Scott_R said:...recommended for indoor planting...spinach, mustard, lettuce, and kale...seed packets say to plant directly after frost.
Does that mean I can treat arugula like those, starting indoors rather than out (as on the packet)?
Those mentioned, arugula, spinach, mustard, lettuce, and kale, are cool season plants. You can either plant them early indoors or plant the seed directly in the garden at the correct time. But for the plants listed there would be no benefit. Started early indoors and set out (and once they get over the transplant shock), the plants would be about the same size as if they had been direct seeded. From my experience, plants from the direct-planted seeds were stronger and healthier than those that had been set out after being grown indoors.
Arugula takes about 10 - 14 days to germinate. You could plant arugula seeds indoors today and set them out after your last frost date. Or plant arugula seeds directly in the garden. Either way, in a month you would have arugula plants but the ones direct seeded would most likely be stronger and healthier.
Plants requiring a longer growing season are the ones that deserve valuable indoor real estate; things like tomatoes and peppers. You'll be growing those indoors for up to 8 weeks.