Do Coffee Grounds Affect Plant Growth?
Gardening has its own set of urban legends and mythologies that are passed down from generation to generation and gardener to gardener. Planting seeds according to phases of the moon, talking to plants to make them grow or adding food coloring to plant water to gradually change their colors are just a few of these urban legends. Another of these urban legends is that tossing used coffee grounds onto plants will stimulate plant growth, and recent scholarly research gives a clue as to whether this legend is accurate or not. Does this Spark an idea?
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The Myth
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Walk into a coffee shop and, more often than not, that shop will give away its spent coffee grounds to anyone who wants them. This is because gardeners sometimes add coffee grounds to compost piles but increasingly apply them directly to plants, claiming that the grounds repel cats, kill slugs, prevent weeds, aerate and acidify the soil, provide nitrogen, attract earthworms and offer many other beneficial effects on plant growth. According to Linda Chalker-Scott of Washington State University, there has been much investigation on the use of other byproducts of coffee production such as coffee hulls and husks, coffee bean pulp and peel and the wastewater generated from coffee production. But until her own, there has been very little research on the effects of coffee grounds on plant growth.
The Science
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Coffee grounds are rich in nitrogen, and any successful gardener knows the critical role of nitrogen in healthy plant growth. Additionally, coffee grounds have a very high carbon to nitrogen ratio, meaning that the nitrogen is in a form that plants can actually absorb. Specialized bacteria and fungi that break down coffee grounds also appear to release compounds that can be quite effective at suppressing common and sometimes destructive plant diseases, including those associated with the fusarium, pythium and sclerotinia fungal species.
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Effects on Plant Growth
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The specific effects of coffee grounds on the growth cycle of plants, however, appears to be quite unpredictable and inconsistent. In her meta-analysis of all current scholarly research on the effects of coffee grounds on plant growth, Chalker-Scott notes that coffee grounds have "enhanced sugar beet seed germination and improved growth and yield of cabbage and soybeans" as well as acting as "an effective replacement for peat moss in producing anthuriums." The same research has suggested that increases in soil nitrogen, moderating soil temperature and increasing soil moisture due to the addition of coffee grounds are thought to be the primary causes of the enhanced growth.
Other Considerations
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But as noted, not all plant species see benefits from the addition of coffee grounds. Seed germination of alfalfa and white and red clovers actually decreased after the addition of coffee grounds. Similarly, Chinese mustard, komatsuna, Italian ryegrass, inch plant, geranium and asparagus fern all showed diminished growth after coffee grounds were used as a mulch around them. A toxin released by the coffee grounds may be responsible for the diminished growth, and this toxin could make coffee grounds an effective herbicide in some circumstances.
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