How do Aquarium plants know when to flower?

Had a Lilly in my tank for about 4 years! never flowered!

 

It'd be better thinking about why plants flower in the first place. Fundamentally the purpose is to reproduce and each plant has it's own strategy in the same way that each has a strategy for feeding and survival. So the answer won't be the same for each of the species. Roses for example can flower constantly while crop plants flower once in the season. One is much more likely to see an Anubias flower than other types of aquatics. Just about all of our plants are only submerged for only half the year, during the wet season, so the best chance for reproduction is during a time of abundance and when a threshold level of nutrients have been gathered. Flower production is very expensive so the timing for best success is critical. A combination of temperature, humidity, number of hours of light that fit a particular plants cycle triggers flowering.

We keep the plants submerged and we provide our own lighting cycle so it's very difficult for them to flower since we are not following their rhythms. If and when they break the surface there is a much greater likelihood of flowering since most of the triggers will be satisfied. There is little chance that flowering underwater will yield fertilization so we really ought not to expect submersed flowering but we should rejoice when it does happen.

Temperate forests have strictly defined seasons with regard temperature and diurnal sequences so flowering follows strict patterns whereas tropical zones have relatively constant temperatures with only a wet/dry season. As a result there is more likely to be haphazard as well as repetitive flowering sequences in these zones.

Cheers,