Light Sensitive Plants That Only Bloom During Dark Winter

Just like the midwinter holidays brighten up gloomy weather, so do the flowering plants commonly associated with them. Some of us may never think about what magic provides us with blooming houseplants at precisely the time when most of the trees, shrubs, and perennials outdoors are sleeping. In fact, it's the light sensitivity of plants like Christmas cacti (Schlumbergera bridgesti), poinsettias (Euphorbia pulcherrima), and kalanchoes (Kalanchoe spp.) that make them into the perfect floral gifts for winter, bearing flowers when the days are at their darkest.

There's a reason we associate these three plants with the end of the year — they're all naturally adapted to flowering when days are short and nights are long. These houseplants are far from being the only species that are affected by the day length, though. In the garden, crops like lettuce and potatoes need more than 12 hours of daylight to bloom. However, for winter bloomers, short days are key. This type of timing is also crucial for chrysanthemums to flower in autumn since they are only triggered to produce buds when dark nights are longer than days.

Knowing that day length determines whether these stars of winter produce buds or not isn't just a fun factoid to pull out at holiday parties. This light sensitivity will also determine whether you're able to get your houseplant to reflower or not. The secret to getting your Christmas cactus to bloom is to provide it with pitch-black nights that are at least 13 hours long, lasting over a period of two months in autumn. Likewise, kalanchoes need six weeks of daily dark periods that last 14 hours long or more. And to get your poinsettia to rebloom, you'll need to adopt the same strategy, but provide 16 hours of total darkness each day for eight weeks in fall.

It can be tricky for many people to trigger winter flowering from Christmas cacti, kalanchoes, and poinsettias in a home setting, since our homes are filled with light sources like lamps, nightlights, televisions, computers, and the glow of status buttons on electrical appliances. Even luminosity from outdoor security lights can prevent total darkness from filling our rooms. Unless you have a room that has no light pollution for the required period, you can place the plant under a box or in an unlighted closet. Don't get confused and think you need to leave them in complete darkness for several weeks straight, though — they still need bright light during the day alongside their long, dark nights.