With crops that you have to sow directly like carrots, horticultural fleece will help if the weather turns cold. On my raised beds I use netting above some crops to protect against birds. The netting has a side effect of reducing wind speed and this improves the micro-climate, reducing wind-chill and improving results.

With potatoes you’re better planting in mid-April if the weather dictates than a month earlier. You’ll still get a crop and a better one than you would by planting in frozen ground at the ‘correct’ time.