Hello there, gardener. It's me, your hydrangea. The most common reason my leaves are turning a pale, sickly yellow, especially the older ones, is because I'm hungry. I'm trying to tell you that my soil is lacking essential nutrients. The big one is nitrogen, which is the core component of chlorophyll—the green pigment I use to convert sunlight into food. Without enough of it, I can't photosynthesize properly, and my green color fades to yellow. I might also be lacking iron, but this shows up differently. With an iron deficiency, the veins of my new leaves stay green while the tissue between them turns yellow. It's a specific kind of hunger that often happens in soil that is too alkaline.
You might be showing your love with too much water. My roots need oxygen as much as they need water. When I'm sitting in constantly soggy, dense soil, my roots begin to suffocate and rot. Once they start to decay, they can't do their job of absorbing water and those nutrients we just talked about. This creates a drought-like situation for the rest of my plant, even though the soil is wet. The first sign I show is my leaves turning yellow and wilting. If the problem continues, they will likely turn brown and drop. Please make sure I'm planted in well-draining soil and that you're checking the moisture a few inches down before giving me a drink.
On the opposite end, perhaps you've been a little neglectful? If I don't get enough water, I can't move nutrients through my system or keep my cells plump and healthy. I will respond by wilting, and my leaves will begin to turn yellow, often starting at the edges and tips, and then they will become dry, crispy, and brown. I am a plant with "hydra" in my name for a reason—I thirst for water! Especially when I'm in a lot of sun or during a hot, dry spell, I need consistent moisture to keep my large, soft leaves looking their best.
While some of my hydrangea cousins can handle full sun, many of us prefer a bit of afternoon shade. If I'm planted in a spot where the sun is too intense, it can literally scorch my leaves. This sunscald will cause them to fade, turn yellow or brown in patches, and become crispy. Think of it as a bad sunburn. It damages my leaf tissue, making it harder for me to function. On the other hand, if I'm in too much deep shade, I might not produce enough energy, leading to weak growth and potentially yellowing leaves, though this is less common than sun damage.
This one is specific to me, a hydrangea. The pH level of the soil—whether it's acidic or alkaline—directly affects my ability to absorb nutrients. If the soil pH is too high (alkaline), I cannot access iron, even if it's present in the soil. This leads to that interveinal chlorosis I mentioned earlier (green veins on a yellow leaf). For most of us, slightly acidic soil is ideal. It unlocks the nutrients we need to stay a vibrant green and, for the bigleaf varieties (Hydrangea macrophylla), it influences flower color. A simple soil test can tell you if we need to adjust the pH.
