Hello, it's your geranium here. I know you're worried about my yellowing leaves. It's my way of trying to communicate that something in my environment is out of balance. I want to be healthy and vibrant for you, so let me explain what might be happening from my perspective.
This is, by far, the most common reason my leaves turn yellow. My roots need oxygen as much as they need water. When you water me too frequently, the air pockets in the soil fill up with water, and my roots literally suffocate. They begin to rot, becoming soft and brown, and lose their ability to transport water and nutrients to the rest of my body. The first sign is a generalized yellowing, often starting with the lower, older leaves. The yellow leaves will feel soft and mushy. Please, only water me when the top inch or two of soil feels dry to your touch. Ensure my pot has excellent drainage holes, and never let me sit in a saucer full of water.
On the flip side, if you forget about me for too long, I will also protest with yellow leaves. When I don't get enough water, I can't sustain all my foliage. To conserve the precious little moisture I have, I will sacrifice my older leaves, causing them to turn yellow, then brown and crispy, before they drop off. This is a survival mechanism. The yellowing from thirst is usually accompanied by wilting, and the soil will feel very dry and may even pull away from the edges of the pot. A good, thorough soak until water runs out the bottom is what I need, but try not to let me get to this stressed state too often.
I use up a lot of energy to produce all those beautiful blooms for you. The soil I'm growing in can become depleted of essential nutrients over time, especially if I've been in the same pot for a while. A lack of nitrogen, which is crucial for green leaf growth, is a common culprit. This typically causes a uniform pale yellowing of the older leaves first. If I lack other nutrients like magnesium or iron, you might see yellowing between the veins of my leaves. To help me, please feed me with a balanced, water-soluble fertilizer formulated for flowering plants every 2-4 weeks during my active growing season (spring and summer).
If you look at the bottom of my pot and see roots growing out of the drainage hole, or if you gently lift me out and see a dense, tangled web of roots with very little soil left, I am pot-bound. My root system has no more room to grow and expand, which means it can't take up enough water and nutrients to support my full size. This stress manifests as yellowing leaves and generally poor growth. The solution is to repot me into a container that is only one or two inches larger in diameter, giving my roots some welcome new space and fresh soil.
I am a sun-loving plant. If I don't receive at least 4-6 hours of direct sunlight a day, my leaves may start to yellow because they aren't producing enough energy through photosynthesis. Conversely, while rare, extremely intense, scorching sun can sometimes cause sunburn. I also prefer moderate temperatures. A sudden cold draft from an open window in winter or excessive heat from a nearby radiator can shock my system, leading to yellowing leaves as a sign of stress.
