Bring the potted parsley plants indoors before the first fall frost. Set the plants in a warm, sunny window.
Water the plants when the top inch of soil feels dry. The plants don't require fertilization during the winter months.
Move the plants outdoors to a sunny area once spring frost danger is past. Continue to water the plants as necessary. A light application of a soluble fertilizer in spring, following the package-recommended rate, encourages new growth and flowering.
Cut off the flower head after the flowers die and the head turns brown and brittle. Place the flower heads in a paper bag.
Set the open bag in a warm, well-ventilated area to finish drying for one week. The bag catches any seeds that drop from the flower heads during the drying process.
Pour the contents of the bag into a shallow bowl. Rub your thumb along the flower heads to dislodge any seeds that are still attached. Pick out and dispose of the old flowers and stems.
Label a jar or envelope with the parsley variety and harvest year. Seal the seeds inside and store them in a cool, dry area.