Neil Sperry

Neil Sperry: Let’s unwrap mysteries of Christmas cacti

Keep your tropical Christmas cactus happy with organic, well-draining soils — and by avoiding heavy, sticky native soil or dense commercial potting soils.
Keep your tropical Christmas cactus happy with organic, well-draining soils — and by avoiding heavy, sticky native soil or dense commercial potting soils. Special to the Star-Telegram

Stop by any garden center about now and you’ll find a good selection of plants for the season. Poinsettias will grab your eyes first, but somewhere amid all that sea of red, you’ll find some exotic little cacti. Appropriately named Christmas cacti or Thanksgiving cacti, these plants are well known for blooming at this season each year.

How do they do that? How do they know? That’s just part of their amazing story. Here are a few facts you may not have known about this cheerful bunch of tropical plants.

Christmas cacti are native to rain forests. They grow in the heavily treed mountains of southeastern Brazil. That’s not exactly where you’d expect to find water-conscious members of the cactus plant family.

Cacti are all succulents, but not all succulents are cacti. The term “cactus” refers to plants from one specific plant family (Cactaceae). Many other plants get thrown in with them, even though they’re not distantly related to true cacti. Agaves, yuccas, crown of thorns, pencil cactus and baseball cactus are all from other plant families entirely.

All cacti, Christmas cacti included, are native to the Western Hemisphere. Many plants that resemble cacti, such as the aforementioned crown of thorns, pencil cactus and baseball cactus, are from the Euphorbia family. Even though they look much more like cacti, they’re actually closely related to poinsettias.

Christmas cacti have no leaves. The green portions that bear their flowers are actually flattened stem segments.

Most members of the cactus family are terrestrial growers, but Christmas cacti are epiphytes. That refers to the fact that they grow suspended from tree trunks and hanging from rocks. Pieces of stems and seeds fall into leaf axils of their support trees and cracks in the rocks. The plants develop roots, and stem growth begins. But even though it may rain 100 inches per year, these plants still have highly evolved ways of surviving without soil and during the dry seasons. After all, they’re cacti!

In keeping with their native habitats, Christmas cacti require highly organic, well-draining soils. Coarse-textured forms of organic matter such as sphagnum peat and finely ground pine bark mulch work well. What you don’t want is our heavy, sticky native soil or dense commercial potting soils.

Bright light is important, but not direct sunlight during the summer. Again, think of their native habitats. They grow within rainforest canopies. A sunny window indoors is good. That translates to bright shade outside.

These are tropical plants that cannot withstand freezing weather. However, they’re also not fond of extreme heat and low humidity. If you grow them outdoors, put them in a spot that isn’t hit by drying summer winds.

Christmas cacti are photoperiodic. That means that they measure the length of the dark period. The plant hormone that initiates flower bud formation is destroyed by light, so if Christmas cacti are exposed to bright light at night it will delay their blooming.

As with poinsettias, you’ll want to give them 14 hours of total and uninterrupted darkness each night beginning in early October. That will bring them into bloom by early December. But read that again carefully — darkness at night. They’ll still need their normal bright light during the daytime.

Unlike poinsettias, Christmas cacti will flower as a result of exposure to temperatures in the low 50s. However, since you have better control over the lights at night, and since you probably don’t want your thermostat set to 55 degrees, it’s usually easiest just to modify their exposure to light.

It’s incredibly easy to start new Christmas cacti. Each stem segment is capable of forming roots. After the plant finishes blooming, carefully trim off pieces of stem with two or three segments. Lay them on a pot filled with the recommended planting mix. Keep the mix moist, and the segments will begin to form roots almost immediately. Once the roots are  1/2-inch long you can pot the cuttings up into the same planting mix, two or three cuttings per 4-inch pot.

Christmas cacti can be grown for years in the same large pots. Eventually it will be necessary that you lift them out and repot them. Choose a larger pot size, and use the same loose potting soil to replant them.

These are plants that commonly are handed down from generation to generation. They become family heirlooms that can be propagated and shared with all the family members who would like to have them. As with favorite recipes, that’s a fine way to develop heartfelt family traditions.

Neil Sperry publishes Gardens magazine and hosts “Texas Gardening” from 8 to 10 a.m. Sundays on WBAP/820 AM. Reach him during those hours at 800-288-9227. Online: