From the perspective of the marigold plant (*Tagetes* spp.), the use of coffee grounds is a complex issue with both significant potential benefits and notable risks. Whether they are "good" depends entirely on how they are applied, as improper use can cause more harm than good.
Used coffee grounds can be a valuable source of key nutrients that marigolds require for vigorous growth and prolific blooming. As they slowly decompose, they release nitrogen, which is essential for healthy green foliage development. Additionally, they contain other minerals like potassium and phosphorus, which are crucial for strong root systems and vibrant flower production. For a marigold, this translates into a steady, gentle feeding rather than a harsh chemical influx. Furthermore, the organic matter improves soil structure, enhancing aeration and water retention, allowing the marigold's roots to breathe and access moisture more effectively.
Despite the benefits, fresh, undried coffee grounds pose immediate threats. Their physical texture is dense and compactable. When applied thickly or mixed into the soil while wet, they can create a water-resistant barrier that hinders root respiration and promotes root rot—a death sentence for a marigold. The widely held belief that they are highly acidic is somewhat overstated for *used* grounds; most of the acidity is brewed out. However, they can still be mildly acidic to neutral. For marigolds, which prefer a neutral pH, a large volume of fresh grounds could potentially alter the soil chemistry enough to lock away essential nutrients, making them unavailable for uptake by the roots.
The marigold plant's preference is clear: coffee grounds must be composted first. Adding them to a compost bin allows them to break down in a controlled environment. This process neutralizes any residual acidity, eliminates the risk of compaction, and integrates the grounds into a balanced humus. This finished compost is what the marigold truly desires. It can be worked into the soil at planting time or used as a top-dressed mulch, providing all the nutritional and textural benefits without any of the associated risks. A thin sprinkling of fully dried, used grounds around the base of the plant (not touching the stem) can also be used as a mild mulch to deter pests like slugs.
It is important to understand that during decomposition, the microorganisms that break down the carbon-rich coffee grounds will consume nitrogen from the surrounding soil to fuel their activity. If grounds are mixed directly into the soil in large quantities, this can temporarily tie up nitrogen, making it deficient for the marigold and potentially causing yellowing leaves (chlorosis). Composting first allows this nitrogen consumption to occur within the compost pile, ensuring the finished product provides nitrogen to the plant instead of robbing it.
