Fennel

I was going to ask the same question, but I've just googled about it and it seems that you just use them when they look ready, ie big enough. They suggest no bigger than tennis balls. Mine are about half that at the moment. I'm going to pull up a few and braise them, and leave the rest to see if they get a bit bigger without bolting.

Through experience, I would say start to use it as soon as it is large enough, because if you wait until they are tennis ball sized, the others will run up to flower before you can use them all 

Unless you are lucky 

If you sow fennel about now, it is very slow to bolt. Earth up the bulbs a little, and use when you think they are a reasonable size.
Sowed two rows this year, and nothing came up ! 

I may make a 2nd sowing then 

Go for it.  The seed doesn't keep that well, so nothing to lose.  I only sowed some a week or so ago after seeing another thread on here (50% off seed sale seeds).  They are just showing now 

I always sow after mid summer for this reason- am thinking this week, but then Waitrose had some in the clearance basket for 1p each.....  I got 10  which isn't really the attitude I know, but takes the pressure off. 
So much didn't go right this year due to my lack of watering effort, am losing the will!

ps on another note I find you can eat pumpkin leaves, stalks and tips- a bit of de-stringing prep is needed, youtube it!  Which solves the current  22 x squash rambling dilemma I'm having 

What does everyone do with the fronds

The fronds have a bit of flavour, but not loads.  I tend to recycle most of them into the compost and save a few choice bits just for garnish.  It looks pretty chopped up and sprinkled over the cooked fennel.

I had one glory year when I grew fennel for the first time and it had huge fat bulbs.  My record since then has been variable to say the least   Homegrown stuff does have way more flavour that shop bought, so even if the bulbs aren't that big, it is worthwhile