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thanksgiving, easter and christmas
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The Thanksgiving and Christmas cacti, AKA crab cacti or claw cacti, are hybrids in genus Schlumbergera. They have previously been placed in genera Epiphyllum, Epiphyllanthus and Zygocactus. Plants consist of flattened stem segments that superficially resemble leaves. They tend to have long stem protuberances along the edges that look like soft green spines. They flower from Fall to mid-Winter, depending on night temperatures or night length. These have petals arranged in a zygomorphic form, united into a tube below, with pistils protruding beyond the tube. They are easy to grow as shade epiphytes, in very loose and well-aerated media. They tolerate periods of drying out but do better kept evenly moist all year. Flowering is initiated by one of two things: Six weeks of fall night temperatures below 55 F / 13C; or, six weeks of completely dark nights of more than 10 hours - like Poinsettias, Euphorbia pulcherrima. Turning on an electric light at night just once during this period will inhibit flowering. This is why Grandma's cactus flowered reliably, whereas the one in your living room won't - Grandma put hers under the window in the root cellar for the winter, where it got only natural light. Your plant in the living room gets artificial light most Fall nights.
Easter cacti are hybrids in genus Rhipsalidopsis. The plants look a lot like those of Schlumbergera except there are usually none of the long stem segment protuberances. They have radially symmetric flowers with petals separate at the bases, and a short pistil in the center. They flower in late Winter or Spring. Their shade epiphyte ancestor species were described as genus Epiphyllum, then moved to Schlumbergera, then Rhipsalis, then Epiphyllopsis, then Rhipsalidopsis, then Hatiora (all the previous based on morphology studies), then back into Schlumbergera based on DNA studies then, after more refined DNA studies, back into Rhipsalidopsis.
Rhipsalidopsis are harder than Schlumbergera to grow for most people. They need to stay evenly moist with no drying out and no long wet periods, or the root system collapses. If this happens stem segments can be rooted. They tolerate less heat than do Schlumbergeras. But, they will flower as house plants, without the need for long nights or chilling.
Both genera are easy from cuttings, though Rhipsalidopsis root best in Spring. Single Schlumbergera stem segments can be rooted in bottle caps of water with a success rate of nearly 100%.