Photo by Melody

PlantFiles: Spotted Knapweed, Ballast Waif
Centaurea biebersteinii

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Family: Asteraceae (ass-ter-AY-see-ee) (Info)
Genus: Centaurea (sen-TAR-ee-uh) (Info)
Species: biebersteinii (by-ber-STY-nee-eye) (Info)

Synonym:Centaurea maculosa
Synonym:Acosta maculosa

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Category:
Biennials
Perennials

Height:
36-48 in. (90-120 cm)

Spacing:
24-36 in. (60-90 cm)

Hardiness:
USDA Zone 3a: to -39.9 C (-40 F)
USDA Zone 3b: to -37.2 C (-35 F)
USDA Zone 4a: to -34.4 C (-30 F)
USDA Zone 4b: to -31.6 C (-25 F)
USDA Zone 5a: to -28.8 C (-20 F)
USDA Zone 5b: to -26.1 C (-15 F)
USDA Zone 6a: to -23.3 C (-10 F)
USDA Zone 6b: to -20.5 C (-5 F)
USDA Zone 7a: to -17.7 C (0 F)
USDA Zone 7b: to -14.9 C (5 F)
USDA Zone 8a: to -12.2 C (10 F)
USDA Zone 8b: to -9.4 C (15 F)
USDA Zone 9a: to -6.6 C (20 F)
USDA Zone 9b: to -3.8 C (25 F)
USDA Zone 10a: to -1.1 C (30 F)
USDA Zone 10b: to 1.7 C (35 F)

Sun Exposure:
Full Sun

Danger:
All parts of plant are poisonous if ingested

Bloom Color:
Magenta (Pink-Purple)

Bloom Time:
Mid Summer
Late Summer/Early Fall

Foliage:
Herbaceous

Other details:
May be a noxious weed or invasive
This plant is attractive to bees, butterflies and/or birds
Drought-tolerant; suitable for xeriscaping
Average Water Needs; Water regularly; do not overwater

Soil pH requirements:
6.1 to 6.5 (mildly acidic)
6.6 to 7.5 (neutral)
7.6 to 7.8 (mildly alkaline)

Patent Information:
Non-patented

Propagation Methods:
From seed; direct sow outdoors in fall
From seed; winter sow in vented containers, coldframe or unheated greenhouse
From seed; direct sow after last frost

Seed Collecting:
Allow seedheads to dry on plants; remove and collect seeds
Properly cleaned, seed can be successfully stored
N/A: plant does not set seed, flowers are sterile, or plants will not come true from seed

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Profile:

No positives
No neutrals
5 negatives

Gardeners' Notes:

RatingAuthorContent
Negative coriaceous On Feb 28, 2014, coriaceous from ROSLINDALE, MA wrote:

This species has been declared a noxious weed and prohibited in 16 states. It grows and is a problem all across North America.

The currently accepted botanical name for this plant is Centaurea stoebe L. ssp micranthos

Negative Joan On Apr 1, 2007, Joan from Belfield, ND (Zone 4a) wrote:

This plant is listed on the North Dakota invasive/troublesome list and this information is being distributed in a guide developed by the ND Weed Control Association and other agencies.

Plant Features
Biennial or short-lived perennial, up to 4 feet tall
Basal rosette is formed first year
Bracts on flower heads black-tipped
Branched stems on upper half of plant
Small stiff hairs give plants a grayish look
Leaves 1 to 4 inches long, rosette leaves are deeply lobed
Purple flowers (one or more) on ends of branches, up to 1 inch wide
Blooms early July through August
Taproot
Spreads by seed only

Distribution
Documented in a few areas mostly along roadsides. Grows under most conditions

Interesting Facts
Suppresses growth of other plants by releasing a toxin (allelopathic)
Flowers resemble garden bachelor buttons
Widespread invasive in western U.S.

Negative 433kfj On Dec 19, 2004, 433kfj from klamath falls, OR (Zone 6a) wrote:

I've never met a more aggressive weed. It'll grow in any place that has soil - doesn't matter what kind - it just needs to be dirt of any kind, but the better, the more profuse. It has a large tap-root, making it nearly impossible to pull unless you catch it when it is small and the ground is still moist. If it is growing in rocky soil - forget pulling. I've read this plant produces a chemical that actually prohibits other plants from growing around it. Our local paper had an artical describing efferts to isolate the chemicals this plant secretes to use as an herbecide! It might produce something safer than Round-Up, but in the mean-time, that's what I'm using on it. I spot-spray it in the early summer while it is still a rossette of leaves on the ground. Later on it shoots up spikes that flower and the concentation of an effective dose is hard to give without the spray flying everywhere. In addition to the ballast explanation for one name, I might add that the "knap"-in Knapweed reffered to shorn sheepwool that was shipped to various locations, spreading the seed.

Negative caron On Nov 25, 2004, caron from Woodland Park, CO (Zone 4b) wrote:

Colorado Class B Noxious Weed. Mandatory eradication in all counties except Clear Creek, Elbert, and La Plata.
All locations of this plant in Colorado should be immediately reported to the Colorado Department of Agriculture.

Negative Terry On Jan 24, 2003, Terry from Murfreesboro, TN (Zone 7a) wrote:

The common name 'Ballast Waif' refers to its introduction to the U.S., via ship's ballast (the name is shared by Lythrum salicaria, aka Purple Loosestrife.)

Like many other species in this genus, the Knapweed is beneficial for foraging honeybees, but it is an aggressive plant that tends to take over other plants in its midst, namely by exhausting the water and nutrients from the soil. A single plant disperses thousands of seeds in a growing season.

One source indicates that one of the major ways this plant is spread is by vehicles picking up seeds when driving through an infested field.

Regional...

This plant has been said to grow in the following regions:

Woodland Park, Colorado
Pinconning, Michigan
Clyde, North Carolina
Belfield, North Dakota
Klamath Falls, Oregon
Great Cacapon, West Virginia



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