Planting Edmonds is a monthly column written by and for local gardeners.

Fall is here! We can tell. The temperatures have dropped, the rains have begun, days are getting shorter, there is the smell of pumpkin spice lattes wafting in the breeze and, at least around here, there is a strangely high prominence of the number 12.

Besides our senses telling us it is fall, so do our calendars. We respond by modifying our behavior — we stock up on firewood and coffee, order warmer clothes and bedding, check our home heating unit and clean the gutters — to prepare for the long winter ahead.

Our garden plants know it is fall as well. They don’t use the smell of pumpkin spice or the number 12 to figure this out, but they have figured it out and are also preparing for winter. How do they do that?

Garden plants need to know what season it is. They don’t go inside a house, wrap themselves in warm blankets, sip coffee by the fire and read books to get through the long Pacific Northwest winter. They stay put outdoors and adapt to the changing weather (temperature, moisture, day length, etc.).

Right now, our garden plants need to start doing many things to prepare for winter — slow their growth, shed cold-sensitive tissue (leaves) and build protective bud scales, for example. In the spring, seeds need to germinate, buds expand and leaves and flowers emerge.

These activities need to be synchronized with the environment. A plant that goes dormant too soon misses out on some great late summer growing weather. A new leaf or flower emerging too early in the spring (“I’m baaaaack!!) will freeze soon after. Plants that are properly adapted to our local environment know how to avoid this fate.

Rhododendrons develop flower buds in mid to late summer and protect them with bud scales for the winter to enable spring flowers.

Not only can plants tell what time of year it is, but they can also tell what time of day it is. They use this knowledge to optimize their growth and development on a daily basis.

There are two highly consistent environmental phenomena useful for measuring time: the daily rhythm of 24 hours and the annual rhythm of 12 months. Plants use these to accurately determine what season it is and what time of day it is. What is the mechanism, you ask?

The measurement of day length by plants is called “photoperiodism.” Plants perceive light by tiny “eyes” all over their structures called photoreceptors. They use a specific photoreceptor called phytochrome that perceives the red-light part of the visual spectrum.

When a phytochrome molecule receives red light, it changes into an active form — like a light-activated switch. This active form then triggers changes in the plants, like flower initiation, bud development and seed germination. This activated phytochrome molecule slowly reverts to the inactive form in the dark.

The phytochrome system measuring day length in plants.

Over the course of a day, there will be some phytochrome molecules activated and some not.
To measure day length, and thus know the season, plants have evolved to measure the ratio of phytochrome molecules in the active form to those in the inactive form. They use this ratio to accurately assess the season.

In summer, there is a lot of light, so a lot of active phytochrome is formed. In the winter, less light equals less activated phytochrome, so the ratio of active to inactive is low.

A way to visualize this process is an hourglass. (The one that instantly pops into my mind is from the “The Wizard of Oz” when the green-faced Wicked Witch of the West marks Dorothy’s certain demise with a comically huge hourglass!)

The grains of sand in the top represent active phytochrome and the grains in the bottom represent inactive phytochrome. Light – specifically red light – keeps the sand from falling, so the ratio of sand in the top to the bottom is high. If there is not much light, more of the sand falls to the bottom reservoir by gravity. In this case, the ratio of sand in the top (active) form to the bottom (inactive) form is low.

With this eloquent photoreceptor switch system, our garden plants accurately determine the time of year. The Autumnal Equinox came on Sept. 22, and they knew it because they knew the ratio of activated to inactivated phytochrome. Pretty cool, huh?

This system, photoperiodism, is best described with flower timing. Plants use photoperiodism to decide when to flower. They continually measure day length and plan accordingly. Some plants – iris, for example – flower after “seeing” a long day. Some, such as chrysanthemums, wait for a short day to flower.

Plant nurseries exploit this system by artificially manipulating how much light their plants receive to control flower timing. Plant shoppers want to see the actual flower on the plant before they purchase it. Commercial nurseries can use retractable blackout cloths over their greenhouses to give plants a shorter day, trigger flowering and ship to your local nursery in full bloom. Artificial lights can, of course, extend the day length.

Photoperiodism isn’t the only way plants know what season it is – I just think it’s the coolest! They also sense other environmental cues such as temperature, moisture, soil chemistry and others. Of these, temperature is the most important. They sense temperature by the changes in their membrane structure.

Membranes are made of fats, and we know that fats change structure with temperature (solid versus melted butter, for example). As with photoperiodism, plants “read” the changes in membrane structure and make appropriate changes to adapt to the changing seasons.

Besides knowing what day of the year, it is, plants also know what time of day it is. They use this to change their daily activities. For example, knowing when to open the little pores (stomata) in their leaves that allow for carbon dioxide to enter (reduce global warming) and oxygen to escape, enabling us to breathe, when to open flower petals and how to adjust their leaf position for optimal photosynthesis.

The internal 24-hour cycle in plants and animals is called the circadian rhythm. For this rhythm, blue light is perceived with photoreceptors called cryptochrome. This perception of blue light keeps the daily clock in synchrony with the environment. Plants need to keep their daily activities synchronized with the daytime to function efficiently.

We humans have circadian rhythms and cryptochrome too. The day/night cycle, synchronized with blue light, affects our body temperature, appetite and sleepiness, to name a few. The blue light from televisions, computer screens, smartphones and gaming consoles used at night misleads our bodies about the time of day and can make it harder to sleep. Jet lag is another good human example of what happens when our circadian rhythms are out of sync with our environment.

Well, MY circadian clock tells me it’s “time” to wrap this column up! Plants have developed a beautiful system to accurately measure what time of year it is (photoperiodism) and use it to make changes synchronized to the current season. This allows them to survive and thrive in our gardens. They also can tell what time of day it is (circadian rhythm) and appropriately adjust and optimize their daily activities.

This winter, as I am putting another log on the fire, sipping a hot beverage under a warm blanket and reading a good book, I will also be comforted knowing that my garden plants have done all the right things to wait out our long, dark, wet winter. It’s about time.

— By Joel Ream

Joel Ream grew up in Spokane and earned a Bachelor of Science in botany at the University of Washington and a Master’s in botany at Michigan State University. Joel spent 37 years as a plant biologist at Monsanto, using plant physiology, biochemistry, and analytics to increase the efficiency of crop production. He also worked on new weed control technologies, regulatory studies to support the safety of new products, greenhouse and field evaluation of new crop varieties, increasing the nutritional value of animal feed, and developing methods to measure grain composition. Joel retired to Edmonds in 2018.