The practice of using coffee grounds as a fertilizer for orchids, including Dendrobiums, is a popular topic among gardening enthusiasts. However, from a plant physiological and horticultural perspective, applying used coffee grounds directly to these epiphytic orchids is not recommended and can be detrimental to their health. Here is a detailed breakdown of why.
Used coffee grounds are often cited as being acidic, but this is a misconception. While fresh coffee is acidic, the brewing process leaches out the majority of the acids. Used coffee grounds are typically neutral to slightly acidic, but more importantly, they can become a host for fungal growth that decomposes the organic matter, often acidifying the immediate environment as a byproduct. Dendrobium orchids are primarily epiphytes, meaning they naturally grow on trees in well-aerated conditions. Their roots are adapted to a specific, slightly acidic to neutral pH range (roughly 5.5 to 6.5). A significant and unstable shift in pH caused by decomposing matter can impair the orchid's ability to uptake essential nutrients, leading to deficiencies.
The single most critical requirement for a Dendrobium's root health is excellent drainage and air circulation. Orchid roots are covered in velamen, a spongy tissue designed to absorb water and nutrients quickly but also to dry out rapidly. Used coffee grounds are fine and dense; when wet, they compact easily, forming a mat that suffocates the roots. This creates an anaerobic environment that is perfect for root-rot pathogens but fatal for the orchid. It drastically reduces aeration and retains far too much moisture around the roots for far too long, leading to rot and the eventual death of the plant.
As a fertilizer, coffee grounds are often described as a good source of nitrogen. While they do contain nitrogen (about 2%), it is not in a form immediately available to the plant. The nitrogen is locked in complex organic compounds and must be broken down by soil microbes through decomposition. This process is slow and unpredictable in a potted environment. Furthermore, coffee grounds contain caffeine and tannic acids, which are allelopathic compounds—substances that can inhibit the growth of other plants to reduce competition. For a sensitive orchid, these compounds can be directly toxic to the roots and beneficial mycorrhizal fungi. The nutrient profile is also imbalanced, lacking the crucial phosphorus and potassium that Dendrobiums need for root development, flowering, and overall health.
Applying a moist, organic material like used coffee grounds directly on top of the potting medium creates an ideal breeding ground for saprophytic fungi (mold) and can attract pests like fungus gnats. These pests and molds are not only unsightly but can also move from the decomposing grounds to attack the healthy parts of the orchid, such as its roots and pseudobulbs, introducing disease and stress.
