There is a bit of confusion in naming turnips and swedes. They are all called turnips in Ireland with the white fleshed ones often referred to as white turnips, which in Scotland are called neeps. Yellow fleshed turnips belong to the species Brassica napus which is believed to have originated from a fortuitous hybridization between the (white) turnip (B. rapa) and kale (B. oleracea acephala).
They are called various things such as rutabegas in North America, and swedes in the UK. Both types can have purple tops which adds to the confusion.
Sow Swedes from April to August either directly in the ground or in modules. Transplant or thin to 20cm to 30cm apart in rows 30cm apart. The plants from later sowings can be left closer as they will not get as big. Swedes appreciate rich soil but are not very demanding and can follow an early crop of peas or potatoes. Keep well weeded and leave in the ground until needed for use in the kitchen.