The Science of Succulent Dormancy
Dormancy is a word that’s thrown around every winter. Plants go to sleep or something; they hibernate like bears, right?
Sort of! In this article, we’ll dissect the subject of dormancy from the perspective of succulents (although this concept applies to pretty much all plants). All the information comes from peer-reviewed, published manuscripts. Sources included at the bottom.
Warning: Science ahead! (It’s pretty well-explained, though.)
What is Succulent Dormancy?
In one sentence, the phenomenon can be explained as:
Dormancy is a temporary period of slowed growth.
What does dormancy look like for succulents? Well, it doesn’t look like much, actually. The vast majority of succulents come from arid regions (Sempervivum being a notable exception). Typical dormancy behavior in those places is not very striking. The plant might close up a little, but not much else. There aren’t many visual indicators of dormancy in succulents.
When are succulents dormant? It depends on what kind it is. Most species in a genus will have the same dormancy period since they’re closely related and generally evolved somewhat near each other.
Succulent dormancy is almost always during winter/summer. This is for several reasons, but mostly because those seasons have dramatic climate differences that are the trigger for dormancy in some genera (the plural of genus).
Do other plants have dormancy periods as well?
Yes, plants other than succulents have dormancy periods. In fact, dormancy is most commonly observed in temperate plants (which most succulents are not).
Every winter trees drop their leaves. Grasses and other herbaceous (which means “not woody”) plants either die off completely or die down to their roots.
Aaaand then they stay that way. Until spring! That’s a process most of us are intimately familiar with – it’s like the sun rising in the east and setting in the west. It’s a given.
But that’s a prime example of plant dormancy. Those plants are sitting out winter because it’s more trouble than it’s worth to keep growing during those harsh conditions. A plant would need special adaptations to prevent the cold from freezing all the water in its cells (like pine trees have).
While temperate plants (and deciduous ones especially) have very noticeable signs of dormancy, the external factors that trigger dormancy are the exact same as they are for succulents. Many of the physiological changes in the plants are the same too!
Bonus Dormancy Fact: Seeds
Guess what? Dormancy, both the concept and the state of being, is not restricted to just mature plants.
Seeds go dormant too! In fact, seeds are dormant most of the time… any time they’re not germinating, actually.
Dormancy is like sleeping, right? Slowed growth, or in some cases, it’s just the bare minimum amount of metabolic activity (which means burning energy to stay alive) to prevent death. It’s a form of stasis – especially when seeds are doing it.
Seed dormancy is also sometimes called embryo dormancy. Some seeds have been dormant for literally thousands of years before they successfully germinated. The record-holder seed was dormant for almost 32,000 years before successfully germinating!
What Causes Succulent Dormancy?
This part is really cool in my opinion. It’s not some vague “Oh, I dunno, they go dormant in winter cause it’s cold or something…” answer. There are three factors that influence dormancy (in all plants).
1. Temperature Fluctuation
The obvious example here is when it starts to get cold and leaves begin to brown and fall off (which doesn’t really happen with succulents). However, higher temperatures can also be a signal that it’s time to go dormant.
This applies to xerophilic (arid-loving) plants specifically. They will go dormant during the extra-dry, extra-hot summer of a desert to conserve water.
For most plants, the temperature trigger is set off by either sustained temperature difference or steadily increasing/decreasing temps over some weeks.
This just means light levels. How much light does a succulent get every day? A plant can actually track the differences in the amount of daylight over time. They even have a few different methods.
One popular one is some plants produce a chemical only at night that accumulates slowly, but is rapidly consumed when the plant senses light. Depending on how long the night lasts (which is an indirect measurement of how long the day lasts) the chemical reaches a greater concentration.
Once it hits a specific threshold, other processes are triggered. That’s how they can coordinate things like every plant in a species flowering at the same time! Plants literally count how long night is to figure out what season it is.
As an interesting side note, that process described above is very sensitive. In experiments with plants that use this technique for flowering, a single 5-minute exposure to light in the middle of the night was enough to totally disrupt the process and they never flowered.
Another super interesting thing about their sensitivity (man, plants are so interesting). In low latitudes, places around the equator, the annual variation in day length is less than an hour of change either way. In those places, plants can detect deviations in daylight in quantities as little as 30 minutes from the average 12ish hours, then use that to synchronize changes like dormancy.
