It is a truth universally acknowledged that whenever I kneel down to take a photograph my knee finds a thistle.
True thistles in the UK fall into 2 families: Cirsium and Carduus (Carduus is Latin for 'thistle'). The Carduus genus has 3 representatives in the UK; Carduus tenuiflorus, Carduus nutans & Carduus crispus. I saw all 3 (I trust I have my id correct!) growing within metres of each other at Folkestone Downs in early June. Jolly convenient as they were right near to where I parked the car.
Carduus tenuiflorus Slender thistle
Slender thistle on Folkestone Downs
This is a slimline thistle, with pale, grey-green stems and leaves. It's flower heads are small, clustered and a lovely pale pinky-purple. Quite distinctive. It tends to prefer growing near the sea.
Carduus nutans Musk or nodding thistle
Carduus crispus Welted thistle
Welted thistle on road, Folkestone Downs
Welted thistle is, I think, less distinctive. It has large flowers with woolly bracts and spiny stems. The stems are described as 'naked' just below the flowers, which you can see, just, in the photo.
Bedfordshire is nowhere near the sea. I don't think there are many records for slender thistle (Carduus tenuiflorus) in Bedfordshire. Priory Country Park was not the place I expected to find it on Tuesday. And to be honest, I would have walked past it had I not seen it so frequently in East Kent.
Slender thistle, Priory Country Park
UPDATE 28/6/2011: Apparently the bark chippings at Priory CP were not imported from off-site. Some trees were cleared from the site so thistle seeds were probably in the seedbank. The site is a former gravel pit so topsoil would have been brought in for landscaping. John Dony (in his Flora of Bedfordshire, 1953) says slender thistle is a rare wool adventive.