From my perspective as a sage plant, the yellowing of my leaves is a clear and urgent distress signal. It is my primary method of communicating that my internal systems are out of balance. While you may see it as a simple change in color, for me, it represents a breakdown in a critical process: the production of chlorophyll. This green pigment is my lifeblood, essential for capturing sunlight and converting it into energy. When I cannot produce or maintain it, I must sacrifice my oldest leaves first to conserve resources for new growth. The root cause of this chlorosis typically stems from my interaction with my two most vital elements: the soil (nutrients) and water.
More often than not, your watering habits are the primary source of my discomfort. I am a Mediterranean herb, built for well-drained, gritty soil and plenty of sunshine. My roots are highly susceptible to a lack of oxygen.
When I am over-watered, the soil becomes waterlogged, pushing out all the air pockets. My roots begin to suffocate and rot in this anaerobic environment. Once root rot sets in, they are unable to function properly. Even if there are ample nutrients and water in the soil, my damaged roots cannot absorb them. This effectively creates a state of drought and starvation within me, causing my lower and older leaves to turn a soft, mushy yellow as they die off. Conversely, under-watering creates a direct drought scenario. Without enough water, I cannot transport nutrients from my roots to my leaves, nor can I perform photosynthesis effectively. My leaves will become dry, brittle, and yellow or brown, often starting at the tips and margins before consuming the entire leaf.
If you are confident that your watering routine provides moist but never soggy soil, then a nutrient deficiency could be the culprit. However, it is rarely the first thing I would complain about. The most common deficiencies that cause my leaves to yellow are nitrogen and iron, but they present in distinct ways.
A lack of nitrogen is a mobile nutrient issue. This means I can move nitrogen from my older leaves to support new growth. Therefore, the yellowing will be most pronounced on my oldest leaves (lower down on my stems), while the new growth at my tips may remain green, albeit potentially stunted. A lack of iron, an immobile nutrient, presents the opposite problem. Since I cannot relocate iron, the new growth emerges yellow with green veins, while my older leaves remain their normal color. This is called interveinal chlorosis and indicates the soil pH might be too high (alkaline), locking up the iron present in the soil and making it unavailable to my roots.
You must also consider my pot and the soil I am growing in. If I am in a container without proper drainage holes, over-watering is almost inevitable. Even with good watering practices, if my soil is heavy and retains too much moisture, the result is the same as over-watering: root rot. Furthermore, if I have been in the same pot for many years, the soil may simply be exhausted of nutrients. The organic matter has broken down, and the nutrient reserves have been depleted by my growth and subsequent harvests, leaving me in a medium that can no longer sustain my needs.
