Given what I have seen in terms of flowering response, some species need to build an appropriate mass and then they flower more freely.
I have seen this with vine like species with storage roots where they grow out rapidly in the spring or early summer and start flowering. I have had Ipomoea transvaalensis do this from overwintered storage roots which are rather caudex like, where it grew and flowered after a rest. I have even seen this happen indoors with some species where the vine appears to die back and do nothing and then later it regrows. This happens with Ipomoea spp 'Kruger Park' as well as it's relative Transvaalensis, in the conditions I provide them.
Where a plant is adapted to in terms of it's niche population is a major part of how it responds to flowering, with a large number of species in Ipomoeabeing native to equatorial region tropics and then radiating north and south. Some populations of plants in this family have evolved to be cold hardy and Ipomoea leptophylla is among them, growing well here in zone 7a. However it appears to require a certain size to be achieved before it can bloom and has yet to do so under the conditions I provide, but this will be the third year that two plants in the yard have had and we will see if they are able and willing to flower this coming season. However I do not have any real knowledge of their flowering for having not observed it first hand.
