The sudden yellowing of leaves on a bonsai tree, known as chlorosis, signals that the plant is under stress. This discoloration occurs when the tree fails to produce sufficient chlorophyll, the green pigment responsible for photosynthesis. Chlorosis is a symptom pointing to an underlying problem with the tree’s health or environment. Because bonsai are miniature trees maintained in small pots, they are highly sensitive to imbalances in their care regimen. Identifying the specific cause of this yellowing is the first step toward successful recovery.
Diagnosis Based on Watering and Soil Health
Improper watering is the most frequent cause of yellow leaves in bonsai. Overwatering is common and leads to root suffocation because saturated soil deprives the roots of necessary oxygen. Leaves yellowed by overwatering are typically soft, may droop, and often fall off without becoming crispy. Prolonged saturation causes root rot, characterized by mushy, dark, and foul-smelling roots, which prevents the uptake of water and nutrients.
Conversely, underwatering causes a different pattern of yellowing, often starting with the lower leaves and moving upward. These leaves tend to feel dry, brittle, or crispy, and the foliage may appear wrinkled or wilted. The soil may pull away from the edges of the pot and feel desert-dry beneath the surface.
Soil composition plays a direct role, as dense, non-porous soil holds too much water, exacerbating overwatering. Bonsai require a fast-draining substrate, such as a mix containing akadama, pumice, or lava rock, to ensure the roots experience a wet-dry cycle. Poor drainage can also be caused by blocked drainage holes, allowing roots to sit in stagnant water. Correcting this involves ensuring the drainage holes are clear or, in severe cases, repotting the tree into a fresh, well-aerated medium.
Diagnosis Based on Light and Temperature Stress
Insufficient light causes a general, uniform paling of the foliage because light is directly related to chlorophyll production. When a bonsai does not receive enough light, its leaves may turn a pale, washed-out yellow, especially on the inner and lower canopy. Indoor bonsai require bright, indirect light for six to eight hours daily, often necessitating placement near a south or east-facing window.
Temperature extremes and sudden changes can induce stress-related yellowing and leaf drop. Excessive heat combined with low humidity can cause the tree to lose moisture faster than its roots can absorb it, leading to yellowing or browning at the leaf edges (scorch). Cold drafts or moving a tropical species to a cooler location can shock the tree, triggering abrupt yellowing and shedding. Gradual acclimation to new conditions is important to mitigate this environmental stress.
Diagnosis Based on Nutrient Shortages
Nutrient deficiencies manifest as distinct patterns of chlorosis that help pinpoint the exact shortage. Nitrogen (N) deficiency, needed for green, leafy growth, causes a uniform yellowing that starts on the older, lower leaves. The tree sacrifices nitrogen from older leaves to support new growth, which may also appear pale and stunted.
Iron (Fe) deficiency is identified by interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins remain green. This pattern appears first on the newest, upper growth because iron is not mobile within the plant. Interveinal chlorosis on older leaves, however, can indicate a magnesium (Mg) deficiency, which may progress to reddish or purple blotches.
Nutrient availability is often tied to soil pH, rather than a lack of the element itself. For example, iron becomes chemically unavailable for root absorption in alkaline soils, even if the mineral is physically present. Most bonsai thrive when the soil pH is slightly acidic (generally between 5.0 and 6.5), which ensures optimal nutrient solubility.
Diagnosis Based on Pests and Pathogens
Pests that feed on the tree’s sap can cause localized yellowing and stippling, disrupting the leaf’s ability to photosynthesize. Spider mites are a common culprit, thriving in hot, dry conditions and causing a fine, speckled yellowing or bronze appearance on the foliage. The clearest sign of a spider mite infestation is fine webbing on the undersides of leaves or between branches.
Scale insects appear as small, immobile, oval-shaped bumps on the stems and leaves, sucking the plant’s sap and leading to localized yellow spots. Scale and aphids excrete honeydew, a sticky, sugary substance that often leads to the growth of black, sooty mold on the leaf surfaces. Fungal pathogens, such as leaf spot, cause yellowing characterized by distinct brown or black spots on the tissue. Prompt inspection and isolation of an affected bonsai are necessary to prevent the spread of these invaders.