Does your world stop when your plant drops a leaf or two?
It is important to keep in mind that for some plants, like the infamous Fiddle Leaf Fig, that leaf drop is a cause for concern, but for other plants, like a Euphorbia, it is nothing to be concerned about. Knowing whether or not leaf shedding is a healthy part of your plant’s lifecyle is important. Some plants seasonally shed leaves, some shed leaves all the time, and some never shed! Many of the tropical houseplants will shed a leaf or two every once in awhile. And some houseplants, like Euphorbs, will shed leaves seasonally, as will temperate plants.
There are also certain situations where plants will shed leaves under stress due to environmental conditions. When a plant is stressed, the leaves will senesce (from the Latin, senex, to age), or fall off to help the plant achieve homeostatic balance.
Let’s explore leaf drop due to insufficient light first:
We may think of leaves as units of production, like plant sugar factories, but just like any other factory, the workers need to be fed! Leaf cells consume about half to two-thirds of the sugars that they make. Photosynthesis alone costs 18 ATP (ATP is the energy currency of the cell) plus two NADPH to generate 36 ATP’s-worth of energy.
ATP = Adenosine triphosphate
NADPH = Nicotinamide adenine dinucleotide phosphate
Although this is a gain, consider that the cell uses most of the gained ATP very quickly, just to sustain itself! The problem with leaves is that although they capture energy to make sugars for the rest of the plant, they are also expensive to maintain.
So, the plant runs into problems if the light dissipates, or if the plant receives less light than it is used to. It has all these hungry leaf cells, but there is not enough light to support them. The plant makes an executive decision to drop the leaf.
Although this might happen quickly in your eyes – “I woke up and my fiddle had dropped 2 leaves!” – it’s a slow process. The plant first needs to suck out all the minerals and nutrients from that leaf and redistribute them throughout the rest of the plant. The leaf will turn yellow, as chlorophyll degrades and photosynthesis shuts down, and may crisp from the edges inwards, those cells being the first to go.
It should be noted that different plants will drop leaves in different ways in response to light. Generally, they will drop random leaves to thin themselves out if not receiving enough light to achieve homeostasis- the idea being that by thinning out the bushiness, more light will hit all the leaves that are left. The plant will stop dropping leaves when the plant is able to sustain the amount of leaves that are left. A good way to keep a plant bushy and full if it’s dropping leaves is to pull off healthy leaves from the bottom of the plant in order to take away the leaves that the plant cannot support so that it does not randomly drop leaves. This is applicable to all plants that you want to keep bushy if you cannot increase the light!
Another reason for leaf drop can be overwatering:
Overwatering is a common reason for a plant to drop a leaf but not by choice. When soil is too wet for too long, water rushes into the cells, which causes them to swell and sometimes burst. This damage continues up the vascular system, bursting cells, as the excess water has nowhere to go, and finds its way up to the leaves!
This explains why the bottom leaves are generally the first to yellow when a plant is overwatered – the lower leaves are the first to be hit by the deluge of water. The cells flood, changing the pH, and diluting the cells, causing them to yellow and turn slightly transparent, while being bloated and mushy. The cells die from the petiole (leaf stalk) outwards, and in the early stages, the outer parts of the leaves might still be green. If left to continue, the stem will likely become mushy and lose its structural integrity, causing the plant to mush over and collapse. Sometimes, the stem will blacken at the base, and mold fungi (different from mushroom fungi) may be seen at the base as well.
Some plants like cacti or succulents have adapted to dry environments, and are adapted to actually sponge as much water as possible – making overwatering them quite easy and dangerous to them. Others have adaptations for wet environments, like ferns which have primitive vascular systems, or mosses, which have no vascular system at all, making overwatering quite difficult, as all of these plants have adapted to allowing as much water as possible to flow through them.
Which brings us to another reason for leaf drop – dryness:
For the opposite extreme – dryness – plants will behave differently based on how many succulent adaptations they have, as well as their general structure.
For all non-woody/non-fibrous plants, turgor pressure (water pressure) is what holds a plant up! Plants do not have skeletons, and instead use well-managed turgor pressure to keep upright. They’re basically a water balloon. Too much water and they burst. Too little, and they shrivel.
When there is not enough water, the cells shrivel, and the plasma membrane pulls back from the cell walls, causing weakness, which on a macro-level causes drooping.
Water escapes the plant through the stomata, or plant pores, through a process called transpiration. Transpiration is the process where sunlight and heat evaporate water from the plant through the stomata, and pull water through the plant, like a big straw. When a plant has been getting a lot of sunlight, transpiration will pull water from the soil until is depleted, and when there is no more water, the plant will dry out and wilt. ALL leaves will droop or curl upwards and inwards, and that droopiness/curling will progress into the leaves crisping at the edges, with the crispiness working its way inwards.
Note that salt stress mimics this, as an imbalance of salts will cause the same symptoms, but generally with more burning. In any case, with dryness, depending on the plant, the leaves may turn yellow too, but a paler yellow than they would turn for overwatering. This is caused by the slow denaturing and degradation of the pigments, with the cell structures and fibers intact.
In more succulent plants, wilting will actually manifest itself as wrinkling – the thickened leaves are so waxy and fortified that they cannot wilt, but they can crenate or wrinkle. Some semi-succulent plants like Dracaenas, will both crisp at the leaf’s edges and shrivel at the leaf’s base. Others like cacti will just shrivel at the base. For succulent plants like the cacti, it is wise if one is inexperienced with watering to wait until they shrivel, and then soak them for only one day then leave them out in the sun to dry, to avoid overwatering.
So next time your houseplant drops a leaf or two outside of its usual shedding, take it as an opportunity to check in with your plant: is it receiving less light than usual? Have you been too heavy on the watering? Is it super dry in your apartment?
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