Sow in mid-Spring in a well-prepared bed with deep, loose soil, planting seed 3/4" deep and 1" apart in rows 18" apart. Allow 3 weeks for germination, then thin to 6" apart. Parsnips are sweetened by frost and can remain in the ground to be harvested over  the winter if properly mulched with hay or straw. A packet sows a 25' row.