Reading the title of this article, you’re probably already making a list in your head: too much water, not enough water, illnesses, burns, undesirables…
Let me stop you right there: we’re talking about the completely NORMAL and NATURAL reasons why plants shed their leaves.
Starting your list again? Once “autumn” has passed, you’re not sure why the leaves are falling? Phew! Good, I was afraid I wouldn’t be able to teach you anything today!
1. My Favorite Time of Year
Well, yes, let’s start with the obvious! In autumn, as the days shorten, the reduction in light triggers the production of abscisic acid, a hormone that causes leaves to fall. Trees that shed their leaves in winter are known as deciduous. But why is this necessary?
Without going into too much detail, plants breathe through their leaves by photosynthesis. They then lose water through stomata, tiny holes in the skin. Without stomata, water would not circulate. Try plugging the end of a straw with your finger and plunging it into the water, and the water won’t rise: you absolutely need an “outlet”.
Humidity plays a very important role, since it’s what allows water to rise up the plant towards the leaves: and a bit like a dry sponge that absorbs more water, dry air facilitates evaporation. If ambient humidity is 100%, water stagnates and doesn’t evaporate (because at 100%, you’re right in the water!). This slows photosynthesis, but also water absorption by the roots. This explains why, on humid days, even if there’s no rain, we need to water our plants less. It’s also why we can put them in a plastic bag when we’re away for long periods.
In winter, when the ground is frozen, the plant can no longer drink, and would soon become dehydrated. To protect itself, it sheds its leaves when the light diminishes, simply preventing water from escaping during the winter.
Deciduous and Coniferous Trees
This is one of the major differences between conifers and broadleaves: conifer leaves stay on the tree through the winter. Why don’t they dry? Because these leaves, more commonly known as needles, are covered with a protective wax, have a much smaller surface area, and stomata are located beneath the leaves, protecting them from the sun and wind. In short, there’s much less evaporation, and the needles are stronger than the leaves, protecting them from frost and the weight of snow.
2. But What About Deciduous Conifers?
Good thinking! Some conifers also lose their leaves in winter. These include larch, but also bald cypress and water fir, among others. Ginkgo biloba is a close relative of the conifers and, although in a family of its own, is also deciduous.
I love nature and its mysteries, and here’s one that has no official answer! There are, of course, a number of hypotheses about this phenomenon, but no certainty as to why some conifers don’t keep their needles in winter. Here, then, are the two commonly accepted, though not mutually exclusive, explanations for the deciduous conifer phenomenon.
These conifers are identified as hardwoods.
Welcome to 2024, where even trees have variable identities! Okay, I’m joking, but at the same time… not so much! Evolution, with a capital “E”, is no stranger to mishaps. Already, proto-donates (3-meter dragonflies) and long necks (dear Little Foot the dinosaur!) have disappeared to make way for frankly useless species like mosquitoes and aloes. No danger of us getting unicorns, eh!
What’s more, dear Evolution has a tendency towards “good enough”, i.e., “it’s good enough to do its job, even if it’s far from perfect”. Seals can’t breathe underwater? Let’s give them nose holes that close. Butterflies only live a few weeks? What the hell? The important thing is that they lay their eggs before they die! Do humans have back pain? Well… that doesn’t stop them from living either, just put some ice on it!
Am I exaggerating? Maybe, but just a little!
Let’s get back to our deciduous conifer hypothesis, shall we? Conifers existed long before deciduous trees. The fact of losing their leaves therefore appeared in the course of evolution. Some conifers would therefore have “continued their evolution” in the footsteps of deciduous trees and started to lose their needles. Just like that. The gene was in the DNA, it was activated and BOOM! A larch that identifies itself as a maple!
The Evolutionary Advantage
It’s also possible that these trees began to lose their needles in response to environmental stress. Larches like moist soil: was evaporation harmful to it in winter, as it is to hardwoods? Does the carpet of dead needles act as a mulch? Its needles are a little more delicate than those of other conifers: are they more sensitive to frost and the weight of snow?
