(Editor's Note: This article was originally published on November 25, 2011. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Both Thanksgiving and Christmas cacti are epiphytes that grow on tree branches in tropical rainforest areas. Their high perch allows water from frequent rainfall to drain quickly. Although native to Brazil, the genus is named after a Frenchman by the name of Frédéric Schlumberger. Members of the genus are easily recognized by the leaflike pads joined one to another like a chain. Flowers appear at the tips of branches and are beautiful during the day but close at night.
Schlumbergera truncata is the scientific name of the Thanksgiving cactus, and the Christmas cactus is a cross between two species and correctly named S. ×buckleyi. Even though S. truncata is the Thanksgiving cactus, it is the one that is usually sold as Christmas cactus. Sellers call it Christmas cactus because it sells best by that name. The true Christmas cactus, S. ×buckleyi, blooms a month or so later than the Thanksgiving cactus and is usually in bloom after the holidays are past.
S. truncata (Thanksgiving cactus) is easy to distinguish from the S. ×buckleyi (Christmas cactus). On Thanksgiving cactus, flowers are asymmetrical and are held out from the ovary so that they are almost horizontal from the tips of segments. Flowers of the Christmas cactus are radially symmetrical and hang down straight from the ovary. Thanksgiving cactus: asymmetrical flowers bend upward at ovary so that they are almost horizontal
S. truncata (Thanksgiving cactus) is easy to distinguish from the S. ×buckleyi (Christmas cactus). On Thanksgiving cactus, flowers are asymmetrical and are held out from the ovary so that they are almost horizontal from the tips of segments. Flowers of the Christmas cactus are radially symmetrical and hang down straight from the ovary.
Thanksgiving cactus: asymmetrical flowers bend upward at ovary so that they are almost horizontalThanksgiving cactus: foliage segments are "toothed,: with soft points or claw-like projections; truncated
Leaf blades and growth characteristics are different, too. Leaf blade edges on Thanksgiving cactus are toothed (dentate) while those on the true Christmas cactus are crenate (scalloped) and do not have the teeth or points along the sides. Typically, the Thanksgiving cactus branches are more upright than those of the arching but pendent branches of Christmas cactus.
Christmas cactus: branches pendentThanksgiving cactus: branches erect
Schlumbergera ×buckleyi (Christmas cactus) is a hybrid between S. russelliana and S. truncata. The cross was made in the late 1840s by English nurseryman William Buckley. Two or three different hybrids of this cross are still available today, and work continues so that Christmas cactus is available in several different colors, all with slightly different characteristics. Because of its late bloom that appears after the Christmas season, this cactus is not usually available commercially. Nonetheless, it is a long-lived pass-along heirloom plant usually obtained from relatives or friends.
Christmas cactus: flowers symmetrical and hanging down straight from ovaryChristmas cactus: segments with no claws or points; scalloped edges
Six species, all native to eastern Brazil, make up the Schlumbergera genus. They can be divided roughly into two groups: 1) those with flattened leaf blades (phylloclades) with scalloped margins (crenate) and truncate bases (blunt-ended; appearing to terminate abruptly as if cut off) with areoles in the margins and apices (S. kautskyi, S. orssichiana, S. russelliana, and S. truncata), and 2) those with cylindrical, terete (circular in cross section) or ovoid leaf blades with areoles distributed over the entire surface (S. opuntioides and S. microsphaerica). In addition, three interspecific hybrids are recognized (S. ×buckleyi, S. ×exotica, and S. ×reginae). Of these, the most commercially available species are either S. truncata or complex hybrids of S. ×buckleyi.
Thanks to friends at Dave's Garden for some of the photographs that helped illustrate my article.