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Some of the plants most closely associated with the holidays are day-length sensitive, and exposure to long periods of darkness is a key to getting them to bloom.

Many people have Thanksgiving or Christmas cacti. A combination of long nights (12 to 14 hours or more of darkness) and cool nighttime temperatures (low to mid-60s) for about six weeks promote flowering, so leaving them outside in the fall is a good way to prepare them to bloom.

Once flower buds have formed, these may drop when temperatures dip below 50 degrees Fahrenheit, so it’s better to bring Thanksgiving or Christmas cacti into a protected area if temperatures are forecast to get very cold. Don’t allow them to experience freezing temperatures at any time.

Many of us treat poinsettias as annual plants, acquired around the holidays and then disposed of, but they are perennials in their native, tropical habitat and can live from year to year. If you’ve held a poinsettia over from last year, it will also need to be exposed to long periods of darkness to develop the characteristic color in its bracts. Compared with Thanksgiving and Christmas cacti, it generally takes a bit longer — eight to 11 weeks — from when poinsettias are first exposed to long nights until they bloom.

Note that light coming from sources such as security lights and automobiles can interfere with the needed periods of darkness, so place poinsettias in a location that’s protected from artificial light or cover them in the evenings. Provide cold protection if temperatures are forecast to get below 40 degrees Fahrenheit.

Let me know if you have questions.

Dr. Mary Helen Ferguson is an Associate Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu, 985-839-7855 (Franklinton) or 985-277-1850 (Hammond).