It’s hard to say with evolution: did a species develop something spontaneously that turned out to be advantageous? Or was a real problem solved by something new? Was it that needles used to be more delicate, then the larch started losing them in winter, or was it that losing its leaves enabled it to make thinner, less energy-intensive ones? A mystery! In short, why do some conifers lose their leaves in winter? We don’t know! Maybe it’s just because a few golden larches look pretty against the other conifers in autumn!
3. Winter Indoors
Even in our homes, winter is hard on our precious plants. Less light, which means less photosynthesis, which means less evaporation… but also very dry air, which tends to dry out the leaves. Except that, without light, they can’t drink as much as they need: your plants aren’t pipes that magically pump water up to the leaves either!
So you need to find the balance between watering your plants enough to keep them drinking, but not too much, otherwise the soil will remain damp and the roots will rot, unable to absorb as much without solar energy. Your plants experience a kind of mild winter, a period of slow growth, even dormancy. They protect themselves from desiccation by losing a few superfluous leaves. So, we’re back to point 1 of this article, but to a lesser extent.
Note that some plants, like the desert rose, can lose all their leaves, even indoors. This is normal, as this plant needs lots and lots of light. If this happens, remember that the reserves in its trunk are sufficient for its winter survival. Watering it will only cause it to rot, as it has no way of evacuating water!
4. Old Leaves
No plant keeps its leaves all its life (except perhaps one species which will be the subject of a future article. Let me know in the comments if you think you know it!)
Think of that tradescantia you keep cutting and repotting every year because the lower stems are receding. Or that dracaena with its endless trunk and green tuft at the end.
On average, I’ve read that leaves live 6 years. But if we consider deciduous trees, and say that the needles of certain conifers can live for 45 years, that’s an estimate that seems to come out of nowhere. So let’s say that some leaves simply live for several years.
Once a certain age is reached, however, the plant sheds them. Why does this happen? Evaporation, but also environmental constraints.
Imagine a palm tree that kept all its leaves: it would be very sensitive to wind and could be uprooted by the slightest storm; older leaves could be damaged, even diseased and useless; humidity levels would be very high in the trunk and could lead to mildew; the internal vascular system would have to be much more complex to connect to each leaf and this could affect its efficiency, even weakening the trunk.
And these are just the problems I could imagine in a few seconds. Chances are, if a plant kept all its leaves and lived long enough to be studied, we’d find other problems!
5. Leaves in the Wrong Place
Although some plants have leaves or even flowers directly on their trunks, such as certain lianas or the cocoa tree, this is not very common.
A Crassula or an apple tree in your living room or backyard will eventually lose the leaves in the center of the tree. I’ve just told you about the life expectancy of a leaf. Isn’t it the same thing? Yes and no.
A leaf on the trunk was once at the end of the branch when the plant was younger, but now it’s in the wrong place. Is it doing as much photosynthesis, hidden by its fellows? Does the tree have to have a special channel right in the middle of its internal network just for “Madame lonely leaf”? I could rename all the reasons above to justify this point. It’s not necessarily old, it’s just… useless.
You know those trees that grew at an angle, hoping to reach more light and make a place for themselves? Well, it’s the same thing. A plant will make more leaves and branches on the more nutritious (sunnier) side and will end up leaning that way, leaving the other “misplaced” leaves and branches to die, or at least not grow.
It’s normal: if you’re hungry, you go to the market, not the hardware store. The tree does the same: it takes the most logical direction and leaves the less nutritious options behind.
Conclusion
The important thing to remember from this article is that, yes, plant leaves can fall off for no apparent reason. Just like your hair or dead skin! If your plant looks healthy, but the few leaves at the bottom are drying out, it may simply be because it’s living its life as a plant. In nature, leaves on the ground have an equally important role to play in the balance of ecosystems, so leaf loss is perfectly normal and necessary.
After all, if all the leaves always stayed on the plants, then the forests, especially where there is no winter, would be so dense that it would be worse than walking through a bowl of salad!
Good idea for an article, thanks for doing it. BtW, why did you delete the 5/18 article on shade plants? I had it open in my browser before heading to the nursery tomorrow. Now it’s gone. I was planning to ask about some of those plants!