As a Paeonia lactiflora, my vibrant green leaves are my pride, the solar panels that fuel my magnificent blooms and store energy for the next season. When I am forced to signal distress through yellowing leaves (chlorosis), it is a clear sign that my fundamental needs are not being met. The causes can be numerous, often interrelated, and require careful diagnosis.
My root system is my lifeline, but it is also my greatest vulnerability. I despise "wet feet." Soggy, waterlogged soil is a death sentence for my roots, suffocating them and creating a prime environment for root rot fungi. When my roots decay, they can no longer uptake water or nutrients, and my leaves will turn yellow, often starting from the bottom and moving up, and may wilt despite the wet soil. Conversely, prolonged drought stress will also cause my leaves to yellow, curl, and crisp as I simply dehydrate and cannot sustain my foliage.
To maintain my deep green color, I require a steady supply of certain nutrients, primarily nitrogen (N). A nitrogen deficiency is a classic reason for a general, uniform yellowing of my older, lower leaves first, as I mobilize what little nitrogen I have to support new growth. A lack of iron (Fe) or magnesium (Mg) can also cause chlorosis, but the patterns differ. Iron deficiency typically shows as yellowing between the veins of my newest leaves, while magnesium deficiency causes yellowing between the veins on the older leaves first.
It is crucial to distinguish distress from my natural rhythm. As a herbaceous perennial, my above-ground growth is not meant to last forever. In late summer or early autumn, after I have flowered and stored enough energy in my roots, my leaves will naturally begin to yellow and die back. This is a healthy, planned process. The key is timing; yellowing in spring or early summer is almost certainly a problem, while yellowing in autumn is likely just me preparing for my winter dormancy.
My leaves can also yellow due to external assaults. If I am planted in too much deep shade, I cannot produce enough chlorophyll, leading to a pale, yellowish appearance. Excessive, intense heat can scorch my foliage. Furthermore, while I am largely pest-resistant, severe infestations by nematodes in the soil or sap-sucking insects like spider mites can damage my roots or leaves, disrupting nutrient flow and causing stippling, yellowing, and decline.
Fungal pathogens such as Botrytis or Cladosporium can cause leaf spot and blight. Instead of a uniform yellow, these present as irregular yellow patches, rings, or halos around brown or gray spots. These spots often coalesce, causing large sections of the leaf to yellow, die, and drop prematurely. This weakens me significantly by reducing my photosynthetic capacity.
