They're easy to grow and a popular winter holiday gift. So, many office and home windowsills hold either a Christmas or Thanksgiving cactus.
Typically, the cactus owners check often through fall and winter for signs of blooms. But, they have no idea why some years bring a wealth of orchid-fragile flowers and others bring a few pink or red buds. They don't know why the plants sometime bloom for the holidays and sometimes bloom later.
"If you know what they need, holiday cacti actually are very predictable. And, compared to many South American jungle plants, they're much more flexible in what they require to bloom," said Ward Upham, horticulturist with Kansas State University Research and Extension.
Although different species, Christmas cactus (Schlumbergera bridgesii) and Thanksgiving cactus (Schlumbergera truncata) have exactly the same needs, Upham said. Both like bright, indirect light, though too much sun can turn leaves yellow. They like common household temperatures and soil that's constantly moist, but not waterlogged.
"Now isn't the time to transplant them, though," he added. "These plants seem to flower best if they're a little pot-bound."
Upham warned that neither will flower at temperatures below 50 degrees. And, neither will flower unless one of the following happens during fall:
"After that, the plants need another nine to 10 weeks for the flowers to finish developing and bloom. So, if you want blooming by one of 2005's winter holidays, this is the time to start thinking about how you can provide the conditions they need," Upham said.