3. Water Availability
This should be fairly intuitive, especially for us succulent growers who are already familiar with the precise requirements of watering your succulents.
In many regions, annual rainfall is concentrated during two or three seasons. Weather tends to follow some broad patterns (or it did before climate change, anyway). Thousands of years of the same has given people and even plants a pretty good idea of what to expect as far as rain is concerned.
Even though succulents are like the camels of the plant world, there’s not much point in trying to eke out a living during the extreme conditions of high summer in a desert (or harsh winters, for that matter). So, instead, they just opt not to.
Of course, plants don’t measure water by annual rainfall. It’s usually either by ambient humidity, which affects the rates of evapotranspiration (water loss through “sweat”) or simply the amount of time since the last good drink.
(4ish… Stress-Induced Dormancy)
You know how biology is messy? It never fits in neat little categories that we want it to.
There’s a fascinating study on orchids that illustrates this pretty well. The researchers defoliated (AKA pulled off all their leaves – rude!) and then shaded some orchids.
Those plants went into dormancy earlier and stayed in it for longer than their undisturbed fellows. However – they didn’t have a significantly increased mortality rate compared to the orchids that were left alone.
The results indicate that dormancy can be used as a response to immediate hardship or disaster to wait until better conditions come around. Apparently, it’s a pretty good survival trick!
Can Dormancy be Artificially Induced or Avoided?
Dormancy is simply a reaction to a combination of the above 3 primary factors. Turn down the heat and the light to convince a plant that it’s winter. A species that is winter dormant will slow down and, conversely, winter growers (which are usually dormant during the summer) will take off.
It’s not like they use a calendar, y’know?
In fact, the simple act of keeping your succulents indoors is mitigating, or possibly negating entirely, the effects of dormancy. The temperature indoors doesn’t drop, not significantly anyway. Changes in watering schedules are optional.
The only thing that really changes is the light level – and that’s assuming the plant is on a windowsill and not under a grow light.
What are the effects of Succulent Dormancy?
So what are these plants actually doing when they’re “dormant”? Slacking off, probably.
There are a lot of things that are going on, some are more pronounced than others. It also varies from genus to genus, and species to species. Here are a couple of examples of things most plants go through when they’re dormant.
- The meristems (the spots where new growth originates from, like at the tip of a branch) are blocked from receiving growth-promoting signals from the plant. That means no new growth, or at least very little.
- A reduced rate of photosynthesis can be measured in fully dormant plants. Most metabolic processes slow down, including respiration. Water may be concentrated in the “succulent” part of the plants (usually the leaves or stem).
- SOME plants, even a few succulents, change the composition of their cytoplasm (cell-juice) to make it more resistant to freezing. Succulents are full of water, of course, which is why cold tends to be so damaging (the cells freeze and burst).
In addition to the physiological effects of dormancy, other behaviors are linked to the onset of dormancy.
A succulent-specific example: many kinds of barrel cactus will only flower after being exposed to colder temperatures and reduced light for a day or two. The beginning of dormancy means it’s time to reproduce, too. Growing those flowers will be one of their last acts for the season.
One thing that’s not linked to dormancy is plant coloration. Succulents are highly prized for their bright, vivacious colors. Those colors are the result of sun-stress. Even in the reduced light of winter, there should be enough direct light for succulents to maintain those colors. If your plants are fading to plain green, get them some more light!
Is Dormancy Necessary for Healthy Plant Growth?
An interesting question, and one of the few where the answer might be different between succulents and other plants.
For some plants, yes, dormancy is an important part of their life cycle. Like in the example above, some plants won’t undergo sexual reproduction (flowering) without it. Some bulb plants, like lilies, will not grow back the next year if the bulb isn’t exposed to cold temperatures for a couple months.
To go further, some seeds won’t germinate until they have been dormant for a period of time in chilly weather. Many woody plants require cold temperatures to harden their roots, which promotes healthy, robust, and stable root systems.
Some people claim that walnut trees will senesce (die) if they go too long without dormancy.
Yet, most of these examples come from plants in temperate regions that experience at least a mild winter. What about in tropical or arid regions, where seasons are different or annual temperature is mostly stable?
