Coffee grounds are a popular organic material often used by gardeners to enrich soil. They contain nutrients and organic matter that can benefit plant growth and soil structure. However, applying them universally is a misconception, as their unique chemical and physical properties can be detrimental to specific plant types. Understanding what makes coffee grounds a complex amendment is the first step in knowing which plants should never receive them.
Chemical and Physical Properties of Coffee Grounds
The effect of spent coffee grounds on soil is determined by their chemical composition and fine physical structure. Used coffee grounds are typically mildly acidic, possessing a pH between 6.5 and 6.8, which is close to neutral. While fresh grounds are more acidic, brewing leaches out much of the acidity. Consistent application of used grounds can still slightly lower the soil’s pH over time.
Coffee grounds are considered a nitrogen-rich “green” component in composting, containing approximately 2% nitrogen by volume, along with smaller amounts of potassium and phosphorus. This nitrogen is bound in proteins and organic molecules and is released slowly as microorganisms break down the material. This slow-release characteristic provides a steady supply of nutrients. However, it can also stimulate microbial activity that temporarily competes with plants for existing soil nitrogen.
The fine, silt-like texture of the grounds significantly impacts soil structure. Applied too thickly to the soil surface, the grounds can lock together and form a dense, water-resistant crust. This compaction impedes water infiltration and reduces soil aeration, preventing moisture and oxygen from reaching plant roots. Coffee grounds also increase the soil’s water-holding capacity, which is problematic in some soils.
Specific Plants That React Negatively to Coffee Grounds
Plants that thrive in alkaline or neutral soil are the most likely to react negatively to coffee grounds due to the slight acidifying effect. Mediterranean herbs such as lavender and rosemary prefer soil with a neutral to alkaline pH, often between 7.0 and 8.0. If the soil becomes too acidic, they can suffer from stunted growth and yellowing leaves. The mild acidity of coffee grounds can disrupt nutrient uptake for these high-pH adapted plants.
Certain garden vegetables and flowering plants also prefer neutral or alkaline conditions. Asparagus and lilacs require a neutral to slightly alkaline environment, and coffee ground application can negatively impact their optimal growth. Roses generally prefer a pH between 6.5 and 7.0. While they tolerate slight acidity, excessive application may push the soil pH too low, resulting in stunted growth and reduced blooming.
Plants requiring sharp drainage or sensitive to excess moisture are poor candidates for coffee ground applications. Cacti and succulents are adapted to arid conditions and are highly susceptible to root rot if soil retains too much moisture. Since coffee grounds increase water holding capacity and can compact soil, they create waterlogged conditions detrimental to these species. Furthermore, young seedlings can be negatively affected by allelopathic chemicals and residual caffeine, which may inhibit germination and early growth.
Common Application Mistakes to Avoid
The method of application is significant in determining whether coffee grounds are helpful or harmful. A common error is applying the grounds in a thick, dense layer directly to the soil surface. When used as heavy mulch, they dry into a hard, impermeable crust that repels water runoff and prevents moisture from reaching the roots below. This compacted layer also restricts the exchange of gases between the soil and the atmosphere, potentially suffocating roots.
Another mistake is using grounds that have not been properly rinsed or filtered from the brewing process. Grounds containing residues of sugar, milk, or artificial sweeteners can attract pests and encourage the growth of harmful molds or fungi. For general use, it is better to compost the grounds first or mix them thoroughly with other organic materials before applying them as a thin layer to the soil.
Applying a large volume of grounds directly against the base of a plant stem is also poor practice. The fine, moisture-retaining nature of the grounds keeps the area around the plant crown constantly wet, which can cause stem rot or fungal issues. Even for acid-loving plants, the grounds should be spread in a thin layer and lightly raked into the top inch of soil or incorporated into a balanced compost mix to prevent compaction and moisture retention problems.