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Much as I love summer, with its sun filled hundred and ten degree-days, it’s a nice break when temperatures drop into the 90s. It gets darker earlier, but sunsets are glorious and everything is perfect for life in the garden. I’m even planning on breaking out my barbeque grill.

For a quite a few plants, the number of daylight hours influences whether or not they’ll flower. Not all plants, obviously. Roses’ll bloom as long as they receive enough light and of course water.

But those who bloom only at a particular time of year are usually responding to the number of daylight hours. Technically, a short day plant needs long periods of darkness in order to do something, like produce flowers or bulbs. Don’t be fooled by this long/short daylength idea. I’ve seen in catalogs that it’s more or less than 12 hours per day. They should know better. It isn’t the same at all.  The plant may require 13 hours of darkness, or it may need 10 hours of dark. What matters is – the plant needs a certain amount of darkness, which makes it a short day plant. The clock isn’t’ so important; the plant makes the determination.

So why would a plant flower only if it receives enough darkness?

Support comes from

  • In nature, why do plants flower? To produce seeds. They’re not interested in whether or not we think they’re pretty.
  • If the plant flowers when days are short, like spring or fall, they’re preparing the next generation of seeds and protecting them from the worst weather to come.
  • In spring, the seeds’ll germinate and be ready for summer.
  • Fall-flowering plants have seeds that may germinate in the fall or just be ready to pop into action come spring. Almost no one germinates in the dead of summer or winter.

Some very pretty ornamentals are short day, cosmos, dahlia and viola, for instance.  Holiday times are full of short day plants. Christmas cactus comes to mind. To be more accurate, pretty much all the tropical flowering cacti flower when nights are long enough. They also have a temperature requirement, but our homes usually meet that need. Kalanchoe, a good hot weather tropical, also produces its flowers under short days.

We’re all familiar with poinsettia. Come the holidays, the whole world is full of those red plants. Even though they’re way overdone, I’m still delighted by them. So bright and cheery! When the season’s finished, we’re left with a plain green plant. But who wants to throw away a perfectly healthy houseplant?

Many people have tried unsuccessfully to bring their poinsettias back into bloom for the holidays. There’ve been a lot of attempts, but one thing that’s guaranteed NOT to work is to put the plant in a paper bag in a closet for a couple of months. I’ve known folks who’ve tried this.  What they got was – a dead poinsettia.

The plant still needs SOME light, in addition to water and fresh air. If the intrepid gardener must get it flowering again, start in October. Put it in a place where it’ll get 14 hours of uninterrupted darkness. That uninterrupted part is important. If the plant senses light, even streetlights, it won’t work.

Chrysanthemums, all kinds of mums, are technically short day plants. You may be saying to yourself – “Wait a minute, I see mums in bloom all year long” and that’s true. Mothers’ day, Independence day, Thanksgiving, Easter – you’re able to buy mums for almost any event.

The wonders of modern lighting! Very few mums are grown out in fields where they’re responding to sunlight. Growers provide artificial light, and provide it at the appropriate length to promote flowering. The plant doesn’t know any better.

Temperature changes also have an effect, but when days are shorter, the weather tends to be cooler, too.

All of this is part of the reason we can have successful winter gardens here in the Mojave. What do we grow in cool temperatures? Lettuce, greens, spinach – they’re all long day plants. Win-win for us!

For KNPR’s Desert Bloom, this is Dr. Angela O’Callaghan of the University of Nevada Cooperative Extension. Happy autumn!

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