Getting poinsettia to bloom again is not complicated but it does demand specific cultural controls.
We’ll assume you’ve grown your poinsettia since last Christmas and you’ve fed it properly with lots of light — it has grown into a small shrub and you want to get it to bloom again. Here’s what you need to know.
Short Day Plants
Poinsettia are referred to as “short day plants” and they require a night time (lack of sunlight) of 11 hours and 45 minutes to initiate flowering. So as our summer days shorten towards fall, the increasing length of night triggers flower formation.
- This assumes a temperature of 65F. If the temperatures are lower (say 62F) then the time of sunlight deprivation will be longer — in this example 13 hours of night.
- Cooler temperatures will require longer nights.
And temperatures above 70F will create a similar situation — too high a temperature requires longer night hours.
Twelve Hours Is Another Key Component
Generally, in the home environment you need night darkness of 12 hours for good and fast flower formation.
And you require this night darkness for approximately 6 weeks starting in mid-September. After this, you only require regular day length (whatever that is in your location)
No Light At All
If you put lights onto the poinsettia leaves (even for a few seconds during this 6 week period) at night -photosynthesis will start and the plant will “wake up” and the dark requirements will not be met.
Research shows that light as low as 1–2 foot candles will trigger growth. There is some data indicating a few seconds of light will put the plant back a hour or more.
(As a sidenote -this is why some greenhouses in urban areas can no longer grow poinsettia. Traffic moving past the greenhouses at night throw headlight beams onto the plants and wake them up and street lights can be a major problem if they don’t use artificial shading on the plants.
Getting a poinsettia to bloom again requires full, bright sunshine. Anything less will reduce flowering. In the home, this translates to a full south window (squeaky clean) or supplemental grow-lighting. Low light levels will also produce small bracts.
Soil should be a high quality soilless mix. No garden soil as the soil has to be well-drained.
House temperatures should be in the 65–70F range during the night and 80F during the day for best flowering.
You might want to sit the pot on a propagation mat for the day to heat up the roots if you can’t heat up the entire room.
Lower temperatures during the daytime will either retard flowering or eliminate it.
Feeding Your Poinsettia
Commercial growers feed with every watering in dilute concentrations but home gardeners can get their poinsettia to bloom again by feeding a balanced houseplant food once a week at full strength. Failure to feed will produce tiny bracts.
Pruning The Poinsettia
Getting a poinsettia to bloom again means proper pruning. In mid-to late summer, cut off the growing tips to force the plant to throw new shoots.
Maintain a high humidity for the plant if possible. Keep feeding.
The plant should throw shoots and by the third week of September, those shoots should be one-inch long.
Keep feeding and watch for insects. It is at this point that whiteflies attack poinsettia and you can control them with soap only until the color starts to show on the bracts.
If you spray soap on coloured poinsettia leaves, they do not respond well (burning).
Other common pests include mealybugs and thrips.
If you maintain the temperature, full light environment, watering and feeding, you should see colour by the middle of November. Any deviation from the guidelines above and flowering will be delayed.
The how-to/ weekly schedule for when to prune, start shading(in a closet) feeding etc is detailed in my ebook below