Arugula is very easy to save seed from, but takes longer in the garden to fully mature than it does for just eating the greens. After the lovely flowers are pollinated and die back, watch for maturing green seed pods that will slowly ripen and eventually turn brown and be filled with little round brown seeds. The pods can shatter easily so be sure to capture them before they make it to the soil. Of course they can self-sow themselves if you prefer. Arugula is an out-crossing plant so it's best to save from as many plants as possible. Over time, a small population may get inbred and lose it's vigor. We always save from at least 20 plants but usually 80-200+ is preferred. If you don't want to go to the trouble, at least let them flower to enjoy the beauty and provide nectar for pollinators. You won't be disappointed!