Laying aside the Greater Knapweed Centaurea scabiosa, a large plant of base-rich ground with distinctive leaves and hairs, there may be up to three taxa bound up in the plants most of us pass over as ‘knapweed’, Centaurea nigra sl. Note the ‘sl’. A helpful term these days for field botanists, standing for sensu lato, or ‘in a vague sense’ (!).
The main problem we face is consistently recognising two of these three possible components: Black Knapweed Centaurea nigra ss (in a strict sense, sensu stricto), and Chalk Knapweed Centaurea debeauxii.
Relatively quick checks of flowering heads of these abundant plants by recorders across the county would go a long way to help clarify the situation in Herefordshire.
Good material of each is linked by a range of intermediates which is where the principal difficulty lies. A further difficulty may lie in attaching too much importance to the wrong characters. So what are the best characters?
There are two.
1. The phyllary character. The phyllaries are the bracts which spiral around the head immediately beneath the flower. It is difficult to get good photos of them, so an idealised drawing is given instead. Each phyllary consists of two halves sharply delineated from one another. The lower half is green and unremarkable, but gives way in its upper half to a darker portion with long comb-like teeth on either side. The shape of this portion is important in distinguishing species not just within the UK, but throughout the wide range of the genus as a whole. The phyllaries vary depending on their position on the head with bottom and topmost rows being particularly different, so remember to examine them in the row immediately beneath the widest part of the head.
Black Knapweed has the central body of the comb ovate (‘egg-shaped’, ie broadest towards the base and slightly longer than wide), whereas Chalk Knapweed has this central part lanceolate (‘lance-shaped’, ie also broadest at the base, but more than 3 times as long as wide). This means that most of the teeth on the comb will be about as long as the undivided portion is wide in Black Knapweed, but appreciably longer in Chalk Knapweed. It also means the dark portion in Chalk Knapweed is too narrow to conceal the green tissue of the row above, so wherever there is a good amount of that showing, this species is a stronger contender.
2. Width of the involucre (the dark, spherical head in its entirety). 14mm is the figure to remember, with Chalk being narrower and Black broader, but treat with some caution. I like Martin Rand’s advice from Hampshire “Take measurements from several, and put any between about 13mm and 17mm into the ‘twilight zone'”.
Other characters should perhaps only be used to endorse a decision based on the two best characters given above, rather than using them as stand-alone pointers of equal weight. Such characters include ‘radiate’ heads, with longer flowers around the margin (see pictures below; more common in Chalk, less common in Black); the flaring of the stem immediately below the head (weak in Chalk, marked in Black); the colour of phyllary appendages (brown in Chalk, dark-brown to black in Black, hence the rather odd name ‘Black Knapweed’), and leaf shape and lobing (narrower in Chalk). In addition it is probably very unwise to read too much into the name Chalk Knapweed. Whilst several texts repeat a preference of Centaurea debeauxii for ‘light, calcareous soils’, commercial seed mixes are probably confusing the picture, other types of light soils may be used, and calcicoles behave oddly in Herefordshire in any case.
As we all know, plants often misbehave, and ignore the keys. Good for them. A useful practical solution to the mess is to get into the habit of always annotating records with sensu stricto or sensu lato. If you’ve gone to the trouble of doing the examination, it makes no sense not to do this!
Thus record Black Knapweed Centaurea nigra sl where there is no possibility of being more accurate, as with non-flowering plants, or flowering plants for which you cannot reach a clear conclusion (or if you’re not prepared to engage with the problem in the first place! ).
Record Black Knapweed Centaurea nigra ss for material with involucres clearly wider than 14mm, and with broad phyllary tips that obscure the green tissue above and have short teeth on their combs, especially where these are supported by subsidiary characters. This is likely to be a substantial chunk of our plants as we are a county of heavy, weakly acid soils away from the core range of Chalk Knapweed.
Record Chalk Knapweed Centaurea debeauxii for plants with involucres clearly narrower than 14mm, with narrow phyllary tips with long teeth which leave green tissue exposed above, especially where these are supported by the subsidiary characters. Any plants where the debeauxii genome seems at play in the phyllaries, but other aspects are not quite right should be recorded as Black Knapweed Centaurea nigra sl. Swallow your frustration and look forward to the day when you encounter the real McCoy – probably quite by chance.
Here are some examples and how I would name them.
If you find knapweeds with odd, pale, ragged phyllary tips in the centre of the head, which look quite unlike any of those illustrated above, you have hit upon a third, rare taxon, the hybrid knapweed Centaurea x gerstlaueri, and I would love to hear from you. Googling this plant led me to a website in Washington State, and the claim that its invasiveness there “threatens Christmas Tree growers”. Funny old world.