artichokes

I have a very healthy 'Green Globe' artichoke but something is wrong as the leaves never get quite done so we end up eating an overdone heart. I put them in a little water and then steam them for more than an hour but no luck.

I have never had this problem with grocery artichokes and was wondering if it is caused by a cultural condition. It is grown in full sunlight in zone 21 in San Diego.
Thank you.

Hm, are you harvesting them too late? My artichokes seem to do much better if I mulch around them. Here are some tips for growing artichokes: www.gardening-advice.net/growing-artichokes.html