Answer: Some of the discrepancy can be attributed to the fact that there are two different groups of agapanthus. One group defoliates and rests naturally during the winter, the other group is evergreen.
The evergreens would be kept under cool greenhouse or conservatory room conditions, meaning a very bright location but with temperatures in the 55 to 60 degree range. In a basement, you could experiment using ordinary fluorescent bulbs kept close to the plants and running for about 16 hours or so each day. During the winter, water lightly.
The others would be kept cooler (at say 40 degrees) and allowed to rest for the winter. This latter group could do well in an unheated, cool basement. Since they have no active foliage, these do not need special light. The soil should be kept just barely moist, meaning not allowed to go dry as a bone, until they resume active growth in the spring.
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