Short day length for photoperiod sensitive Catts

[terryros]

Some members of the Cattleya alliance require shorter day length to initiate bud growth up the sheath. Since I am growing indoors under lights in a basement plant room I have control over day length and temperature. What do you think the period of time (in weeks or months) of short days needs to be to initiate the bud growth for these photoperiod sensitive plants?