There is a dearth in published research on this subject, unfortunately. We can look to the methods of commercial growers for an answer, however.
Most succulents are grown in commercial greenhouses in a perennially warm, sunny place (hello, California) or in a heated, lighted greenhouse elsewhere. Most of our home-grown propagations are in similar conditions. These indoor plants may have never been properly dormant their whole lives.
And they seem to be doing okay.
I posit that most plants, especially succulents or tropicals, will be satisfied to continue a growth cycle indefinitely without dormancy.
With the caveat, however, that certain behaviors of plants (like sexual reproduction) might never occur or only very infrequently. Good thing we can propagate succulents asexually, eh?
Under optimal growth conditions, without allowing for a dormant phase, you can expect steady vegetative growth, if nothing else.
Is Dormancy Incremental / On a Spectrum?
Another interesting question. The answer is: sorta, probably.
Few things in nature are binary. Black and white doesn’t happen all that often, most biological processes are on a scale of gray.
A succulent that is dormant might grow a new leaf or some of those aerial roots. That doesn’t mean that plant isn’t dormant. Dormancy is a period of slowed growth, not the complete cessation of the creation of new cells. An organism would die pretty quickly without any kind of cellular maintenance.
It’s not like any animal or plant really totally shuts down during a hibernation/dormancy/brumation/aestivation. There’s still some metabolic processes occurring, which leaves open the possibility for a gradient of activity.
So: Can you make your succulent just a little bit dormant? Yeah.
Is there a reason to? Not unless it’s necessary for flowering or something.
How to Care for a Dormant Succulent
This is probably the reason you came here, and since you sat through all the rest of that science-nonsense, we’ll make it quick.
Dormant plants are not actively growing. They need less of everything, but they still need a little.
Light For Dormant Succulents
This is the thing that should change the least.
Succulents still want plenty of light, even in their dormant state. If the average amount of light a succulent should receive is about 12 hours a day (and it is), it can be reduced to 10 or even 8 hours.
You could go lower than that, but then you risk the loss of color (which isn’t harmful, just sad) or etiolation (also not harmful, but even more sad because it’s irreversible).
Light in the winter is generally a little less intense, less powerful. Succulents can handle it either way, so don’t go out of your way to protect them from bright, direct light.
Fertilizer for Dormant Succulents
Water for Dormant Succulents
This is the big one. When a succulent is dormant, it needs significantly less water. Overwatering is a common problem for new succulent owners, and dormancy exacerbates the issue.
It’s difficult or impossible to give broad, sweeping watering recommendations. It’s highly dependent on your setup – the plant, the soil, the pot, and the climate. Some plants, like Kalanchoe are very thirsty. Lithops, on the other hand, only need to be watered a few times a year (and winter is not one of those times).
The rule of thumb I use is to water succulents half as often as you normally do during their dormancy season. As with any change in care, closely monitor their health for changes.
That brings up the question, though: “When is my X succulent dormant?”
List of Succulent Genera and their Dormancy Seasons
Use this as a reference, but not an absolute guide. Individual species may have different dormant seasons than the rest of their genus (although genus-level dormancy is a good place to start).
|Summer Dormant||Winter Dormant|
Sources Used :
- Rolf Borchert, Guillermo Rivera; Photoperiodic control of seasonal development and dormancy in tropical stem-succulent trees, Tree Physiology, Volume 21, Issue 4, 1 March 2001, Pages 213–221, https://doi.org/10.1093/treephys/21.4.213
- Shefferson, R. P., Kull, T. and Tali, K. (2005), ADULT WHOLE‐PLANT DORMANCY INDUCED BY STRESS IN LONG‐LIVED ORCHIDS. Ecology, 86: 3099-3104. doi:10.1890/05-0586
- Viémont J.-D, and Crabbé J. Dormancy in Plants: from Whole Plant Behaviour to Cellular Control. CABI Pub., 2007.
- Antje Rohde, Rishikesh P. Bhalerao, Plant dormancy in the perennial context, Trends in Plant Science, Volume 12, Issue 5, 2007,Pages 217-223,ISSN 1360-1385, https://doi.org/10.1016/j.tplants.2007.03.012.