Gadenin日本語Get the appBright light all year, direct sun in the cool months
This is a succulent from a hot, dry climate, and it is far greedier for light than the average flowering pot plant. In poor light the stems stretch, the leaves space out along them and the plant will not flower no matter what else you do. A south or west window is the right place for most of the year.
The one qualification is midsummer glass. Behind a south window in full summer sun the leaf edges bleach and then scorch to papery brown patches, and those marks are permanent. From late spring to early autumn, move it back a couple of feet or put a sheer curtain between it and the glass. In autumn, winter and early spring, give it every hour of direct sun you can.
| Position | Suitability | What happens there |
|---|---|---|
| South window, autumn to early spring | The best | Compact growth, thick leaves, red edges on the foliage |
| South window, midsummer | Too harsh | Bleached and scorched leaf edges. Pull it back two feet |
| East window | Very good | Morning sun without midsummer scorch. Safe all year |
| North window | Poor | Stems stretch, leaves thin, few or no flowers |
| Six feet into a room with a lamp on all evening | Will not flower | Grows on quietly and never sets a bud |
Why the shop plant flowers once and never again
A kalanchoe is a short-day plant. It only starts building flower buds when the dark period runs to about fourteen hours a night, and it needs that to repeat night after night for roughly six weeks before any bud is committed. Growers deliver that by blacking out the greenhouse on a timer, which is why the plant you brought home was covered in flower.
In an ordinary living room the plant never gets those nights again. A ceiling light switched on for two hours after dusk cuts the dark period to nine or ten hours, and the plant reads that as summer. It keeps making leaves, the stems lengthen, and a year later you have a tall green thing that has never flowered. Nothing is wrong with it, and no feed or fertilizer will change it. The dark is the missing ingredient.
Even brief light breaks the count. A hall light spilling under a door, a street lamp through thin curtains or a television in the same room for an hour is enough to reset the plant for that night.
How to give it six weeks of long nights
The treatment is simple and slightly tedious: for six weeks the plant needs fourteen hours of complete darkness and ten hours of bright light every single day. Most people do it with a cardboard box and a note in a diary.
- Pick a start point and count six weeks Begin in early autumn if you want flowers by midwinter, or in midwinter for spring flowers. It takes about six weeks of treatment plus another four to six weeks for the buds to open, so work back from when you want it in flower.
- Cover it every evening at the same time Put a cardboard box or a black bin liner over the plant at around the same hour each evening and take it off in the morning. Fourteen hours covered, ten hours uncovered. A kitchen timer or a phone alarm makes it far more likely you will actually keep it up.
- Make the cover genuinely light-tight A box with a gap at the base is not dark. Weight the edges down, or use a cupboard with a solid door instead of a box. If light gets in, the six weeks count for nothing and you will not know until no buds appear.
- Give it full light for the other ten hours Darkness alone does nothing. During the uncovered part of the day the plant needs the brightest window in the house, otherwise it has no energy to build the buds it has been triggered to make.
- Stop covering once you can see buds When small green bud clusters are visible at the tips of the shoots, the treatment has worked and you can stop. Move the plant back to its window and let the buds develop and open in normal light.
Temperature and airflow
Warmth matters less than light, but the limits are firm at both ends. Below about 50F (10C) growth stops and the leaves go soft and blotchy; a real frost turns them to mush overnight. Above about 86F (30C) in a closed room the plant stalls and existing buds may drop instead of opening.
The bud-setting period has a narrower band than the rest of the year. Nights around 59 to 64F (15 to 18C) are close to ideal for building buds, and a room that stays above about 75F (24C) all night can stop the process even when the darkness is perfect. A cool bedroom or a spare room is usually better for those six weeks than a warm living room.
| Season | Where to stand it | Temperature to hold |
|---|---|---|
| Spring | South or east window, full sun | Above 50F (10C) at night |
| Summer | Back from the glass, or behind a sheer curtain | Below 86F (30C) with air moving |
| Autumn | Brightest window, and start the dark treatment | Nights of 59 to 64F (15 to 18C) |
| Winter | Full sun at a window, off a cold sill at night | Never below 50F (10C) |
A cold window sill behind closed curtains on a winter night can be ten degrees colder than the room. Move the plant onto a table for the night, or draw the curtains behind it rather than in front of it.
Reading the plant to check the position is right
A kalanchoe tells you about its light very clearly, because the shape of new growth changes within weeks of a move. Look at the length of stem between one pair of leaves and the next: short and stubby means the light is good, long and bare means it is not.
| What you see | What it means | What to do |
|---|---|---|
| Long bare stem between leaf pairs | Not enough light | Move to a brighter window and cut it back in spring |
| Leaves large, thin and deep green | Low light, or too much nitrogen | More light, and stop feeding until growth firms up |
| Thick leaves with reddish edges | Strong light. The plant is happy | Nothing. This is the target |
| Bleached or papery patches on the leaf face | Scorched by summer glass | Move back from the window. Marks do not heal |
| Healthy plant that never flowers | Nights too short or too warm | Six weeks of fourteen-hour nights in a cool room |