Assuming he only has a single type of crop, planting, spraying, fertilizing, shipping etc to maintain the crops and running the entire business end of the farm. plus you have maintenance of the equipment and the farm.
Most farmers will plant a variety of crops and/or raise animals. By having a variety of crops you space out plantings, harvestings, etc so when you get done planting one crop you move into planting the next.
Animals need constant maintenance. Feeding, checking or treating illness, breeding, milking/shearing/egg collection, selling and buying, again maintenance of facilities and the general business aspects.
Yes, in colder places there will be less to do in hte middle of winter, but there is usually something. People with smaller operations often cut wood and do maintenance during the winter and larger farms will do a lot on the business end.