Darkness = color: secrets for holiday cacti blooms, poinsettias
Shorter days, cooler temperatures unlock the magic of these plants.
Updated
A Thanksgiving cactus shows off its upright stems, pointed leaves, and outward-facing flowers. Photo by Sue Franck
BY SUE FRANCK
Contributing Writer
As our days grow shorter and darkness settles in earlier, our gardens may be winding down, but inside our homes, some plants are just getting ready for their grand seasonal debut!
Thanksgiving cactus, Christmas cactus, and poinsettias bring welcome bursts of color during the darker months of the year. But have you ever wondered what triggers these plants to bloom or develop their brilliant holiday colors? The secret lies in darkness, cooler temperatures, and a little gardening know-how.
The Magic of Short Days
Thanksgiving cactus, Christmas cactus, and poinsettias are known as short-day plants. Despite their name, these plants actually respond to long periods of uninterrupted darkness rather than shorter days. To encourage blooming or color changes, they need approximately 12–14 hours of complete darkness each day over several consecutive weeks. The amount of time varies depending on the plant variety.
Thanksgiving or Christmas Cactus? Know the Difference!
Thanksgiving cacti are much more common than true Christmas cacti and are frequently sold or referred to as Christmas cacti. Although they look remarkably similar, a closer look at their stems, leaves, flowers, and even pollen can help you tell them apart.
Thanksgiving Cactus
Stems grow mostly upright, with the last leaf segments arching slightly downward.
Leaf segments have 2–4 pointed projections along their edges.
Flowers point outward from the ends of the stems.
Pollen is bright yellow.
Christmas Cactus
Stems and leaves have a cascading, downward-growing habit.
Leaf segments have smooth, scalloped edges without pointed projections.
Flowers droop downward like little bells.
Pollen is purplish-brown or pinkish-purple.
What's the dif? Thanksgiving cacti (left) have outward pointing flowers with bright yellow pollen. Photo by Stephanie Loomis/Unsplash. The bell-like blossoms of Christmas cacti (right) droop downward with purplish pollen. Photo by Yoksel Zok/Unsplash
How Much Darkness Is Enough?
Not all holiday plants follow the same schedule. Each has its own requirements for the uninterrupted darkness needed to produce flowers or develop colorful bracts (the modified leaves that give poinsettias their familiar red appearance).
Here is a general guide:
Thanksgiving cactus 4-6 weeks
Christmas cactus 6-8 weeks
Poinsettia 8-10 weeks
Easter cactus 8-12 weeks
Keep the Darkness Truly Dark!
Here is where gardeners sometimes run into trouble. During the daily dark period, even a brief exposure to light can interrupt the process of setting flower buds on cacti or changing the color of poinsettia leaves. That includes sunlight, household lights, and even the glow from a television or cell phone!
Fortunately, there are a couple of ways to meet these plants' darkness requirements:
Use natural daylight: Place plants in a room that receives only natural daylight. Since daylight hours are shorter than 12 hours after the fall equinox in our area, this can be an easy way to provide the necessary darkness.
Create an artificial nighttime: Place plants in a completely dark closet, cover them with a sturdy cardboard box, or use a dark fabric covering that blocks all light.
If you choose the second option, remember that these plants still need 10–12 hours of light every day. Bring them out of the darkness consistently and place them where they receive bright, indirect light, preferably near an east-, west-, or south-facing window.
Once cactus flower buds have formed or poinsettia bracts have begun changing color, the strict darkness schedule is no longer necessary. You can move your plants into their favorite display locations and enjoy the holiday color!
Temperature Makes a Difference, Too
Darkness alone isn't enough. Temperature plays an important role in encouraging buds to form and poinsettia leaves to develop their vibrant colors.
Aim for daytime temperatures between 65–75°F and nighttime temperatures between 50–55°F. Nighttime temperatures of 60–65°F may also work, provided plants receive 14 hours of uninterrupted darkness.
Keep plants away from chilly window drafts and excessive heat from furnace vents. Sudden temperature fluctuations can stress plants and interfere with flowering or color development.
