It has been proven that coffee and coffee grounds make plants grow faster. However, no one is certain whether the growth has anything to do with caffeine or if the other ingredients found in coffee are responsible.
Recycled coffee grounds make an ideal fertilizer because they contain nitrogen, potassium and phosphorus – ingredients that are known to enhance plant growth. However, placing pure caffeine on plants does not have the same effect as coffee grounds. In some studies, plants that were fertilized with pure caffeine exhibited retarded growth and did not thrive like plants fertilized with a coffee ground mixture.