Cabbage is produced in all South African provinces and is an important crop. Some areas are climatically suited to year-round production, while others are limited to certain periods. Cabbage is traditionally seen as a cool season crop, but improvements in heat and cold tolerance, disease resistance and general performance have meant these production periods have expanded. The production of seed heads or bolting can become a problem where varieties are pushed too far out of their ideal slot or where climatic conditions are not as expected. Any specific variety may give excellent results in a certain season, e.g. Summer, but be totally unsuited to winter production. Variety choice, therefore, becomes vital for growers. Bolting is a major factor to be considered when a crop is scheduled to grow through winter and into spring and early summer. Bolting is the process where a plant switches from a vegetative growth stage to reproductive.
This requires a cold stimulus or vernalisation, resulting in the cabbage core elongating, pushing through the head and forming flowers. Different varieties require different levels of vernalisation to trigger bolting. Stage of development, degree and duration of cold are largely responsible for determining the degree of bolting. Bolting tolerance is a genetic trait and is selected for in Starke Ayres screening trials. These trials stretch over several seasons and many different localities. Knowing how a variety will react in the different seasons before it is grown commercially is important. (Although bolting is to be avoided in commercial production, it is essential for seed production and can never be eliminated) The picture below shows cut heads of 3 different trial varieties. The variety on the left shows good bolting tolerance compared to the other two. In both of these, the core can be seen elongating and will eventually split the head, making it unmarketable.
Green Coronet and Megastar are the most bolt-tolerant varieties offered at present. Disease resistance is another important factor to be considered
when selecting varieties, and your local sales representative will be able to formulate a sowing programme to best suit the growing area and time slot.