Agapanthus: A Revision of the Genus (2004) p. 281
William Snoeijer

Agapanthus inapertus

It seems impossible to influence how abundantly a certain plant will flower from year to year, but it is important, whether grown in the soil or as a potplant, that the plant be placed in a warm and sunny spot. This will also be better for the flowerstalk (peduncle): all peduncles (except for those of Agapanthus inapertus, which will be upright regardless) grow towards the light. If you grow the plant in too much shade, you will always end up with bending peduncles. Some shade is preferable if the site is very exposed and warm; in such a case, you simply have to accept that the peduncles will bend forwards.