''Centaurea nigra'' is a species of flowering plant in the family

Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae w ...

known by the common names lesser knapweed, common knapweed and black knapweed. A local vernacular name is hardheads. It is native to Europe but it is known on other continents as an

introduced species An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...

and often a

noxious weed A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or liv ...

. Although the plant is often unwanted by landowners because it is considered a

weed A weed is a plant considered undesirable in a particular situation, "a plant in the wrong place", or a plant growing where it is not wanted.Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24. ...

by many, it provides a great deal of

nectar Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists ...

for

pollinators A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the maj ...

. It was rated in the top five for most nectar production (nectar per unit cover per year) in a UK plants survey. It also placed second as a producer of nectar sugar per floral unit, among the meadow perennials, in another study in Britain.

Description

It is a

herbaceous Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

growing up to about a metre in height. The leaves are up to long, usually deeply lobed, and hairy. The lower leaves are stalked, whilst the upper ones are stalkless. The

inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed o ...

contains a few

flower heads A pseudanthium (Greek for "false flower"; ) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, compos ...

, each a hemisphere of black or brown bristly

phyllaries In botanical terminology, a phyllary, also known an involucral bract or tegule, is a single bract of the involucre of a composite flower. The involucre is the grouping of bracts together. Phyllaries are reduced leaf-like structures that form one or ...

. Each head bears many small bright purple flowers. The fruit is a tan, hairy

achene An achene (; ), also sometimes called akene and occasionally achenium or achenocarp, is a type of simple dry fruit produced by many species of flowering plants. Achenes are monocarpellate (formed from one carpel) and indehiscent (they do not ope ...

2 or 3 millimetres long, sometimes with a tiny, dark pappus. It flowers from July until September. The flowers sometimes are yellow or white

Wildlife value

In Europe, the plant is an important source of food for the

European goldfinch The European goldfinch or simply the goldfinch (''Carduelis carduelis'') is a small passerine bird in the finch family that is native to Europe, North Africa and western and central Asia. It has been introduced to other areas, including Austral ...

honey bee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current co ...

, lime-speck pug moth, and the following butterflies:

large skipper The large skipper (''Ochlodes sylvanus'') is a butterfly of the family Hesperiidae. Taxonomy It was long known as '' Ochlodes venatus'', but this is a Far Eastern relative. There is still some dispute whether this species should be considered a ...

meadow brown The meadow brown (''Maniola jurtina'') is a butterfly found in the Palearctic realm. Its range includes Europe south of 62°N, Russia eastwards to the Urals, Asia Minor, Iraq, Iran, North Africa and the Canary Islands. The larvae feed on grasse ...

small heath Small Heath is an area in south-east Birmingham, West Midlands, England situated on and around the Coventry Road about from the city centre. History Small Heath, which has been settled and used since Roman times, sits on top of a small hill. Th ...

painted lady ''Vanessa cardui'' is the most widespread of all butterfly species. It is commonly called the painted lady, or formerly in North America the cosmopolitan. Description File:Vanessa cardui MHNT CUT 2013 3 14 Pontfaverger-Moronvilliers Dos. ...

peacock Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...

, red admiral,

small copper ''Lycaena phlaeas'', the small copper, American copper, or common copper, is a butterfly of the Lycaenids or gossamer-winged butterfly family. According to Guppy and Shepard (2001), its specific name ''phlaeas'' is said to be derived eithe ...

and

small skipper The small skipper (''Thymelicus sylvestris'') is a butterfly of the family Hesperiidae. Appearance It has a rusty orange colour to the wings, upper body and the tips of the antennae. The body is silvery white below and it has a wingspan of 25� ...

. In the UK study published by the peer-reviewed journal PLOS One, the plant was even more noteworthy for being the top producer of both nectar and pollen, when the amount of each were compared with the pollen production of the highest nectar producers and vice versa. Of the six species that produced more nectar than this plant, none of them produced a significant amount of pollen. The top producer of nectar was

ragwort ''Jacobaea vulgaris'', syn. ''Senecio jacobaea'', is a very common wild flower in the family Asteraceae that is native to northern Eurasia, usually in dry, open places, and has also been widely distributed as a weed elsewhere. Common names inc ...

and the top producer of pollen was

corn poppy ''Papaver rhoeas'', with common names including common poppy, corn poppy, corn rose, field poppy, Flanders poppy, and red poppy, is an annual herbaceous species of flowering plant in the poppy family Papaveraceae. It is a temperate native with a ...

Similar species

Brown knapweed (''

Centaurea jacea ''Centaurea jacea'', brown knapweed or brownray knapweed, is a species of herbaceous perennial plants in the genus ''Centaurea'' native to dry meadows and open woodland throughout Europe Europe is a large peninsula conventionally conside ...

'') is different in having pale brown bract appendages, no pappus. Flowers August until September. '' Centaurea ×moncktonii'', is a fertile hybrid between black knapweed and brown knapweed.

References

Natural England Website description

External links

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USDA Plants Profile
{{Taxonbar, from=Q1851639 nigra Plants described in 1753 Taxa named by Carl Linnaeus Flora of Maghreb