Longer nights and lower temperatures make Alaska ideal for holiday plants and cacti

My dad and I had a running discussion about what makes Thanksgiving and Christmas cacti set blooms in the fall. I maintained that it was because nights were finally longer than days. He claimed it was the lower temperatures.

In the end, I was right — not that it gave me any pleasure, just blooms! He won the debate when it came to cyclamen, however, as well as jasminum polyanthum. These are plants that do respond to the lower temperatures. It is true that Thanksgiving and Christmas cactus, Schlumbergera spp., hold their buds and flowers better if temperatures are lower, so I will give my dad some credit.

Anyhow, what triggers some plants to flower is scientifically referred to as photoperiodism. This is simply the duration of light a plant is exposed to in a day. Interestingly, it is actually the amount of darkness that controls flowering of most of these flowers, not the duration of light.

The so-called holiday cactuses are not the only houseplants that bloom if you take advantage of the shortening days. There are plenty of others. As an Alaskan, you owe it to yourself and family to own a few of these.

At the top of the list is the poinsettia. There are always folks who try to replicate in succeeding years the bracts that were there at time of purchase. It takes total darkness for 13 hours to set the tiny little yellow flowers and develop their accompanying red bracts. Alas, I am a bit late in reminding the few readers who keep theirs over to start as it takes about two months of treatment.

If you start today, you should get some color on poinsettia by Christmas. It always amazes me when I hear 15 minutes of light during the night will stop the whole process. Cover yours with a box from, say, 5 p.m. to 8 a.m. to ensure there is no exposure to light. Keep your plant in a room where it is 60 to 70 degrees.

Next, chrysanthemum, chrysanthemum spp., flower in the fall, triggered by the shortening days. They are on sale now, morphed into a Thanksgiving display plants. These probably had their buds set by manipulation of the light in greenhouses this spring to be ready a month before its holiday.


If you have a mum carried over from last year, treat it like a poinsettia with 14 hours of darkness and see if you can’t get it to set buds. If you have a blooming plant now, treat it like a houseplant this winter after you remove that foil and its spent blooms. Let it dry out in between watering.

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Then there are kalanchoe. While it is usually too much bother for most of us to force old poinsettias and mums, kalanchoe are different. They are easy to grow all year around. Expose them to nights of 13 hours of darkness, however, and they will produce red, pink, yellow, orange, salmon and white flowers.

There are a whole host of different kinds of kalanchoe. Their flowers last for a long time. Leaf edging and color can differ, too. This is a good “collection” plant. You can grow them from seed.

Lots of foliage begonias bloom during the winter months because of the longer nights. These are much smaller flowers than the tuberous and fibrous begonias we grow in the summer. Diminutive, sure, but beautiful.

Next, clivias bloom as a result of longer nights. These are big, lily flowering plants. They live forever. They also need cool and dry conditions. Put yours up against a window in an unused room here in Alaska and it will dazzle you with huge flowers.

I should note the short day plants we grow indoors for blossoms are only part of the photoperiodism story. Many of our outdoor crops are affected by the length of days — as well as the coolness of the temperatures, dad. Cauliflower sets buds because of shortening days. And, there are short day and long day broccoli varieties. Ah, but this is a whole other column and someone already did it.

Finally, just because a plant is a short day bloomer doesn’t mean you can skip lights. These plants like bright, indirect light during the shorter daylight hours. Get your lights set up.

Jeff’s Alaska Garden Calendar:

Alaska Botanical Garden: All manner of workshops from wreath-making to gourd lamps. Early bird gets the place in these limited classes.

Spider mites and thrips: Neem oil mixes work. Apply every 3 to 5 days to get all the critters.

Jeff Lowenfels

Jeff Lowenfels has written a weekly gardening column for the ADN for more than 45 years. His columns won the 2022 gold medal at the Garden Communicators International conference. He is the author of a series of books on organic gardening available at Amazon and elsewhere. He co-hosts the "Teaming With Microbes" podcast.