Attention to darkness and temperature is key for vibrant-colored poinsettias. Photo by Irish83/Unsplash
Water Wisely for Healthy Holiday Plants
Watering needs also change during the cooler months, and each of these holiday favorites has slightly different preferences.
Poinsettias: Keep the soil consistently moist to the touch, but never soggy. Allow the soil to become slightly less moist before watering again, but don't let it dry out completely.
Christmas cactus: Allow the top inch of soil to dry before watering. Avoid letting the plant become excessively dry, as leaves may shrivel and developing flower buds may drop.
Thanksgiving cactus: Allow the top 1–2 inches of soil to dry before watering again.
One rule applies to all three plants: Never let them sit in standing water! Waterlogged roots can suffocate and develop root rot, causing leaves to turn yellow and fall off.
If your plants are sitting inside decorative foil or pot covers, remove the covers before watering. Allow excess water to drain completely before putting the covers back on, usually after 10–15 minutes. Always empty any water collected in the saucer.
There is also a difference in the recommended watering method. Poinsettias do best with top watering because their dense root systems may not become thoroughly moistened with bottom watering. Thanksgiving and Christmas cacti are better candidates for bottom watering, which helps reduce excess moisture around their stems and the risk of fungal problems and stem rot.
A Little Patience Brings a Lot of Color
Getting holiday plants to bloom or develop their brilliant colors doesn't require a green thumb or complicated equipment. It simply takes an understanding of what nature provides during this time of year: longer nights, cooler temperatures, and a consistent routine.
With a little planning and attention to their individual needs, you can encourage your Thanksgiving and Christmas cacti to burst into bloom and your poinsettias to show off their festive colors. After all, when the outdoors becomes dull and dreary, there is nothing quite like bringing a little holiday color indoors!
November Tip Box
~Now is the time to set your Thanksgiving and Christmas cacti and poinsettias plants in a closet or room that is limited to only natural daylight without any artificial light from lamps or ceiling fixtures if you want them to bloom for the holidays.
~Forcing these plants to bloom requires a very strict daily dark and light routine, so setting a timer or alarm is suggested.
~ Interested in learning more? Online searches offer many options, but research-based gardening information is most reliable when found on .edu and .org websites, including WSU Master Gardeners. You can also get personalized local advice by emailing WSU Grant-Adams Master Gardeners at ga.mgvolunteers@wsu.edu.
ALL ABOUT SUE
An Ephrata-area resident since 1959, Sue Franck is a retired dental hygienist and certified WSU Master Gardener for Grant-Adams counties. Happiest when getting her hands dirty, Sue operates her seedling business, Blissful Blooms, and has been a vendor at the Moses Lake Farmers Market since 2022, selling vegetable and flower starts. Her gardening column will appear monthly at GCJ.news.
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Darkness = color: secrets for holiday cacti blooms, poinsettias
Shorter days, cooler temperatures unlock the magic of these plants.
BY SUE FRANCK
Contributing Writer
As our days grow shorter and darkness settles in earlier, our gardens may be winding down, but inside our homes, some plants are just getting ready for their grand seasonal debut!
Thanksgiving cactus, Christmas cactus, and poinsettias bring welcome bursts of color during the darker months of the year. But have you ever wondered what triggers these plants to bloom or develop their brilliant holiday colors? The secret lies in darkness, cooler temperatures, and a little gardening know-how.
The Magic of Short Days
Thanksgiving cactus, Christmas cactus, and poinsettias are known as short-day plants. Despite their name, these plants actually respond to long periods of uninterrupted darkness rather than shorter days. To encourage blooming or color changes, they need approximately 12–14 hours of complete darkness each day over several consecutive weeks. The amount of time varies depending on the plant variety.
Thanksgiving or Christmas Cactus? Know the Difference!
Thanksgiving cacti are much more common than true Christmas cacti and are frequently sold or referred to as Christmas cacti. Although they look remarkably similar, a closer look at their stems, leaves, flowers, and even pollen can help you tell them apart.
