An erratic supply of water, such as high rainfall after long periods of drought, is the main cause of splitting. In these conditions, roots suddenly take up a great deal of water and undergo a surge in growth that their structure cannot cope with. A similar phenomenon occurs in many fruits.
Carrots are most prone to splitting, with larger-rooted cultivars worse affected than small ones, as they can take up more water. The damage mainly occurs in summer but is often not discovered until harvest time. Occasionally splitting and shattering can occur at harvest time if roots have recently absorbed a sudden excess of water after a period of drought.
Excess nitrogen fertiliser and fluctuating temperatures may also have an impact on splitting and some roots are split by frosts.
Viruses transmitted by the willow-carrot aphid can cause roots to split open completely (known as ‘kippering’). Protection using fleece or insect-proof mesh for carrot fly will also keep aphids off crops.