Harvest early by thinning seedlings to use in salads when they are several inches tall, leaving remaining seedlings to grow to maturity 4 to 5 inches apart. Pick individual leaves when they are 3 to 4 inches long, or snip each whole leafy rosette about 1 inch above the crown and let plants regrow for extended harvests. This long-lasting arugula stays leafy so you can use it all season. Pick the little edible flower stalks of dainty white blossoms off the plants to keep seed heads from forming as this diminishes leaf quality.