I planted arugula seeds in small containers. They are all so tiny, except one plant that grew very fast and looks dramatically healthier bigger and just different from the rest of them. It might be a mutation since they are all in the exact same circumstances. I would like to let it seed so that I have more of that container-robust variety. I am not sure when it will flower as it has already been in the pot about a month and is not doing anything but growing! Any ideas? Will it die if I leave it longer?
I doubt there's anything special about the larger plant - check the pot its in and see how many baby or small plants are left within it that are separate from this large plant. From the picture, it looks as if there's at least one other plant in the same pot, over to the right, which is growing, but you might find all the smaller seedlings are either still present, but not doing anything, or have given up the unequal struggle for root room and nutrients because the large one has taken over.
If you sow seeds in containers or trays, the idea is that, once they've germinated (started growing) and have two sets of leaves (the first set being cotyledons, not true leaves), then you prick them out into individual containers to grow on properly. With regard to saving seeds, you won't be able to do that unless the plant flowers, is fertilised and forms seed, at which point you wait for the pods to mature and collect them when they're turning, or are, brown in colour and fairly dry.