From my perspective, this is the most common reason my leaves turn yellow and detach. My roots are not like those of a thirsty fern; they are adapted to store water within my thick, fleshy leaves and stems. When you provide too much water too frequently, the soil around my roots becomes saturated for prolonged periods. This suffocates my root system, preventing it from absorbing the oxygen it desperately needs. The cells in my roots begin to die and rot. Once this root rot sets in, my entire vascular system is compromised. I can no longer transport water and nutrients to my leaves effectively. The oldest leaves, being the most expendable, are the first to be sacrificed. They turn a soft, mushy yellow and fall off with the slightest touch as I try to conserve energy and resources for my core survival.
While less frequent than overwatering, a severe and prolonged lack of water can also cause my leaves to yellow and drop, though the presentation is different. My survival strategy is to hoard water. When my soil becomes bone-dry for an extended period, I must tap into the reserves stored in my leaves to stay alive. I begin to reabsorb the moisture from my oldest leaves first. As the water and nutrients are pulled back into my stem and roots, the leaves lose their turgor pressure and chlorophyll, causing them to become thin, wrinkled, flat, and a crispy, papery yellow or brown before they eventually fall off. This is a last-ditch effort to survive the drought you have subjected me to.
As a succulent, I am a sun-loving organism. I thrive in bright, direct light. When you place me in a spot with insufficient light, my very physiology is thrown into disarray. I require intense sunlight to power photosynthesis, the process that creates my food (energy). In low light, my food production plummets. I cannot support all of my foliage, so I must make a choice. I will begin to shed leaves that are no longer energetically productive to conserve my limited resources for new growth. These leaves often turn a pale, generalized yellow before dropping. Furthermore, without adequate light, my internodes (the spaces between leaves) will stretch out abnormally as I desperately reach for a light source, a condition you call etiolation, which further weakens my structure.
My needs are specific when it comes to my soil environment. I require a gritty, extremely well-draining mix. If you have planted me in standard, moisture-retentive potting soil, you have essentially placed my roots in a wet sponge. Even with careful watering, this dense soil will not dry out quickly enough for my liking, creating a constant state of mild waterlogging around my roots. This chronic stress inhibits root function and mimics the effects of overwatering, leading to a slow, steady yellowing and leaf drop. My pot is also part of my foundation; without a drainage hole, any excess water has nowhere to go, pooling at the bottom and guaranteeing root rot.
Sudden changes in my environment can also trigger a stress response. If you have recently moved me to a new location with drastically different light or temperature, or if I have been exposed to a cold draft or frost, I may react by shedding leaves. This is a shock to my system. Additionally, while we are less prone to them, pests like mealybugs or scale can attach themselves to my leaves and stems. They pierce my skin and suck out my nutrient-rich sap, directly damaging the leaf tissue and causing localized yellowing, weakness, and leaf loss as they steal the energy I worked so hard to produce.
