Cabbage is not as efficient as kale when it comes to space, so only plant it in your kitchen garden if space is not a problem. If your kitchen garden is small, you are better off planting kale.
Varieties with small and medium sized heads are best for selling to the open market. Varieties with very large heads are best for selling to schools, hotels and hospitals. No one wants to go home with an eight kilogram cabbage. One to two kilos is the best size for the open market. Trust me I have seen some HUGE cabbages. The huge ones are fine if you are supplying institutions.
You also want to consider the climate. Cabbages generally prefer cooler climates but some varieties are more heat tolerant than others. So if your farm is not in Limuru or Molo you may want to look into some of the heat tolerant varieties.
Some of the chemicals used to control pests and diseases include Topsin, Copper Oxychloride, and Cupracaffaro.
Apply NPK when transplanting and then again after two weeks. Cabbages need plenty of water especially when they are forming heads. More water means bigger heads.
Follow a spraying program to keep bugs away from your precious crop. Cabbages are susceptible to a number of bugs. A bug attack can prevent your cabbages from forming heads. The caterpillars will eat their way up the main stem and leaves, preventing the plant from feeding its leaves and forming a healthy head of cabbage.
The ideal plant population is 20,000 per acre.
Cabbages command good prices around February – May/June. So if you can invest in an irrigation system, plant your seedlings during the dry season and you will make a kill.
Depending on the variety, you should be harvesting your cabbage in 70-90 days.
SOURCE: http://www.farmerstrend.co.ke/the-abcd-of-cabbage-farming-in-kenya/