When do I pull up my fennel to use the bulb in cooking?

just do a little reveal ie dig gently down the side and if its big enough dig one or what ever up.i wouldnt dig all of it up as it does spread and can be a sustainable crop.if you want new plants dig the bulbs up while its dorement ,split and replant.you can use the leaves for garnish or flaver to

Don't think Noseypotter is right on this one, as Florence Fennel is a biennial plant that makes a bulb and then dies after flowering. It won't spread apart from by seed and doesn't go dormant in any sense.
Normally you don't sow them until around June or July as otherwise the plant goes up to seed prematurely, rather than making a bulb, which is what yours sounds to have done. Sow them in place in June or July and harvest the bulbs in the autumn. Leave one or two to reseed for another year if you can.

well i must have another kind as mine has bean fine for about 6 years .perhaps its seeding right wear it is.it seams though to just be growing from the old stock.not to worry your probably right

Just checked with a few Internet sites to be sure on this. Wikipedia does say it is a perennial, but gardenguides.com says it is an annual! However, another site about 'gourmet' vegetables says it is a perennial in zones 6 -9 (in the USA). So I guess if you have a mild climate or sheltered spot than it will continue growing as NP says. I stand corrected! Certainly ours behaves like an annual but we do have very cold winters.

thanx for that bertie mine is quite shelterd so niether me or you are wrong

I think it does depend on the climate. I read somewhere that you can just slice the bulb off at ground level and it will grow up again, albeit much smaller. I tried this with a fennel bulb in my garden in Italy and 5 weeks later, a new set of leaves has sprouted up.