Christmas cactus drop flower buds because of drought stress, fluctuating temperatures or due to artificial light interrupting the 12 hours of darkness required for the development of flower buds.
Christmas cactus are ‘short day’ plants which means they require more consecutive hours of darkness then light to form flower buds.
In order to establish why the buds fall from Christmas cacti it is important to know the optimal conditions for bud formation.
This is because the cause of bud drop is usually due to a sudden change in conditions or a significant deviation from the optimal growing environment.
The Optimal Conditions for Christmas Cactus Flower bud Formation
The flower buds of Christmas cactus form most often in September.
The following conditions are required to ensure your flower buds develop properly:
- 12 or more hours of uninterrupted darkness from September onward every evening for at least 6 weeks. Christmas cactus are short day plants that require more hours of darkness then light during bud formation. This is due to adaptations to detect seasonal change in its environment which stimulates flowering.
- Cooler temperatures during bud formation then usual. The optimal temperature for bud formation is around 60°F (15°C) during the 12 or more hours of darkness. This temperature change emulates the cooler seasonal temperatures at this time of year in its native Brazilian environment.
- Bright indirect light during the day gives the Christmas cactus then energy required for bud formation. Too much shade can contribute to flower buds falling off.
- Watering around once per week so that the surface of the soil is dry to the touch between bouts of watering is the optimal balance of moisture, for Christmas cactus during bud formation. Mist the plant once or twice per week and keep it out of any significant air currents.
- Do not apply any fertilizer from late Summer when the flower buds are forming. Additional fertilizer can stimulate foliage growth rather then the formation of buds.
- A consistent environment without moving the plant or turning it around. Christmas cactus get very accustomed to their surroundings and do not like to be moved when the buds are developing.
If your Christmas cactus experiences conditions contrary to this, then this is the likely cause of flowers not forming or dropping off.
Keep reading to learn why your buds are falling off before they can display flowers….
(I have listed the causes of bud drop in order of which is most common).
Underwatering and Low Humidity Causes Buds to Drop off
Christmas cactus are native to mountains rainforest in Brazil here they thrive in high humidity and frequent rainfall.
This is in stark contrast to other desert dwelling species of cactus which thrive in hot and dry conditions with little rainfall.
If the humidity is too low in your home the flower buds drop off Christmas cactus during their formation.
Houses tend to have low humidity anyway, but there are some things that can exacerbate the lack of humidity and increase the risk of bud drop, such as:
- Air currents from air conditioning.
- Fluctuating heat and airflow from forced air.
- Draughty areas of the house.
- Convention currents caused by sources of heat during Winter.
Try and locate your Cactus in a relatively still area of the house (although moving the plant during bud formation is not advised).
To counteract these causes of low humidity and to mimic the conditions of the cactus rainforest habitat, use a spray mist on the leaves two or three times a week.
This helps to reduce water loss (transpiration) from the leaves to mitigate drought stress and creates more favorable conditions for the Christmas cactus to develop its flowers.
Underwatering…
Because people associate cacti with drought tolerance and desert conditions, the topical Christmas cactus is often underwatered which causes drought stress and harms the formation of flower buds.
The correct balance of watering is critical during to avoid flower buds dropping.
Typically watering around once per week so that the soil is evenly moist but the surface is dry to the touch between bouts of watering, meets the moisture requirements of the cactus and avoid any problems with overwatering or underwatering.
(To learn more, read my article for how often and how much to water Christmas cactus and for all the best watering practices).
Interrupted Darkness Interferes with the Formation of Flowers
Christmas cactus are relatively unusual in that they are ‘short day‘ plants which means they rely on shortening day length to stimulate the formation of flowers.
There must be more hours of darkness (more then 12 hours) then light for your Christmas cactus to form flowers properly.
If you turn on a bright light in the middle of the night, even for a short time, this interrupts the plants period of darkness and can either cause the plant to not display flowers or the developing buds fall off.
If it is difficult to find a room in your home that is typically dark without any artificial light for more then 12 hours then I recommend using a light excluding cloth over your Christmas cactus for the 6 weeks or so whilst the buds are developing.
(If your Cactus does not display any flowers at all then read my article why is my Christmas cactus not blooming for the solutions).
Moving the Christmas Cactus can Cause Flower buds to drop off
A common mistake which results in bud drop is relocating the cactus whilst the flowers are developing.
Christmas cactus requires quite a specific sequence of conditions for flowering, so moving the plant can disrupt the environment to which the plant is familiar which causes flower bud drop.
A change in air flow is often a culprit of bud drop, as is a difference in the direct of the light source.
The Christmas cactus prefers to develop its buds with a consistent direction of light.
Turning the plant around or moving it to a different window sill changes the direct of light. The flower buds often grow towards the new source of brightest light then drop off as a response.
Ensure that your cactus stays in one consistent place during the flower bud formation from around September and try not to move it or turn it around for any reason to reduce stress and prevent the buds from dropping.
Saturated Soil Causes Flower Buds to Drop
Whilst Christmas cactus are rainforest plants they are epiphytic which means they grow on other trees and off the ground which means they do not tolerate their roots being in Damp or boggy soil.
The key is to keep the soil evenly moist but well draining.
Too much moisture around the roots can cause root rot, an overall drooping appearance and flower buds to drop.
There are several factors that cause of damp soil and result in buds falling off:
- Overwatering. Typically watering once per week is enough during bud formation.
- Pots without drainage holes in the base. Christmas cactus does not tolerate its roots in standing water. Ensure that your pot has drainage holes in the base and that they do not become blocked with compacted soil or roots.
- The use of saucers, trays and decorative outer pots. Excess water should be able to escape freely from the bottom of the pot. If the saucer, tray or outer pot pools water then the soil can stay too damp and the flower buds fall off due to stress.
Ensure that your potted Christmas cactus plants have good drainage to maintain the optimal balance of moisture during bud formation to avoid the flower buds falling off.
(If your Christmas cactus is excessively drooping or wilting read my article why is my Christmas cactus wilting for the solution).
Fluctuating Temperatures
Christmas cactus are fairly adaptable but fluctuating temperatures whilst the flower buds are forming often causes them to drop off.
Christmas Cactus are native to mountainous Brazilian rainforest regions and they are accustomed to cooler temperatures and shorter days whilst the flower buds develop.
Ideally try to keep the temperature at around 60°F (15°C) from September time for around 6 weeks. Any temperatures that are considerably different to this can be the cause of the flower buds to drop.
Once the flowers have emerged, a temperature of 68°F (20°C) if thought to prolong the flowering time.
(Read my article, how to revive a dying Christmas cactus).
Key Takeaways:
- The reasons for Christmas cactus dropping its flower buds are drought stress, fluctuating temperatures, overwatering and too much artificial light at night.
- Christmas cactus require evenly moist soil, higher humidity and more hours of darkness then light for 6 weeks from September to form buds.
- If you relocate the cactus during flower bud formation and change the direction of brightest light then the flower buds often try to turn towards more light and then drop off as a sign of stress.
- Keep Christmas cactus at lower temperatures during bud formation and spray the leaves to increase humidity.