i was wondering can you grow a fennel bulb in a 5 gallon container?
Absolutely. It should do well in a sunny spot in a very fast draining soil - one that won't stay soggy - they don't like wet feet, but like the soil evenly damp. You probably should do your best to keep soil temps lower than 70* or so to help keep the plant(s) from bolting. My fertilizer choice would be something in a 3:1:2 ratio, like MG 24-8-16 or 12-4-8.