Gadenin日本語Get the appThe red parts are bracts, not flowers
What everyone calls the flower is a rosette of modified leaves called bracts. The actual flowers are the small yellow-green beads clustered in the center, and they are barely worth looking at. Once you know this, the whole business of getting the plant to color again makes sense: you are not making it bloom, you are making a set of ordinary green leaves change color.
It matters practically too. Bracts last far longer than petals, which is why a poinsettia holds its display for two or three months. And when you look at the yellow centers you can read how fresh the plant is: tight, unopened beads still dusted with pollen mean the display has weeks left, while shed centers leaving bare green stubs mean the bracts will start dropping soon.
| Part | What it actually is | What it tells you |
|---|---|---|
| Red, pink or cream rosette | Colored bracts, modified leaves | Lasts two to three months in a cool room |
| Yellow-green beads in the center | The true flowers, called cyathia | Still closed means the display is young |
| Dark green leaves below | Ordinary foliage | Should stay on the plant all winter |
| Bare green stubs in the center | Flowers already shed | The bracts will fade within a few weeks |
Long nights are what trigger the color
A poinsettia colors in response to night length, not to cold, feed or anything you can buy. It needs roughly fourteen hours of continuous darkness every night for eight to ten weeks, with bright light during the remaining ten hours. Outdoors in its native climate the shortening autumn nights do this automatically. Indoors, a lamp switched on in the evening cancels it completely.
The word to concentrate on is continuous. Two minutes of light in the middle of the dark period is enough to reset the clock for that night, and a plant that gets interrupted most nights will simply stay green through the whole winter. This single point is why most home attempts fail, and it is not a matter of degree: near enough is not good enough.
Streetlight through a window, a hallway light left on, a television standby glow, a phone screen in the room. All of them count as light for this purpose. A poinsettia on a windowsill under a streetlamp will never color.
How to run the dark treatment
Start counting back from when you want the color. Eight weeks gives you pale, partly colored bracts; ten weeks gives a full display. Begin in mid autumn, about ten weeks before midwinter, and expect the first pink flush after four or five weeks.
- Choose one dark place and use it every night A cupboard, an unused wardrobe, a spare room with a blackout blind, or a large cardboard box turned over the plant. It must be genuinely dark when you close it, not merely dim. Check by shutting yourself in for a minute with the door closed.
- Cover it at the same time each evening Put it away at around five in the afternoon and bring it out at around seven or eight the next morning. Fourteen hours dark, ten hours bright. Setting an alarm on your phone for both ends is worth the trouble, because one forgotten evening costs you a night.
- Give it real light during the day The daytime half matters as much as the night. Stand it in the brightest window you have for the ten light hours. A plant kept dark fourteen hours and dim for the rest starves and drops its lower leaves while the bracts are forming.
- Keep it warm and watered as usual Aim for 64 to 70F (18 to 21C) during the treatment. Water when the surface of the compost dries, exactly as you would at any other time. Do not let it dry to wilting inside a dark cupboard where you cannot see it.
- Stop once the bracts have colored When the top bracts are fully colored, usually after eight to ten weeks, stop the covering and move the plant to its display position. The color holds from there without further darkness.
Why it stays green, and what to check
If autumn passes and the top leaves stay green, the cause is almost always light at night. Work through the list below in order, because they are ranked by how often they turn out to be the answer.
| Symptom | Likely cause | What to change |
|---|---|---|
| Entirely green, no hint of pink | The dark period is being interrupted | Find the light source and move the plant |
| Pale pink, never deepens | Fewer than eight weeks of treatment | Start two weeks earlier next autumn |
| Only the top few bracts color | Not enough daytime light | Brighter position for the ten light hours |
| Colored, then leaves fell | Too cold or a draft during treatment | Hold 64 to 70F (18 to 21C) in the dark place |
| Small bracts on long bare stems | Not pruned back in late spring | Cut back hard after the old display fades |
What to write down for next year
The dark treatment is the one job on this plant where records pay for themselves immediately, because you only get one attempt per year and you will not remember the details twelve months later.
Note the date you started covering the plant, the date the first pink appeared in the top bracts, and the date the color was full. Next autumn you count back from the date you want the display and you know your own plant's timing rather than a general figure. Also note where you kept it and whether you had to move it, because a place that turned out to have a light leak is worth ruling out permanently.