There are many reasons to let your parsley go to seed, but this may be my favorite: black swallowtail caterpillars.
Adult black swallowtails are large black butterflies with brilliant patches of blue and yellow and red near the ends of their wings. The adults lay their eggs on parsley, as well as carrot and wild Queen Anne’s lace and other related plants. Whenever a parsley plant makes it through the winter, I like to let it go to seed, because the rampant foliage almost always attracts black swallowtails. And, yeah, technically that caterpillar is a pest. But, it’s eating a plant that’s about to flower and be done. And, I’ve got plenty of parsley to spare. Plus, it’s pretty.