The ZZ plant, scientifically known as Zamioculcas zamiifolia, is a popular indoor plant prized for its glossy foliage and remarkable resilience. It tolerates low light and infrequent watering. When black spots appear on the plant’s upright, succulent stalks (petioles), it is a cause for concern. These spots can represent either normal development or a serious underlying health issue. The distinction lies in a careful visual and tactile examination.
Distinguishing Normal Coloration from Disease
The first step is to determine if the black spots are benign scarring or a symptom of disease. Natural markings on a ZZ plant are common and are not a sign of distress. These normal spots typically present as small, dry, dark brown or black scars, often near the base of the petiole. They feel hard and firm to the touch, similar to the surrounding tissue, and will not spread or change over time.
In contrast, black spots indicating a health problem look and feel different. These harmful lesions are often soft, mushy, or weeping, and they tend to spread rapidly on the petiole. Affected areas may appear slimy or sunken, indicating the breakdown of plant tissue. This problematic discoloration is frequently accompanied by other symptoms, such as the yellowing of leaves or the entire petiole collapsing.
The Leading Cause: Moisture Stress and Rot
When the black spots are soft and spreading, the underlying cause is almost always moisture stress leading to rot. The ZZ plant stores water in large underground structures called rhizomes. These rhizomes are highly susceptible to root rot, which is primarily caused by overwatering and soil that retains too much moisture.
When the soil remains waterlogged for an extended period, oxygen is displaced, creating an anaerobic environment around the rhizomes and roots. This lack of oxygen encourages the proliferation of fungal or bacterial pathogens responsible for soft rot. These pathogens attack the water-storing tissues, turning the normally firm rhizomes and roots into a soft, black, mushy substance. The rot then progresses upward through the vascular system of the petiole, manifesting as soft, black lesions on the stalk. Cold exposure can act as a secondary stressor, making the plant’s tissues more vulnerable.
Remedial Steps and Proper Maintenance
Addressing rot requires immediate action to save the plant. The first step is to quarantine the plant and remove all visibly affected, mushy, or blackened petioles. Cut them off cleanly with a sterilized tool, ensuring the cut is made slightly above the soil line.
Root Inspection and Trimming
Gently unpot the plant to fully inspect the rhizomes and roots. Healthy roots should be firm and white, while rotten parts will be black, brown, or slimy. Any soft or discolored part of the rhizome or root system must be trimmed away using sterile shears until only healthy, firm tissue remains. After trimming, allow the exposed root system to air-dry for several hours or overnight to form a protective callus before repotting. Repot the plant into a new pot with excellent drainage holes and fresh, fast-draining soil, such as a succulent or cactus mix.
Adjusting Watering Practices
The most significant long-term correction involves adjusting the watering routine to prevent recurrence. ZZ plants must be allowed to dry out completely between waterings. The soil should be bone-dry several inches down before water is applied again. When watering, drench the soil until water flows through the drainage holes, then discard any standing water from the saucer immediately. This practice ensures the rhizomes are never sitting in stagnant moisture.