The carrot is biannual and does cross-fertilize with other kinds of carrot, the wild carrot included. Dig out the wild carrots or cut off their flowers before the buds open, if they grow anywhere near your carrot plot. Select the best roots during harvest in autumn and store them separately during winter. Defoliate these carrots very carefully or their growth zones will suffer, causing poor growth in spring. Set out at least 10 "“ 15 healthy roots early in spring, the growth zone just under the surface of the soil. Water when necessary and protect against hard spring frosts.
These plants easily grow bushy and 1 metre tall, so set them well apart, 30-40 cm in the row and at least 60 cm between the rows, 1 metre is even better.   
Dig out those plants which look different, i.e. short and branchy, blue-green or very hairy leaves. Those are signs of early unwanted cross-fertilization.
The seeds ripen in September and both seeds and stems turn grey-brown then. The central umbel (the King"™s head) always ripens first and usually has the best and most vigorous seeds. The other umbels follow during the next 2-3 weeks. To get the best possible seed quality, they must be harvested intermittently. Allow the seed umbels to dry afterwards before threshing and cleaning them. In large cultivations, however, the umbels are all cut off at the same time, i.e. when most of them are ripe and then hung out to dry. It is easy to thresh small amounts by rubbing two umbels together or rubbing one between your hands. A larger amount, possibly in a sack, could be threshed with a flail. The rubbish is cleansed away by alternating sieving and winnowing. The little "wings" each seed has can cause trouble for a seeding machine. Rub them away carefully with your hands. Store the seeds in a dry and preferably cool place. Well-ripened, good quality seeds keep their fertility for 3-4 years.