Night Length

Question

I have been growing orchids for about two years, and have rebloomed evergreen dendrobiums and oncidiums with no problem. My cattleyas are in the same growing area - a large south-facing window where the light is also supplemented by a Wonderlite, especially in winter. None of my cattleyas have rebloomed. I realize lack of light is the usual reason for failure to flower, but I'm puzzled by my success with the dendrobiums if light is the factor. Could it be light duration? I do reduce the duration in winter but perhaps not enough. The area gets warm temperatures, rarely below 62 F on winter nights. —Aline M. Kuntz
 

Answer

Since you have pre-empted my first guess, which would be lack of light (it sounds like they receive plenty), I would then suspect that your plants are receiving insufficient day-night differential. Especially if you have selected plants that bloom in winter into spring, long nights are particularly necessary for blooming. Unless the plants receive at least 10 to 12 uninterrupted hours of night, they may not bloom. Lack of uninterrupted darkness may be the key. If the room in which they are grown has even so much as a reading light close to the plants, it will interrupt their night and provide a short-night treatment. The warmer winter nights may also be a contributory factor, but a minor one. —Ned Nash