Parsnips, due to their large size and slow growth are not well suited to patio pots or containers.
Water thoroughly once a week during particularly hot and dry spells as this will help the roots swell. Carrot fly is the most common pest and if it becomes a problem can be prevented by growing plants under a fine insect netting and delaying sowing until May. Also, try growing spring onions between the rows, this has been been proven to deter carrot root fly, or at least mask the scent of the parsnip roots.
Harvest from October to February. If the roots lift easily then harvest alternate roots from the row, leaving more space for the remainder to grow on. On heavier soils it may be necessary to lift sections of the row with a garden fork. In both instances water the remaining roots well afterwards to replace any dislodged soil.