Knapweeds & Allies
What are they?
The knapweeds are closely related to the thistles but most of them are spineless, so they rather resemble thistles in their flowers but have simple or compound, spineless leaves. This is a rather large and complex group of often rather similar plants and botanists are still reviewing the species limits of some of these plants, resulting in occasional changes to the names and the 'splitting' of new species; thus, some plants may have different names here to the ones used in less up-to-date books.
Where are they found?
These are mostly perennials of open, grassy places, roadsides and sometimes rough ground in urban areas. Some are grown as annuals in 'wildflower' mixes and may appear temporarily in cultivated strips, sown by farmers/landowners or local authorities.
Identification
The shape of the leaves is important (though sometimes variable), while the presence or absence of spines can also be useful. As with most members of the Asteraceae, the appearance of the phyllaries - the green or brown, outer part of the flowerhead - is important and these should always be checked. Note that most species can sometimes produce plants with white flowers.
Slender Knapweed Centaurea debeauxii
Native and probably the commonest native knapweed in most kinds of grassy places on drier, nutrient-poor soils and undeveloped habitats. Flowers June to September. The basal part of the flowerhead is relatively narrow and parallel-sided in outline and the phyllaries are relatively open, such that their greenish bases are easily seen in the lower half of the flowerhead. Leaves are variable and may be simple or deeply toothed or lobed. Also often referred to as Chalk Knapweed, a name which has little relevance to the species as a whole.
See also the second paragraph under Common Knapweed (below).
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Common Knapweed Centaurea nigra
(Black or Lesser Knapweed) Native and introduced. As a native, probably most common in valley bottoms on damper soils, but also very widely sown as a constituent of 'wildflower mixes' on roadsides, re-wilding projects and greening projects around new builds. Many of these plants are from non-native seed sources. Flowers June to September. The basal part of the flowerhead is relatively broad and rounded in outline and the phyllaries overlap strongly, such that their greenish bases are difficult to see in the lower half of the flowerhead. Occasionally the flowers have 'rayed' forms which resemble Greater Knapweed but can be told from each other by their leaves; such plants seem to be typically from introduced seed mixes. Leaves are variable and may be simple or deeply toothed or lobed.
The situation with Slender and Common Knapweeds is greatly confused and problematic. Many references now split these as two separate species (which I have followed here), but this recent decision seems largely to be based on the opinion of a single, non-peer reviewed publication, which itself is based on the thoughts of a single observer in one small part of the country. In truth, the great majority of these knapweeds that occur in our area are intermediate between the two extremes and are doubtfully identifiable as either Slender or as Common Knapweed. This situation has perhaps come about through the abundant introduction of non-native plants which has resulted in a mass of hybrid forms. Many plants are perhaps best called Common Knapweed with the understanding that this name best represents an aggregate of forms involving both Centaurea debeauxii and C. nigra.
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Greater Knapweed Centaurea scabiosa
Native. Widespread on sandy or chalky soils but uncommon in the east of the region on heavier soils. Flowers June to September. Best told from Common and Slender Knapweeds by deeply dissected leaves and the larger flowerheads with distinctive phyllaries.
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Brown Knapweed Centaurea jacea
Introduced from mainland Europe. Formerly more common but now probably extinct in the region except as a rare and short-lived casual from seed mixes. Flowers July to September. The basal part of the flowerhead has brown, strongly overlapping phyllaries with pale, papery margins.
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Red Star-thistle Centaurea calcitrapa
Introduced from southern Europe. Formerly more common but now probably extinct in the region except as a rare and short-lived casual from seed mixes. Flowers July to September. A sprawling, many-stemmed plant with deeply cut leaves. The phyllaries bear stout, straw-coloured spines that elongate and spread outwards in a star-like formation before the flowerheads expand.
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Rough Star-thistle Centaurea aspera
Introduced from southern Europe. Established on Weybourne cliffs in the 1970s but now probably extinct in the region. Flowers July to September. Lower leaves deeply lobed, becoming simple and unlobed in the upper part of the plant. The phyllaries bear strongly recurved, triple spines at their tips.
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Persian Cornflower Psephellus dealbatus
Introduced from western Asia as a garden ornamental and twice recorded in our region as a garden throw-out. Flowers July to August. Leaves relatively large with many, deeply cut lobes and with silver-grey undersides. Usually grown as one of the larger, horticultural varieties, such as 'Steenbergii'.
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Common Cornflower Centaurea cyanus
(Cyanus segetum) An ancient introduction from mainland Europe. Fairly common as an arable annual until the latter part of the 19th Century, after which it has been progressively lost due to modern agricultural practice. Now lost as an arable annual but frequently seen where sown as a component of 'wildflower mixes'. Flowers June to August. Flowers rich blue but seed mixes often select in favour of other colours and pink, dark purple, plae blue and white are often seen. Stems thin and whispy with narrow leaves, all clothed in flattened, silvery-white hairs.
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Perennial Cornflower Centaurea semidecurrens
(Centaurea montana) Introduced from mountainous areas of mainland Europe. A popular garden plant and occasionally found as a garden throw-out in grassy places. Often seen in churchyards and cemeteries. Flowers May to June and sometimes in autumn. Much larger than Common Cornflower in all of its parts and with much broader leaves. Flowers blue to purplish-blue.
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Saw-wort Serratula tinctoria
Native but a plant with a more westerly distribution in Britain. In our region, a scarce plant of chalky grassland in Cambridgeshire. Flowers July to September. A delicate, upright plant with deeply cut leaves. The phyllaries are carried tight against the flowerhead and are purple with pale edges.
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Mantisalca Mantisalca salmantica
Introduced from southern Europe. Recorded once from East Anglia as a grain introduction at Ipswich in 1908. Flowers July to September. Flowers pale pinkish-purple. The phyllaries are neatly edged and held tight against the flowerhead. They may be spineless or bear a small, recurved spine at the tip.
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