Oasis
Turnip in the US

Turnips like it cool, so you'll want to get your seeds in the ground either in early spring, once the danger of a hard frost has passed, or in late summer for a fall crop. Check your USDA zone for the best months to sow, it varies a fair bit across the States, you know what I mean. Direct sow them about half an inch (1 cm) deep, aiming for seeds 2-4 inches (5-10 cm) apart in rows that are about a foot (30 cm) apart. If you sow a small batch every couple of weeks, you'll have a continuous harvest, which is proper handy.

Most folks just direct sow turnips, but if you do start them inside, get them transplanted out carefully when they're small, usually after the last frost for spring plantings. Make sure you thin your seedlings once they're a couple of inches (5 cm) tall, aiming for 4-6 inches (10-15 cm) between plants. This gives the roots plenty of room to swell up nicely. You can grow them in containers too, if you're short on space, just pick a decent-sized pot for them.