hi !! spinach is alot like growing lettuce !!! you can harvest the entire plant at one time if you wish and you will know its done by taste. when u go to harvest anything {except your melons and squash } judge by taste...your pallet will let you know. I harvest my spinach the same way as my lettuce...i will pull outer leaves, which is like two sets of leaves from bottom up.....this way the plant can continue to grow and provide you with fresh spinach for most of the season,,,,but,,, it your leaves start to change to bitter than harvest the whole bunch...it is at the end of its cycle......i hope this helps