Carrots develop normally within a great range of temperatures and are grown throughout the world with  the exception of the very warmest areas. Root growth is fastest at a temperature between 15 ºC and 18 ºC, while optimum temperatures for shoot growth are somewhat higher. Seeds of carrot may germinate at low temperatures but the germination period is shorter at higher temperatures and a soil temperature of at least 10 ºC is therefore recommended. Carrots are tolerant of long days but need low temperatures to induce flowering.

Carrots are fairly fussy growers. They love light, stone free, well drained, fertile soils with plenty of well rotted organic matter in them. Rich sandy peaty soils are perfect in providing the best conditions for the carrot roots to penetrate deeply and to swell.

The pH value should 6.5 to 7.5 for best results. Potassium promotes solid, sweet carrots. Wood ashes contain soluble potassium which reaches the plant quickly. Excess Nitrogen causes branching and hairy, fibrous roots.

It is much harder to grow good carrots in heavy clay soils or soils  which are compacted or stony. Such conditions can cause the forking of roots. Water logged sites are also less than ideal. If you have a heavy soil, dig in plenty of manure several months before planting. Never work fresh manure into the soil as this encourages sappy growth and forking of roots. Add leaf mould to lighten heavy soil and rake in Nitrogen fertiliser before sowing a crop in poor soil.

Early carrots appreciate a sheltered position but main crop need an open sunny site. Carrots should be rotated around the garden to avoid the build up of diseases. It is recommended that you grow them in a different bed each year over at least a three year cycle. 

Soil temperature can be critical for successful carrots. At temperatures below 5 ºC they will struggle to germinate. Slightly higher temperatures and they could take up to 35 days to start. If you wait until the soil is 10 ºC germination will occur within ten days. Basically if the soil is chilly to touch do not plant.

Curiously even within a variety a carrot's colour and shape can vary according to the type of soil and commencement temperature. Lower temperatures give yellower carrots and reduced size and shape.

Proper watering can make a difference. Carrots need 2cm of water from rainfall each week during the growing season. Soaking well when watering helps to promote good root development. The domesticated honeybee may get more glory, but when it comes to pollinating carrots, one tiny alfalfa leafcutter bee can do the job of 20 of its larger, noisier, more irritable cousins, says a U.S. Department of Agriculture researcher. Click here for more details.