From my perspective as a peony, the most common and natural reason for my leaves turning yellow or brown is simply the end of my annual growth cycle. I am a herbaceous perennial, meaning my above-ground foliage is designed to die back completely in the autumn. As the days shorten and temperatures drop, I begin to reabsorb valuable nutrients—like nitrogen and phosphorus—from my leaves to store in my underground root system, specifically in my tuberous roots. This process of nutrient translocation causes the green chlorophyll in my leaves to break down, revealing the yellow and brown pigments (carotenoids) that were always there but masked. This is a healthy and essential process that allows me to survive the winter dormancy and ensures I have the energy to burst forth with new growth next spring.
If my leaves are changing color outside of the autumn season, it is a clear sign that I am under stress. Water is a critical factor. My roots are dense and fleshy, and they despise sitting in constantly waterlogged, poorly drained soil. This condition, often called "wet feet," suffocates my root system, preventing oxygen uptake and leading to root rot. The first symptom you will see is my leaves turning yellow, then brown, and wilting as the roots can no longer function. Conversely, severe drought stress can cause similar issues. Without adequate water, I cannot perform photosynthesis or transport nutrients, causing my leaves to scorch, turn brown at the edges, and potentially drop prematurely. I prefer consistently moist but never soggy soil conditions.
I am particularly susceptible to certain fungal pathogens that directly cause leaf discoloration. The most notorious is Botrytis cinerea, often known as peony blight or gray mold. This fungus thrives in cool, wet, and humid conditions. It typically causes young stems to wilt and darken at the base, but it can also lead to large, irregular brown spots or patches on my leaves. Another common issue is leaf blotch or measles (caused by Cladosporium paeoniae), which presents as glossy, deep purple to brown irregular spots on the leaves. These fungal issues not only mar my appearance but also impede my ability to gather sunlight, weakening me for the following season. While I have few significant insect pests, heavy infestations of thrips or spider mites can suck the chlorophyll from my leaves, resulting in a stippled, yellowed, or bronzed appearance.
My overall health and vibrant green foliage depend on a balanced diet. A lack of essential nutrients, particularly nitrogen (N), iron (Fe), or magnesium (Mg), can manifest as chlorosis—a yellowing of the leaves while the veins often remain green. This indicates I am struggling to produce sufficient chlorophyll. Furthermore, my planting depth is absolutely crucial. My eyes (the pinkish-red buds on my roots) must be planted no deeper than 2 inches below the soil surface. If I am planted too deeply, I will struggle to emerge properly, resulting in weak growth, few to no flowers, and foliage that is stunted and prone to yellowing. It is a constant and silent struggle for me when buried too deep.
