Christmas cactus seem like something out of a fantasy tale. The small, green branch segments stay turtle-green all summer, but just before the holidays they burst into an amazing display of brilliant blossoms that can last for months. You can take steps to help your Christmas cactus bloom, but not every piece of advice you hear will prove useful. For example, if you've heard that giving a Christmas cactus castor oil is a good idea, you'll want to separate fact from fiction. Below, some information about this gorgeous plant... or just skip to the bottom for the "castor oil tip."
It's a bit of magic straight out of Hogwarts. The so-called Christmas cactus (Schlumbergera bridesii or Schlumbergera truncata) looks like a succulent with many green, segmented branches when you buy it. Then, as the Christmas holiday season arrives, it bursts into pink, red, orange, or yellow blossoms. This amazing display has triggered all sorts of amateur theories on the reasons behind the seasonal bloom and "tips" for making sure your cactus flowers.
The truth is quite different from the myth. The Christmas cactus isn't a cactus at all, nor does it share the growth requirements of a desert succulent. In fact, the plant is an epiphyte hailing from the shady, wet forests of southeastern Brazil. Like all epiphytes, these plants do not sink roots into the ground, but dwell in tree branch crotches where dead leaves mix it up with moss.
Unlike desert plants, Christmas cactus doesn't like to dry out, nor does it do well with direct sun. The plant needs an always-moist potting soil rich in organic material, with increased irrigation when this "cactus" flowers. As an epiphyte, a Christmas cactus does best when it is pot-bound, so leave it in a small container as long as you possibly can. Full sun will burn the pale green branches, so think indirect light.
The Method Behind the Magic
So why does this epiphyte blaze into flower as the holidays approach? It has nothing whatsoever to do with Santa Claus. The only reason Christmas cactus bloom in late November is because, in the wild, the plants flower when the weather is cool and the days are short. Before the cactus is put on the market, the breeders replicate these conditions for 40 days. They keep the plant cool (under 60 degrees F) and give it total darkness for some 16 hours a day, alternating with eight hours of light.
You'll have to replicate this in subsequent years to make your plant repeat the performance. Place the Christmas cactus in a cool room in September and keep in in the absolute dark for 16 hours a day. The plant should not get air drafts, either cold or hot. Keep your eye out for the appearance of tiny flower buds about four weeks after you begin the regime. At this point, you can cease the dark/light procedure and move the plant to a spot where you can enjoy its bloom.
The Castor Oil "Tip"
If your cactus bloomed well the first year but never repeated the show, you may have turned to the internet to find solutions. One "tip" making the rounds instructs you to add a few tablespoons of castor oil to the plant's pot in October to trigger a holiday bloom.
While a little castor oil applied in October may not kill the plant, it is unlikely to trigger a bloom either. The bloom depends on light and temperature, not fertilizer. While castor oil and castor meal are rich in nitrogen, it is unclear that epiphytes benefit from or require nitrogen to grow. Even if nitrogen fertilizer does aid the Christmas cactus, the time for fertilizer is spring and summer, not fall. And any balanced fertilizer will do just as well if not better.