It will not flower
A plant that grows well and never flowers is usually short of light, or short of the trigger its species needs. What each of the common houseplants actually wants.
What it usually is
1 Not enough light
| How to tell | Healthy green growth, good size, no buds, in a spot with no direct sun. Flowering costs far more than leaf making, and a plant that cannot pay simply does not. |
|---|---|
| The fix | Move it to the brightest position the species will take. For most flowering houseplants that means a window with some direct sun in the morning or late afternoon. Expect the change to take a season rather than a fortnight. |
2 Too much nitrogen
| How to tell | Large, soft, dark green leaves on a plant fed with an all purpose feed through the year, and no buds at all. |
|---|---|
| The fix | Switch to a feed higher in potassium, the kind sold for tomatoes, through the growing season. Nitrogen builds leaves, potassium builds flowers, and a plant given plenty of the first has no reason to attempt the second. |
3 The trigger the species needs has not happened
| How to tell | Look up what your plant needs. Christmas cactus wants long uninterrupted nights and cool weeks in autumn. Moth orchids want a drop of about ten degrees between day and night for a fortnight. Bulbs want a cold rest. Nothing you do about light or feed substitutes for these. |
|---|---|
| The fix | Give it the trigger deliberately. Move the orchid to a cooler room or nearer a window at night in autumn. Put the Christmas cactus somewhere genuinely dark from evening to morning for six weeks. Then return it and wait. |
4 The plant is too young, or too comfortable
| How to tell | A young plant that has never flowered, or one repotted recently into a much larger pot and now growing vigorously. |
|---|---|
| The fix | Wait, and stop potting on. Many species flower only when the roots are snug, and a plant given room to grow will grow rather than flower. Leave it in the same pot for another year. |
Through the year. Most houseplants set buds in response to the lengthening or shortening of the day, so the work is done months before the flowers appear. Autumn conditions decide a winter flowering, and winter light decides a spring one.
Flowering is expensive. A leaf pays for itself over its life; a flower never does, in energy terms, and a plant only attempts one when it has a surplus and a reason. Almost every case of a healthy plant that will not flower comes down to one of those two being missing.
The surplus is light, and this is where most indoor flowering fails. A plant that makes decent leaves in a north facing room has enough light to live and not enough to bloom. There is no feed, no fertiliser and no trick that substitutes for it. If you want flowers from a plant indoors, it needs the best window in the house, and for most species that means real sun for part of the day.
The reason is the trigger, and it differs by species in ways that are worth looking up rather than guessing. For the plant most people want in bloom at the holidays, see plants for the festive season. A Christmas cactus sets buds when the nights are long and unbroken and the room is cool, which is why one standing under a lamp that is on until midnight never flowers. A moth orchid sets a spike after a fortnight with about ten degrees between its warmest and coolest hours, which is what an autumn windowsill provides and a centrally heated hallway does not. An amaryllis wants a dry rest. A hoya wants to be left alone on the same spur, which is why cutting off the old flower stalk costs you next year’s flowers.
Feeding is the third piece and the easiest to get backwards. All purpose feeds are built around nitrogen, which grows leaves. A plant you want flowers from wants potassium, and the cheapest source is ordinary tomato feed, used at the strength on the bottle from spring until the flowers open.
Last, patience and pot size. Many houseplants flower better slightly potbound, because a plant with room to grow will always choose growing. If yours has just gone into a pot two sizes up and is throwing out leaves, that is your answer, and the fix is to leave it alone for a year.
Entries that name it
What the journal says about this symptom on each plant.
| African violet | Not enough light, a pot that is too big, or too much nitrogen. Give it ten to twelve hours of bright indirect light, keep it in a small shallow pot, and use a feed made for African violets. |
|---|---|
| Bird of paradise | Too little light, a plant that is not yet mature, or a pot that is too large. Move it to full sun, feed through the growing season, and leave it potbound. A seed grown plant needs four to six years and a division two or three. |
| Cape leadwort | Too much shade, or a hard prune done too close to the flowering season instead of at the end of winter. Move it to full sun if the site allows, and save the hard cut for late winter, before new growth starts, so it does not remove the wood the coming flowers grow from. |
| Christmas cactus | The plant has not had the long dark nights and cool weeks it needs to set buds. From about six weeks before you want flowers, give it twelve to fourteen hours of complete darkness each night and keep it near 15 degrees, watering sparingly. |
| Escallonia | A hard prune done in early spring, which removes the previous season's wood before it has flowered, or too much shade. Move the pruning to straight after flowering finishes from now on, and check the site gets a genuinely full day of sun. Both are more common causes than a pest or nutrient problem. |
| Flamingo flower | Not enough light, or a pot much larger than the roots. Move it to bright indirect light and leave it slightly potbound. A feed higher in phosphorus through spring and summer helps once the light is right. |
| Laurustinus | A hard prune at the wrong time of year, since it flowers on the previous season's wood, or a heavy nitrogen feed pushing leaf growth instead of flowers. Prune only straight after flowering finishes, and skip the fertiliser if the soil is already reasonable. An established plant needs very little of either to flower well. |
| Lavender | Too little sun, too much feed, or a plant that was cut back too hard into old wood. Full sun, no feed, and a light prune each year straight after flowering rather than a hard one in spring. |
| Moth orchid | Not enough light, or no drop in night temperature to trigger a spike. Move it to a brighter window, and give it a few weeks in autumn where nights are about 5 degrees cooler than days. |
| Peace lily | Not enough light, or a plant that is too young. Move to brighter indirect light; mature plants flower in spring and again in late summer. |
| Rosemary | Too little sun, or too much feed and water. Full sun and leaner treatment. Rosemary flowers best when it is slightly hungry and dry. |
| Spider plant | A pot with too much room, or too much feed. Leave it slightly potbound and halve the feed. Spider plants throw runners when they are comfortable but not indulged. |
| Tomato | Too little light, or too much nitrogen feed. Move it into full sun and switch to a high potassium tomato feed once the plant is growing well. |
| Wax plant | Not enough light, a plant that is too young, a pot that is too large, or the flower stalks having been cut off. Move it to bright indirect light, leave it potbound, and never remove the short woody stalks the flowers came from, because the next year's flowers come from the same ones. |
Questions from the margin
- Why won't my houseplant flower?
- Almost always light. Flowering costs far more than making leaves, and a plant that grows well in a dim room has enough light to live on and not enough to bloom. No feed substitutes for it.
- What feed makes houseplants flower?
- Something higher in potassium than nitrogen, which is what ordinary tomato feed is. All purpose feeds are built around nitrogen, which grows leaves, and a plant given plenty of it has no reason to attempt flowers.
- Why won't my orchid flower again?
- Moth orchids set a spike after about a fortnight with roughly ten degrees between day and night. A centrally heated room never gives them that, which is why an orchid moved to a cooler spot or nearer a window in autumn so often reflowers.
Checked against RHS, Orchids: encouraging reflowering; RHS, Schlumbergera: getting them to flower.