haggard, and can be rejuvenated by cutting back a third to half of the plant. 

Propagation

Centaurea can be propagated either by using cuttings or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the cutting method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. To propagate your Knapweed, follow the instructions below:

Propagating Centaurea by Cutting
  1. Gather your materials. You'll need a clean, sanitized, sharp pair of garden shears, rooting hormone, a pencil, and a 3-inch pot filled with well-draining compost or other organic material. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand.
  2. In spring or early summertime, choose the plant that you'd like to propagate. Ensure that this plant has had a good watering the night before, as this will improve the chances your cutting will take root. Cut one or more 4-6 inch portion(s) of the end of a non-blooming branch and strip the leaves off of the lower half. Make a hole in your planting medium by pushing your pencil into the soil. Dip the end of the stem in rooting hormone, and plant it in your pot. Fill in soil around the cutting to support it, but do not pack it down too much.
  3. Moisten with water, and ensure that the pot has good drainage. Your pot should be kept in warm, sunny conditions indoors (and not in direct sunlight). Water whenever your soil is dry under the top layer. After about four to six weeks have passed, your cutting should be rooted. When you notice new growth, you'll know you have roots. 
  4. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring, when you can transplant it to its new home. Ensure that the last frost of the year has occurred before doing this, as it can seriously damage your new flowers.

Propagating Centaurea by Seed
  1. Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand. You will also, after a few weeks, need individual 3-inch pots for each seedling that grows. 
  2. Harvest your seeds. The seed heads on Knapweed plants are pretty easy to spot, they're left behind after flowers have faded. Once they have dried out and begun to release seeds, you can cut them off of the plant, placing them in a paper bag. Harvest as many as you can, and place the paper bag in a dry, cool, dark area. The bag can be shaken once in a while to help dislodge seeds. Once they have been released, take them out of the bag and put them in a container so you can work with them.
  3. Water your prepared seed tray or pot filled with soil, and ensure that good drainage is occurring. Place your seeds in your prepared pot or seed tray about a quarter inch to a half-inch under the soil surface. Cover them with soil and water with your spray bottle. Place your tray of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well. ​Annuals take 7-10 days to germinate, and perennials take between 15 and 30 days to germinate from sowing. Once you begin to see seedlings sprout and they are able to be handled, transplant them to their own individual 3-inch pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down with the tines of your fork about an inch and a half, gently working the seedling up until it is out. Then, replant the seedling in its individual pot.
  4. Keep the seedling's potting medium moist, but not wet, by spraying the soil with your spray bottle. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.
 
Propagating Centaurea by Division
This type of propagation is relatively simple and is done with perennial species only; you will need a clean shovel, mulch, fresh soil (any kind will do, though it does need to be well-draining), compost, and pots (only necessary if you will be saving divided plants for transporting elsewhere). Division is best done in spring or fall (after the plant has entered dormancy and been cut to the ground, but before it has begun its new spring growth), and needs to be done every two to three years regardless of whether you have the goal of propagating the plants for best performance, especially if Knapweed is being grown in a container. To propagate by division, follow these steps:
  1. Gather your materials. Using a shovel, dig up your existing plant. Start from the outside and work your way in to avoid damaging the roots. Once you have worked it up out of the ground, lay it out.
  2. Start to tease apart the roots with your gloved fingers. Once the roots are untangled, separate your plant into segments, ensuring that each one has at least one thick root section to grow from.
  3. Prepare your pot(s) or new planting site(s) with amended, well-draining soil. You can either bring in new soil or simply amend your existing soil with compost to complete this task. Then, plant your divisions and water thoroughly. 
  4. Apply a thick layer of mulch over the new planting to protect it from cold weather as it acclimates to its new home. Then, enjoy watching it sprout.
Centaurea dried seed heads can be cut off of plants and harvested for their seeds.
Centaurea seeds are typically light brown to tan in color, and are relatively small.

Problems and Pests

Centaurea plants are typically relatively pest and disease free in gardens, though they can have issues from time to time with:
  • Aphids
  • Powdery Mildew
  • Rust
For more information on these pests and diseases, including how to treat and prevent them, consult the following links:
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Gallery

Some species of Knapweed have curiously thin, spidery petals.
Knapweed will draw many pollinators, including bees and butterflies, to the garden.
This garden features Centaurea as a lawn-adjacent pollinator plant.
Cornflowers come in many different shades and flower compositions, including double blooms.
This informal garden bed features Centaurea alongside white Aquilegia and light pink-mauve Chaerophyllum.
The lush foliage of Centaurea plants fills in bare garden spaces well.
This natural, meadow-style garden features yellow-flowered Centaurea macrocephala.
Centaurea cineraria has blue-grey foliage and is commonly called 'Dusty Miller'.
Knapweed flowers have an intricate design that is gorgeous when viewed up-close.

Videos

A great introduction to this genus of flowering plants.
A growing and care guide for these plants.
All about Cornflowers.
Learn some quick facts about these easy-care flowers.

Resources

How to Grow and Care for Cornflower (Bachelor's Button)
Centaurea montana
Centaureas: The Best Varieties to Grow and How to Care for Them
How to Plant and Grow Centaurea
Wikipedia: List of Centaurea Species
Wikipedia: Centaurea
The New Sunset Western Garden Book: The Ultimate Gardening Guide (2012) - Pages 230 - 231, Centaurea
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