From my perspective, rooted firmly in the soil, the most common reason my lower leaves begin to yellow is a simple imbalance of water. My roots are my mouth, and they need a consistent but careful drink. If the soil becomes too dry and parched, my roots cannot uptake the water they need to transport nutrients and maintain my cells' turgor pressure. This causes my oldest leaves to sacrifice themselves, turning yellow and withering to conserve water for my more vital parts—my stem and the developing flower bud. Conversely, if my roots are drowning in soggy, waterlogged soil, they begin to suffocate and rot. Without healthy roots, I cannot absorb water or nutrients at all, leading to a similar yellowing, often accompanied by a wilted, drooping appearance despite the wet soil.
Building my towering structure and massive flower head requires a immense amount of energy and specific building blocks. The yellowing of your leaves is often my direct way of communicating a deficiency. If the yellowing begins on the older, lower leaves and moves upward, I am likely starving for nitrogen. Nitrogen is the key component of chlorophyll, the green pigment essential for photosynthesis. Without it, my leaves fade to yellow as I pull the remaining nitrogen from my older growth to support new, young leaves and the flower. If you see yellowing between the veins of my newer leaves, I may be lacking iron. A general pale yellowing across my entire being could indicate a need for sulfur, magnesium, or other trace elements that are crucial for my metabolic functions.
It is important to remember my natural biology. I am an annual plant, which means my entire life's purpose is to grow, flower, set seed, and then complete my cycle. As my magnificent flower begins to fade and my seeds mature, my energy is diverted entirely to reproduction. There is no need to sustain the large, solar-panel leaves that supported my vegetative growth. Therefore, it is a natural and expected process for my leaves to yellow, brown, and die back as I near the end of my life. This is not a cause for alarm but a sign of my success. The yellowing will typically progress upward from the bottom leaves as my biological resources are reallocated to ensuring the next generation within the seed head.
Sometimes, the yellowing is a distress signal caused by unseen attackers. Below the soil, microscopic nematodes can be feeding on my roots, damaging my ability to uptake water and nutrients, which manifests as yellowing and stunting above ground. Fungal diseases like Verticillium wilt or downy mildew can invade my system, clogging my vascular tissues and preventing the flow of life-sustaining water, leading to patches or sections of yellow and wilted leaves. Aphids or spider mites might be sucking the sap directly from my leaves and stems, draining me of my nutrients and causing a stippled yellow appearance. In these cases, the yellowing is often accompanied by other signs like wilting, spots, or visible insects.
