Christmas Cactus: Why It Won't Bloom, and the Trick That Works

Almost everything people assume about this plant is wrong, starting with the name.

It is not a desert cactus. Schlumbergera comes from the coastal mountain forests of southeast Brazil, where it grows on tree branches and rock faces in dappled shade with high humidity. Treat it like a desert cactus and it will sulk. Treat it like a jungle plant and it will outlive you, which is why so many are passed down through families.

Why it will not bloom

It needs long nights and cool temperatures, and a normal warm lit house gives it neither.

Schlumbergera is what botanists call a short-day plant. It sets flower buds when nights get long and temperatures drop, which is its signal that the season has turned. In a house held at a steady 21°C with a lamp on until eleven at night, that signal never arrives.

Here is the routine, starting six to eight weeks before you want flowers, so mid-October for Christmas:

1. Give it 12 to 14 hours of complete darkness every night. A spare room, a cupboard, or a box over it. Complete means complete, since even a streetlight or a hallway lamp can break the cycle.Darkness
2. Keep nights cool, ideally 10 to 15°C or 50 to 60°F. A cool spare room or an unheated porch works. Cool nights alone will sometimes do it even without the darkness routine.Cool nights
3. Water less during this period. Let the top few centimetres dry between waterings. Not bone dry, just leaner than usual.Slight drought
4. Stop feeding entirely.No fertiliser
5. Keep it up for six weeks or until you see small buds at the tips of the segments.Six weeks
6. Once buds appear, bring it back into normal light and warmth, and stop moving it.Then leave it alone

That last point matters more than it sounds. Once buds have set, this plant drops them at the slightest provocation.

Bud drop, which is the other great frustration

Buds form, you get excited, and a week later they are on the carpet.

The causes, roughly in order of how often they are to blame: moving the plant, a sudden temperature change, a draught from a door or a heating vent, letting it dry out completely, overwatering, and ripening fruit nearby releasing ethylene gas.

Pick a spot before buds set and leave the plant there. Do not rotate it. Do not move it to the table for guests and back again. It is a plant with strong opinions about being disturbed.

Ordinary care, the rest of the year

Light. Bright indirect. An east window is close to perfect. Direct summer sun bleaches the segments to a reddish purple, which is stress rather than colour.

Water. More than a desert cactus, less than a fern. Water when the top third of the soil is dry, then thoroughly. It is a jungle plant and does not want to be parched, but it rots readily in soggy soil.

Humidity. Appreciates it. A pebble tray helps, particularly in a heated house in winter.

Soil and pot. Free draining. Ordinary compost with a good handful of perlite or orchid bark. Keep it pot bound, since it flowers better when snug and resents a big pot.

Feeding. Half strength every month or so from spring to late summer. Stop in early autumn to help trigger the bud cycle.

Which one do you actually own?

Most plants sold as Christmas cactus are Thanksgiving cactus, and once you see it you cannot unsee it.

Thanksgiving cactus (Schlumbergera truncata) has segments with sharp, pointed, claw-like teeth along the edges. Blooms roughly November. This is the one in almost every shop.

Christmas cactus (Schlumbergera bridgesii) has segments with rounded, scalloped edges. Blooms roughly December. Less common in trade than the name suggests.

Easter cactus (Rhipsalidopsis gaertneri) has rounded segments with small bristles at the tips, and star-shaped rather than tubular flowers. Blooms spring.

Look at the edge of a segment. Pointed claws means Thanksgiving, scalloped waves means Christmas. It matters, because it changes when to start the darkness routine by about a month.

Propagating, and the good news about pets

Snap off a piece two or three segments long, let the broken end dry for a day or two, then push it a centimetre into moist mix. Roots in a few weeks. This is how the family heirloom ones spread, and it is nearly foolproof.

And a rarity in this category: Christmas cactus is non toxic to cats and dogs. The ASPCA lists Schlumbergera as safe, which puts it in a small group alongside spider plants, ferns and true palms. Compare that with poinsettia, amaryllis and holly, which all cause problems, and it is a genuinely good choice for a house with animals.

If you are unsure whether yours is a Christmas, Thanksgiving or Easter cactus, photograph a segment edge clearly and Identium will tell you which, which sets the timing for the whole bloom routine.