Thanksgiving Cactus
Christmas Cactus
How Much Darkness Is Enough?
Not all holiday plants follow the same schedule. Each has its own requirements for the uninterrupted darkness needed to produce flowers or develop colorful bracts (the modified leaves that give poinsettias their familiar red appearance).
Here is a general guide:
Thanksgiving cactus 4-6 weeks
Christmas cactus 6-8 weeks
Poinsettia 8-10 weeks
Easter cactus 8-12 weeks
Keep the Darkness Truly Dark!
Here is where gardeners sometimes run into trouble. During the daily dark period, even a brief exposure to light can interrupt the process of setting flower buds on cacti or changing the color of poinsettia leaves. That includes sunlight, household lights, and even the glow from a television or cell phone!
Fortunately, there are a couple of ways to meet these plants' darkness requirements:
If you choose the second option, remember that these plants still need 10–12 hours of light every day. Bring them out of the darkness consistently and place them where they receive bright, indirect light, preferably near an east-, west-, or south-facing window.
Once cactus flower buds have formed or poinsettia bracts have begun changing color, the strict darkness schedule is no longer necessary. You can move your plants into their favorite display locations and enjoy the holiday color!
Temperature Makes a Difference, Too
Darkness alone isn't enough. Temperature plays an important role in encouraging buds to form and poinsettia leaves to develop their vibrant colors.
Aim for daytime temperatures between 65–75°F and nighttime temperatures between 50–55°F. Nighttime temperatures of 60–65°F may also work, provided plants receive 14 hours of uninterrupted darkness.
Keep plants away from chilly window drafts and excessive heat from furnace vents. Sudden temperature fluctuations can stress plants and interfere with flowering or color development.
Water Wisely for Healthy Holiday Plants
Watering needs also change during the cooler months, and each of these holiday favorites has slightly different preferences.
One rule applies to all three plants: Never let them sit in standing water! Waterlogged roots can suffocate and develop root rot, causing leaves to turn yellow and fall off.
If your plants are sitting inside decorative foil or pot covers, remove the covers before watering. Allow excess water to drain completely before putting the covers back on, usually after 10–15 minutes. Always empty any water collected in the saucer.
There is also a difference in the recommended watering method. Poinsettias do best with top watering because their dense root systems may not become thoroughly moistened with bottom watering. Thanksgiving and Christmas cacti are better candidates for bottom watering, which helps reduce excess moisture around their stems and the risk of fungal problems and stem rot.
A Little Patience Brings a Lot of Color
Getting holiday plants to bloom or develop their brilliant colors doesn't require a green thumb or complicated equipment. It simply takes an understanding of what nature provides during this time of year: longer nights, cooler temperatures, and a consistent routine.
With a little planning and attention to their individual needs, you can encourage your Thanksgiving and Christmas cacti to burst into bloom and your poinsettias to show off their festive colors. After all, when the outdoors becomes dull and dreary, there is nothing quite like bringing a little holiday color indoors!
November Tip Box
~Now is the time to set your Thanksgiving and Christmas cacti and poinsettias plants in a closet or room that is limited to only natural daylight without any artificial light from lamps or ceiling fixtures if you want them to bloom for the holidays.
~Forcing these plants to bloom requires a very strict daily dark and light routine, so setting a timer or alarm is suggested.
~ Interested in learning more? Online searches offer many options, but research-based gardening information is most reliable when found on .edu and .org websites, including WSU Master Gardeners. You can also get personalized local advice by emailing WSU Grant-Adams Master Gardeners at ga.mgvolunteers@wsu.edu.
ALL ABOUT SUE
An Ephrata-area resident since 1959, Sue Franck is a retired dental hygienist and certified WSU Master Gardener for Grant-Adams counties. Happiest when getting her hands dirty, Sue operates her seedling business, Blissful Blooms, and has been a vendor at the Moses Lake Farmers Market since 2022, selling vegetable and flower starts. Her gardening column will appear monthly at GCJ.news.