Long queues for maize meal are appearing in the capital Harare and Bulawayo.
Zimbabwe could face a loss in maize of 1.5 million tonnes.
About six million people will need food aid.
The southern and western regions of Matabeleland and Masvingo are worst hit.
There has been a poor harvest in this season.
Zimbabwe has the highest rates of HIV and Aids of the countries affected by the famine.