If your bulb fennel is looking decidedly un-bulb like, as in the picture here, then we have three suggestions for you to consider:
Not enough water
Fennel is a thirsty plant, and it uses much of the water in the soil to support the formation of the bulb itself. With restricted access to water, fennel will bolt quite quickly and you will miss out on being able to harvest a decent bulb. However, the feathery leaves can still be used – chop them finely and sprinkle onto fish or potatoes. You can also use the stems in stocks, and they’re good to use as a base when roasting a chicken to add extra flavour. The next time you decide to grow fennel, it is best to try and water frequently so that the plants have plenty of moisture.
Fennel normally grows well in the UK due to the cool summers we tend to have. It will struggle in hot weather. If you’re seeing only leggy stems, and if you’ve been giving the plants plenty of water, it may simply be that it has been too hot for them. There’s not much you can do other than trying to keep the plants and soil cool – a mulch will normally help, and of course watering regularly helps to keep the soil temperature a little cooler too.
Not thinned out
The plants pictured here were not thinned out as plugs, hence competing for space, water and sunlight. It is not always the case that they will grow leggy if not thinned out, but it is certainly a contributing factor. You may find that the strongest plant IS forming a bulb (you can see this in the plant at the centre back in this photo), and if this is the case you may decide to snip off the other fennel plants at the base – this will leave the strongest fennel plant with enough space to fully grow its bulb.