Question: I work in an office and we often throw out coffee at the end of the day. Could we use this to water our office plants?
Answer: Honestly, I don’t think this has ever been carefully studied, but logically speaking, there appears to be no reason why you couldn’t water your houseplants with coffee, at least occasionally. And many people do. Of course, you’d have to let it cool first!
What’s in It?
Liquid coffee is mostly water, something all plants need. It also contains hundreds of different compounds, some of which are good for plants (minerals, for example), others probably harmful (caffeine, etc.) and most, quite innocuous. But all are highly diluted and even harmful ones will break down rapidly in contact with microbes in the potting mix.
Watering with coffee likely has little in the way of benefits for your plants … besides offering them moisture, of course. Perhaps it could be seen as a very, very weak fertilizer.
Also, coffee is acidic, with a pH of about 5. But then, so is rainwater (usually about 5.6), although acid rain can be much lower. Eventually, over many, many months of watering with coffee, this might increase the soil acidity slightly and that could be to the benefit of acid-loving plants like African violets, phalaenopsis, azaleas and begonias. But the effect will be minimal: it’s very hard to significantly change the pH of a substrate through watering.
It’s probably best not to water with coffee to which sugar and dairy products have been added. Or with flavored coffees either. They contain extra compounds that would have to be broken down and they could overwhelm the limited microbes found in containers, leading to undesirable odors, mushrooms, fungus gnats, etc.
Again, I know of no official guidelines as to how much coffee you can safely use in watering plants (it would make a great science project for a biology class, though!), but hearsay evidences suggests a weekly watering with diluted coffee (I’d guess half water and half coffee) appears to do no harm.
Or pour the coffee on outdoor plants. In a garden, there are usually many more plants to work with and you could empty a daily percolator full here and there while doing no damage whatsoever … and probably a modicum of good!