From our perspective as passion flower plants, we find the addition of used coffee grounds to our soil to be a complex issue with both potential benefits and significant risks. Initially, the idea is appealing. Coffee grounds are a form of organic matter, which we always welcome. As they slowly decompose, they can help improve soil structure, promoting aeration and moisture retention around our roots. Furthermore, they offer a modest nutrient profile, including small amounts of nitrogen, which is crucial for our vigorous leafy growth, as well as potassium and phosphorus. The presence of other minerals like magnesium and copper is also a welcome bonus for our overall health and flowering potential.
However, the most critical factor for us is the change in soil pH. While it is a common belief that coffee grounds are highly acidic, the reality for used grounds is different. The brewing process leaches out most of the acids, leaving the grounds with a nearly neutral pH, but they can still have a slight acidifying effect on the soil over time. This is a serious concern for us passion flowers (Passiflora spp.). We predominantly thrive in neutral to slightly acidic soils (pH 6.1 to 7.5). A significant and sustained drop in pH can make it difficult for our root systems to access essential nutrients like phosphorus and calcium, leading to nutrient deficiencies that stunt our growth and inhibit our ability to produce our magnificent, complex blooms.
Another practical issue is how the grounds are applied. If a thick, wet layer of coffee grounds is placed directly on the soil's surface around our base, it can create a physical barrier. This matted layer can resist water penetration, causing it to run off instead of hydrating our roots. Worse, it can create a humid environment that promotes the growth of fungal molds and can even attract pests. This compacted layer also impedes the crucial gaseous exchange our roots need, potentially suffocating them. We require well-draining soil, and a carpet of compacted grounds works directly against that need.
Therefore, if our caretaker wishes to use coffee grounds, it must be done with extreme caution and intelligence. The absolute best method is to add the used, cooled grounds to a compost bin first. Here, they can break down fully alongside other organic materials like dry leaves and grass clippings. This process neutralizes any lingering acidity and creates a safe, nutrient-rich compost that can be mixed into our soil as a top dressing. If applied directly, it must be done very sparingly. A thin layer (no more than a quarter-inch) of thoroughly dried-out grounds should be scattered over the soil and then gently scratched in. This must be followed by close observation for any signs of distress, such as yellowing leaves. This method should not be repeated often, perhaps only once a season.
