Does caffeine speed up a plant's growth?

Caffeine has not been proven to have any effect on a plant's growth. However, coffee grounds have shown to be beneficial for plants because coffee has a high concentration of nutrients that are important in plant growth.

Coffee grounds are a good source of nitrogen, an essential nutrient for plants. Coffee grounds also include phosphorus, potassium, calcium and other beneficial nutrients. In addition, coffee is slightly acidic, and many plants need a certain amount of acidity in the soil to grow well. As an added benefit, coffee grounds are a good repellent for slugs and other pests that can damage plants. While caffeine may not result in a direct improvement in plant growth, coffee grounds can be a good alternative to fertilizer.