Calathea plants are popular for their striking, patterned foliage. Despite their beauty, these tropical plants are sensitive and often develop brown spots on their leaves. Understanding the causes is the first step toward resolving this common issue. This article will guide you through identifying, diagnosing, and addressing brown spots on your Calathea plants.
Primary Causes of Brown Spots
Brown spots on Calathea leaves signal an imbalance in their environment or care.
Insufficient humidity is a common factor, as Calatheas are native to tropical rainforests where humidity levels are consistently high. When the air around them is too dry, their leaves lose moisture rapidly, resulting in crispy, brown edges.
Water quality and watering habits also significantly influence leaf health. Tap water often contains chemicals like chlorine and fluoride, as well as dissolved salts and minerals. These can accumulate in the soil and burn the sensitive roots and leaf tips, leading to browning, particularly along the leaf margins. Both underwatering and overwatering can cause brown spots. Underwatering leads to dry, crispy patches, while overwatering results in mushy, dark brown areas due to root rot.
Light exposure plays a role in the development of brown spots. Calatheas prefer bright, indirect light, mimicking their natural understory habitat. Too much direct sunlight can scorch their delicate leaves, causing bleached or crispy brown spots. Conversely, insufficient light can stress the plant, making it more vulnerable to other problems that manifest as browning.
Pest infestations are another potential cause of brown spots. Tiny pests such as spider mites, thrips, and aphids feed on plant cells, leading to brown stippling or patches on the leaves. These pests often thrive in dry conditions and can quickly spread. Fungal or bacterial issues can also appear as distinct brown spots, sometimes with a yellow halo, indicating a leaf spot disease.
Diagnosing and Addressing Brown Spots
Identifying the specific cause of brown spots involves closely examining their appearance and location on the plant. Crispy brown edges or tips often point to low humidity or underwatering. If the brown spots are more central on the leaf surface, they might indicate sunburn, pest activity, or disease. Mushy, dark brown spots, especially accompanied by wilting despite moist soil, are strong indicators of overwatering and potential root rot.
Addressing Humidity Issues
To address low humidity, increase the moisture in the air around your Calathea. Using a humidifier is an effective method, aiming for humidity levels between 50-70%. Placing the plant on a pebble tray filled with water, ensuring the pot does not sit directly in the water, can also help create a localized humid microclimate. Grouping Calatheas with other plants can contribute to slightly higher humidity.
Addressing Water Issues
For issues related to water quality, switch from tap water to distilled, filtered, or rainwater. If using tap water is unavoidable, allowing it to sit in an open container overnight can help dissipate some chlorine. When watering, ensure you do so thoroughly, allowing water to drain from the bottom. Only water when the top inch or two of the soil feels dry to the touch. Periodically flushing the soil by watering heavily helps remove accumulated salts.
If root rot is suspected, carefully remove the plant from its pot. Gently wash away old soil, and trim any mushy or black roots with sterilized scissors. Repot the plant in fresh, well-draining soil. Ensure the pot has drainage holes to prevent future waterlogging.
Addressing Light Issues
Addressing light exposure issues means moving your Calathea to a location that receives bright, indirect light. This might involve placing it a few feet away from a window or using sheer curtains to diffuse intense sunlight. Avoid locations with direct, harsh sun exposure, which can scorch the leaves. Consistent temperatures between 65-80°F (18-27°C) are preferred, so keep plants away from cold drafts or heating vents.
Addressing Pest and Disease Issues
Pest infestations require prompt action. Regularly inspect your plant, particularly the undersides of leaves, for signs of pests like webbing or tiny insects. If pests are present, wipe the leaves with a damp cloth to remove them. For more persistent infestations, insecticidal soap or neem oil can be applied following product instructions. Isolate affected plants to prevent pests from spreading to other houseplants.
If fungal or bacterial leaf spots are identified, immediately remove any affected leaves to prevent further spread. Improving air circulation around the plant can help. Avoid wetting the leaves when watering, as moisture promotes fungal growth.
Pruning Affected Leaves
When brown spots or sections appear on leaves, they will not revert to green. For aesthetic purposes and to redirect the plant’s energy, you can prune off the affected parts. Use sharp, clean scissors or pruning shears to cut along the natural curve of the leaf or remove entire damaged leaves at their base. Do not remove more than 20% of the foliage at one time.
Other Considerations
During the growing season, a light, balanced fertilization can support overall plant health and resilience. However, avoid over-fertilizing, as this can lead to salt buildup and leaf burn. Maintaining a stable and consistent environment, including steady temperatures and appropriate light levels, helps prevent stress. Regular inspection for early signs of distress allows for timely intervention.