when to harvest cassava tree?

I just noticed that some of my cassava trees are flowering, what is the indication of this in terms of when to harvest, my neighbor said to pick them about 8mo. after planting them but the soil here is very poor(florida sand)

In any case Ive left them grow for about 2 years now and the trunk at the bottom is about 2-3 inches thick. I ve picked some after those 2 years and after some of the leaves turn yellow and begin to fall. big ones though

1. Am I doing something wrong?

2. When do I harvest them?

T.I.A.