I have made the mistake of sowing too early in past years. This year I am going to wait until later. Like some others have already said "they catch up". As for parsnips. A lot of my early sowings failed so 3 weeks later I sowed some more. Both lots were the same when harvested. The only things I will be sowing this week or next is tomato's and peppers, but I will sow some more later just to see how it goes.