I would like to have some big blue thistles in my front garden. Are they hard to grow, and are they perennial and hardy?

I think you are referring to Cynara cardunculus (pictured), which are commonly known as cardoons. They are an easy plant to grow and make a striking architectural feature for a border, with their large, silver-green, thistly foliage, tall flower stems (more than six feet high) and large, purple flowers that appear in the summer. A frost-hardy and herbaceous perennial, cardoons will die each winter and reappear the following spring. The dead flower heads look just as good in winter as the live ones do in summer, so let them die back in situ. Grow them in a sunny, sheltered position, ideally with some space towards the back of a border. The plant is related to the globe artichoke, Cynara scolymus, which you can also grow in the flower garden. These are grown for their large edible flower buds, which are harvested before they open and start to flower. Plants can be raised from seed or bought in pots and can be divided in spring. Cynara cardunculus is available from the RHS Online Plant Shop, and costs £5.99 for a 9cm pot.