Tag Archive: day length

Finding varieties of spinach and beets that won’t bolt. Watching the leaves turn color, even when there hasn’t yet been a frost. Trying to get last year’s Christmas cactus or poinsettia to bloom. Solving the mystery of who spilled the perfume in the meteorology lab. What do they have in common? The answer is a phenomenon called photoperiodism, which helps plants keep track of when it’s time to bloom.

Kalanchoe and night-blooming jasmine

Night Blooming Jasmine, right, and Kalanchoe, left.

Many plants seem to “know” how long the day is. Depending on the variety, they may refuse to bloom unless the length of day is to their liking. For varieties bred at lower latitudes, this may pose problems for Alaskans, since plants such as spinach and beets often take our long summer days as a signal that they must bloom at once rather than grow the leaves and roots we want. Similar problems arise in late summer with imported perennials, shrubs, and trees, which refuse to prepare for winter dormancy while the days are still as long as they are here in early September. But how do eyeless, clockless, plants know how long the day is?

The answer is found in a chemical, called phytochrome, produced by plants. Phytochrome changes form when red light shines on it, and slowly changes back to its original form when it is in the dark. The plant “counts” the hours of darkness by how much of the phytochrome has changed back to its dark form before it is changed again by light. The chemical’s reaction to light is very fast, so that even a short burst of light may turn all the plant’s phytochrome back to the light form, and the plant will start all over again timing how long the night is. So a plant that blooms when days are long, like spinach, is really responding to short nights, while winter-blooming plants such as poinsettias, kalanchoes, and Christmas cactus are responding to long nights. A single minute of light in the middle of the dark period is enough to “reset the clock” in some of these plants and make them think they have had two short nights (long days) instead of one long one.

Some long-night plants will bloom just as well if they are given cold nights. Many Christmas cactus are in this group. Others, like kalanchoes, will only be satisfied with long, uninterrupted, nights, which may be difficult to supply in our lighted homes. A trick that works with soybeans (and might work with winter-blooming house plants) is to place a black envelope over a single leaf near the growing point for 16 hours a day. The covered leaf makes enough of the night form of phytochrome to convince the whole plant that winter has come. If you try this with a poinsettia or kalanchoe, let me know if it works.

Some plants are even trickier in their requirements, and one of these led to the “spilled perfume” mystery. I had a night-blooming jasmine in the meteorology lab that blooms on long nights when they follow short nights. It normally has a major burst of bloom in October or November, but if someone comes in during a winter night and turns on the lights just once, that single pair of “short nights” convinces it that another summer has gone by and it blooms again about 6 weeks afterwards. The flowers are inconspicuous but have a powerful jasmine odor when they open at night. When it bloomed last year, nobody thought to mention to me that they were puzzled by the odor. It took several days before anyone could figure out where the perfume was coming from!

Author’s note: this post is recycled from one that was on the Alaska Science Forum when I was writing it in the late 80’s. I’m having a busy weekend — concert, horse show, public reading and writers’ group, and I just didn’t have time to write a new science article..

I went to an Alaska Writers Guild meeting Tuesday night, and mentioned Friday’s post on the effect of orbital tilt. This led to a discussion of day length, and I realized that while I knew some planets had really weird day lengths, I wasn’t sure which ones. (I thought it was the inner planets, which turned out to be right.) So as long as I was looking the information up, I thought I’d share it.


Mercury (Wikimedia)
Mercury turns out to be the planet whose days are longer than its years. For many years the planet was thought to keep the same side facing the sun all the time: one rotation about its axis relative to the stars for each revolution around the sun. We now know it rotates three times for each two revolutions around the sun, making its days a year and a half long. Luckily it’s a short year (88 Earth days.) Its tilt, by the way, is so near zero it is hard to measure. (Its closeness to the sun doesn’t help.)


Venus, Hubble photo
Venus is the really weird one. Its rotation is in the opposite direction from its revolution around the sun, so from the surface of Venus, the sun would appear to rise in the west! At perihelion the sun may actually appear to stand still or go backward in the sky. That is, it would if you could see the sun through the sulfuric acid clouds. A Venusian day is long, however: 116.75 Earth days. A Venusian year is 1.92 Venusian days or 224.65 Earth days long. The tilt of its axis is only about 3.4°.


Mars (Hubble)Mars is easily the least different from Earth when it comes to day length: 24 hours 39 minutes and slightly more than 35 seconds. This is more precise than is generally stated for the other planets, quite simply because Mars is the planet with human-piloted rovers on its surface, and to have daylight, these pilots must work on Martian days (or sols) even though they are located on Earth. (Pilot may not be quite the right word, given that radio communications take 4 to 20 minutes to get to Mars.) Its axial tilt is also similar to Earth’s: 25.2°. A Mars year is 1.8809 Earth years.


Jupiter (Hubble)Jupiter has the fastest rotation rate, and thus the shortest day, of any of the planets: slightly less than 10 hours. Why the vagueness? All we can see of Jupiter is the cloud tops, and those rotate at slightly different speeds at different latitudes. It is clear, however that Jupiter’s days are very short, especially compared with its year length of 11.86 Earth years. Its axial tilt is small, only 3.13°.


Saturn (Hubble)Saturn, like Jupiter, rotates fast and the rotation seems to vary with latitude but is slightly more than 10 hours. The year, however, is over twice the length of Jupiter’s – 29.46 years. The axial tilt is relatively large: 26.73°, which is why the visibility of Saturn’s rings from Earth varies so much. Seasonality is probably weakened by internal heating and the large distance from the sun.


Uranus (Hubble)Uranus rotates slower than the gas giants but still faster than earth, with a day length of 17 hours, 14 minutes. Its year is 84 Earth years long. It is a few years past an equinox (2007) and won’t reach another solstice until 2028. There is some question as to which is the north pole, since its axis is either tilted at 97.77° with normal rotation or 82.14° with retrograde rotation.


Neptune (Hubble)Neptune has a day length of roughly 16.11 hours. Very roughly – Neptune has even more variation in rotation rate of the cloud tops with latitude than does Jupiter, with apparent rotation periods varying from 12 hours at the poles to 18 hours at the equator. Its tilt is a little larger than earth’s, about 28.32°, which should give it pronounced seasons, though not as pronounced as those of Uranus! It year is roughly 164.8 years.

All of this variation is just in our own solar system. What else may be out there?