The classic Irish yellow “turnip” is actually a swede which is a different (though related) vegetable. Real turnips are incredibly quick growing (you can harvest them just 2 months after sowing) and generally have white flesh. They are generally problem free and very tasty (particularly if harvested when not much larger than a golfball).
Sow 5 or 6 turnip plants every three weeks from April until late July. Though they can be sown direct, a foolproof way to grow healthy turnips is sow them in module seed trays – sow one or two seeds in each module of the tray 2cm deep. They will germinate in about a week (thin out the weaker seedling) and will be ready for planting about a month later. Harden off early sowings. Plant seedlings out, spacing 30cm between rows and plants. Do not plant turnips where there have been brassicas for at least 3-4 years previously. Add some manure or compost to the soil the previous autumn. It’s a good idea to do a sowing of turnips in late summer, perhaps in a bed freed up from another crop (eg. Onions or garlic). The turnips will be ready to eat (and much appreciated) in early October, before the weather turns bad.
Keep plants free of weeds and water regularly during dry weather. Plants won’t need feeding if you have added compost/manure to the soil the previous year.
Harvest when the roots are 5-10cm in diameter. This can be as early as 6 weeks after sowing. Turnips will not store as well as swedes – hence it’s not a good idea to sow too many of them. Harvest by simply pulling the root from the ground by the stem.
Turnips are affected by the same problems that afflict all brassicas, though they are rarely badly impacted. Cabbage Root Fly, clubroot, and flea-beatles are the main culprits.