The use of coffee grounds for orchids is a topic of much debate among hobbyists. From a plant science perspective, the answer is nuanced and hinges on understanding the specific needs of orchids versus the properties of coffee grounds.
Proponents of using coffee grounds point to their nutrient content. Coffee grounds contain small amounts of nitrogen, a crucial macronutrient for vegetative growth, as well as potassium, phosphorus, and trace minerals. They are also slightly acidic, with a pH often ranging from 6.5 to 6.8. For orchids like Phalaenopsis that prefer a slightly acidic growing medium (pH 5.5-6.5), this could theoretically be beneficial. Furthermore, coffee grounds are an organic material, and their decomposition could improve the structure of some potting mixes by increasing organic matter content.
Despite the potential benefits, the risks overwhelmingly outweigh them for most home growers. The primary concern is moisture retention. Fresh coffee grounds are fine and compact easily, creating a dense, moisture-logged environment around the orchid's roots. Most epiphytic orchids have roots adapted to absorb moisture from the air and require excellent aeration around them. Soggy, compacted grounds will quickly lead to root rot, the leading cause of death in potted orchids. Additionally, as microbes break down the high-carbon grounds, they can temporarily tie up nitrogen in the soil, making it unavailable to the plant.
While often cited as acidic, used coffee grounds are actually nearly neutral pH and can even become alkaline as they decompose. This pH shift is unpredictable and unlikely to provide the stable, slightly acidic environment orchids need. More critically, coffee grounds can contribute to harmful salt buildup in the potting medium. The brewing process concentrates soluble salts in the grounds. As these salts accumulate, they alter the osmotic pressure around the roots, making it difficult for the plant to absorb water and nutrients, effectively leading to drought stress and fertilizer burn even when the medium is wet.
From a purely botanical standpoint, applying coffee grounds directly to orchids is a high-risk practice with very little proven reward. The physical properties of the grounds directly oppose the core cultural requirements of epiphytic orchids: a loose, fast-draining, and well-aerated root environment. The potential for salt accumulation and root suffocation presents a direct threat to plant health. While a minuscule amount of well-composted grounds incorporated into a very coarse, open mix might be tolerated by some robust varieties, the margin for error is exceptionally slim. For the vast majority of growers, the potential for severe harm makes this a myth not worth testing.
