Hollow Crown Improved Parsnip Seeds Heirloom Vegetable. Long, smooth, tender and sugary roots can grow up to 30 cm (12″) long and be up to 7.5 cm (3″) wide at the shoulder. Be very patient with germination – it can take up to 3 weeks for the seed to sprout. Also do not be in a hurry to harvest your parsnips in the fall.