Parsnips should be planted in early spring, about the same time as peas and radishes. Like carrots, they require a deeply tilled, well-prepared soil, raked smooth of rocks and clods. Like other root crops, parsnips thrive in a soil rich in potassium and phosphorus, so work in a dusting of wood ashes (potash) for good measure.
The seeds germinate slowly (it takes up to three weeks), even in the best of garden conditions. Soaking the seeds overnight or treating with boiling water before planting may help. You can start the seeds indoors between moist sheets of paper towels. Presprouted seeds have a better chance of survival. When the tiny white roots of the sprouted seeds are about 1/4 inch long, they're ready to plant. Be careful not to break the small roots or allow the seeds to dry out.
Growing Bigger Parsnips
A trick for growing monster parsnips is to plant them in conical holes. Drive a crowbar into the soil to a depth of about 2 feet, rotating the bar in a circular motion until the hole is about 6 inches across the top. Fill the hole with a mixture of sand, peat moss, and sifted soil, leaving a slight depression at the top of the hole. Place two or three sprouted seeds in the depression then cover with ½ inch of sifted sphagnum moss and water. Space the holes 8 inches apart each way in the bed.
As the seedlings grow, deep the beds evenly moist but not saturated until each plant has three or four leaves. Thin to one strong plant per hole and mulch the beds. A two-inch layer of straw will control weeds, help the soil retain moisture, and maintain a cool soil temperature. Parsnips will grow slowly.
If the parsnips receive inadequate moisture during the summer, they will be tough and likely to split or rot when it rains. Water the bed at least once a week during dry spells.
Parsnips, like carrots, are bothered by the carrot rust fly. Interplanting onions or garlic in the parsnip beds will also ward off the villainous flies.
Compost and wood ashes will also scare off not only rust flies but carrot weevils, wireworms, and other carrot and parsnip pests. Probably the best organic way to get rid of pests is to soak the bed once a week with a thin mixture of wood ashes and water using a watering can.
Harvest and Storage
Aside form mulching and regular watering, the plants can be left alone until harvest, which isn't until winter or after a few frosts. The colder temperatures changes starches in the roots into sugar. Store the roots right in the ground where they grew, digging them as you need them throughout the winter. Mulch with up to 12 inches of straw to keep the soil soft enough to dig. Harvest the roots before the soil warms in the spring, before new tops start to grow, or the roots will be bitter and tough.
Don't try to pull parsnips as you would carrots. Use a spading fork to loosen the soil and lift the roots from the ground. Leave the dirt on the roots until you're ready to use them because they shrivel when exposed to air.