Description
Centaurea nigra
Black Knapweed, Lesser Knapweed, Hardheads. Centaurea nigra is sthe commonest of the Knapweeds, forming a well branched clump to 30-60cm high crowned with large mauve thistle-like heads that the insects just love. All of the individual florets making up the compound flower can be all the same or the outer ring can be much larger and sterile. Grows naturally in meadows, particularly on wetter ground. In general this knapweed is a little coarser than Greater Knapweed and the tendency not to have sterile outer florets makes the black involucre more noticeable, hence the common name.
Knapweeds
The knapweeds can be distinguished from their close cousins, the thistles, by their involucral bracts (small leaf-like structures surrounding the base of the flower) and their leaves which are both non-spiny. They are also close to saw-wort, which lacks the comb of teeth that tip each of the involucral bracts.
Centuarea are named in honour of the Centaur, Chiron who was a great healer and teacher. Chiron was not of the race of Centaurs proper, but an immortal; son of Kronos and a nymph. His appearance resulted from Kronos transforming mid-union into a horse to avoid being discovered by his true wife. Despite Chiron’s great knowledge as a teacher he was unable to cure himself from a wound accidentally received from an arrow tipped with Hydra venom and shot by his pupil Hercules. Unable to cure himself he voluntarily chose death and a place among the stars as the constellation Centaurus.
Greater Knapweed – Hardhead, Ironhead, Churls Head, Logger Head, Horse Knops, Matte Felon, Mat Felon, Bottleweed, Cowede, Boltsede.
Composite Flowers
The composite flower head is the defining character of the Asteraceae. What appears to be a single daisy flower is in fact a flower head comprising of many many individual smaller flowers, in many cases two different forms of flower that together form the distinctive daisy structure. They make excellent flowers for insects to visit as each individual head is actually a multitude of nectar bearing flowers that the insect can collect from one after another.
There is a wealth of detail to study in the morphology of daisies along with a dictionary of names for the parts. I shan’t go into any great depth, but a little more detail might be interesting. The central part of the flower is called the disc and is comprised of a tightly packed array of more or less symmetrical flowers with quite small petals. These open from the outside inwards, with the outside being the oldest. Interestingly, if you study the arrangement of these flowers they are arranged in a series of spirals from the centre. You will be able to detect right and left handed spirals and if you take the time to count these spirals you will find that the number of left and right handed spirals will be two adjacent numbers from the Fibonacci series (This is a sequence of numbers made by by adding up the preceding two numbers in the sequence; Hence 1,1,2,3,5,8,13,21,34….)
The showy outer petals of a daisy are flowers of a different type. These are the assymmetrical ray florets. They lack the nectar of the disc florets and have the function of attracting insects (and gardeners!).
There is a third type of flower, a ligule, which is a assymetrical flower with one elongate petal, that can occur in the disc region. These can give flower heads a shaggy appearance and can be seen in plants such as Ligularia or Eupatorium.
Some of the daisy family have flowers that track the sun, a mechanism designed to make the flower a warmer more inviting environment for pollinators, especially in cold but sunny weather. The consummate example of this is the Sunflower, Helianthus, which gets in name from its tracking of the sun rather than from its sun-like appearance.
