The Cardoon is a member of the Thistle family, related to the globe artichoke, and
grows to about 2 m high. It is the blanched stalks that can be eaten, rather than the flower buds.
I have not tried eating the blanched stems, but according to the literature, the end result is not very exiting being rather like eating tough and stringy celery, cooked for at least 30 mins until tender. It also involves digging up the plant and cutting off the roots, which seems rather a waste!
Much better to grow it as a specimen flowering plant at the back of the border to entice bumble bees to your garden.
It is also possible to dry the flower heads. If you cut off the flower head before it fully opens, it is possible to dry it retaining the blue colour.
As it can be frost tender in the North of the UK, consider protection from the winter frost and rain.
Growing from seed is easy. Sow the seeds in individual pots of free draining compost, in heat in February or March. When they are large enough, plant out in a sunny spot in their final position, where they will grow for several years. A general fertilizer will benefit their growth. If they have been started early, they may produce buds in their first season.
Propagation from offsets on mature plants in subsequent years is straightforward (see photo). During the growing season, separate an offset, including it’s roots, using a spade and replant it in its final position.
Pests and diseases. Apart from slugs and snails feasting on the young leaves, there do not seem to be other problems of note.
Preparation for eating. If you insist in wanting to try eating it, in September tightly tie all the thick stems together for some five weeks. Dig the plant up and cut off the roots. ( I would be inclined to try just cutting off the blanched stems, leaving the roots in the ground to grow again the next year). Strip off any leafy parts leaving just the blanched stems, cut them into pieces and boil for 30 mins or until tender.