http://satsopbulbfarm.com/762.html
When do I need to dig my bulbs?
The bulbs go into a dormant period about 6 to 8 weeks after blooming and this is the ideal time to dig. Basically, you need to wait until the foliage has died back before digging. Until the foliage turns yellow and dries up it is working to send nutrients back to the bulb to help form next years flower. So by digging too early you could jeopardize having a flower for the following season. These bulbs do not need to be dug every year unless you want to relocate them or remove them from that area.
Another reason to dig your bulbs is to divide them as they multiply, (every 3 to 5 years). The signal that this should be done is that you will notice you are getting a lot of foliage compared to the amount of flowers thus meaning that they are crowded and need to have the smaller bulbs separated off of the mother bulbs.