Begonia is a vast genus of plants, encompassing over 2,000 different species and hybrids popular as ornamental houseplants and garden additions. These plants are cherished for their striking foliage and vibrant flowers, ranging from the fibrous-rooted wax begonia to the ornate, rhizomatous Rex varieties. When a begonia’s leaves begin to develop brown spots or margins, it signals the plant is experiencing stress or an environmental imbalance. This browning, known as necrosis, is the death of leaf tissue, and understanding the location and texture of this damage helps diagnose the underlying problem.
Moisture Management: The Most Common Culprit
Improper watering is the most frequent cause of brown leaves on begonias. When a plant is severely underwatered, the lack of moisture causes the leaf tissue to desiccate. This results in dry, brittle, and crispy brown edges or tips, often accompanied by wilting or curling as the plant conserves moisture.
Overwatering, conversely, can lead to mushy, dark brown or black patches, especially with poor drainage. Consistently soggy soil deprives the roots of oxygen, creating an environment for root rot. When roots are compromised, they can no longer absorb water and nutrients, causing the leaves to brown and wilt. This damage manifests as yellowing leaves with soft, dark brown or black patches near the tips or margins.
To avoid both extremes, a precise watering routine is necessary, starting by checking the soil. Water thoroughly only when the top one to two inches of soil feel dry to the touch, rather than following a strict schedule. Ensuring the pot has adequate drainage holes is essential. Standing water in the saucer should be discarded after about 15 minutes to prevent the roots from sitting in excess moisture.
Environmental Triggers: Light and Air Quality
Begonias thrive in bright, filtered conditions, and exposure to intense, direct sunlight commonly causes leaf browning. Harsh solar radiation can scorch the leaf surface, leading to bleached, pale, or brittle brown patches that appear suddenly on exposed areas. Moving the plant a few feet back from a south or west-facing window, or placing it behind a sheer curtain, provides protection from leaf burn.
Low atmospheric moisture, or low humidity, is a significant environmental trigger, especially for begonias kept indoors when heating systems dry the air. Low humidity encourages excessive transpiration, causing the plant to lose water faster than its roots can supply it. This stress results in dry, crispy brown tips and edges, which is distinct from the larger scorched patches caused by direct sun.
Increasing the ambient humidity using a room humidifier or a pebble tray filled with water can alleviate this issue. Sudden temperature fluctuations, such as being placed near a cold draft or a heat vent, can stress the leaves, leading to discoloration or dark brown spots. Maintaining a consistent room temperature away from drafts helps stabilize the plant’s environment.
Biological and Nutritional Causes
Biological and nutritional factors also result in leaf browning. Certain pests, such as spider mites, mealybugs, and foliar nematodes, feed on the plant’s sap or tissue, causing localized damage that turns brown. Spider mite damage often appears as tiny stippling that can coalesce into large, discolored, or bronzed patches. Foliar nematodes cause irregular brown lesions that may be mistaken for a disease.
Diseases often manifest as distinct brown spots symptomatic of a fungal or bacterial infection. Fungal issues like Botrytis blight, which thrives in damp, cool conditions, create tan or brown spots that can be covered with a fuzzy gray mold. Bacterial leaf spot, caused by pathogens like Xanthomonas, presents as water-soaked, irregular brown spots, often ringed by a yellow halo. These issues are worsened by poor air circulation or consistently wet foliage.
Chemical imbalances can also cause leaf necrosis. Over-fertilization, or “fertilizer burn,” occurs when excess mineral salts accumulate in the soil, drawing moisture away from the roots and leaves. This stress results in brown, scorched-looking margins and tips on the foliage. Conversely, a deficiency in an element like potassium causes older leaves to develop a distinct burn along the margins before the entire leaf turns brown. If fertilizer burn is suspected, flushing the soil with large volumes of clean water helps wash away the excess salts.