Can you water plants with coffee? Gardening experts weigh in

Can you water plants with coffee? You've probably heard this question, especially if you work somewhere that goes through a lot of coffee, or if you yourself are wondering whether just putting it in your food bin is the best use for it. You may have heard that coffee is full of nutrients and shouldn't be discarded. Instead, it should be used as fertilizer for your plants.

Is this really true? While watering plants correctly is essential to keeping them alive and healthy, will watering them with coffee improve their condition? Here's what garden experts say. 

Can you water plants with coffee?

The answer is 'yes', but a couple of caveats. First of all, you'll have to curb your enthusiasm in terms of just how beneficial coffee is for plants. As Larry Hodgson, also known as The Laidback Gardener, bluntly puts it, 'liquid coffee is mostly water.' It is true that '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.' Ultimately, though, the fact that it's diluted means that 'even harmful ones will break down rapidly in contact with microbes in the potting mix.'

This is both a good thing – you probably won't kill your plant with coffee so long as you make sure it's cool before watering – and a bad thing if you were hoping for magical results. Yes, coffee contains nitrogen, but in small amounts that are unlikely to make much of a difference to either your best indoor plants or your garden borders. 

If you do decide to occasionally use coffee to water your plants, make sure it's black, without any sugar or milk added. As Larry cautions, dairy and sugar '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.'

Is it better to use coffee grounds than liquid coffee?

You may wonder whether mixing coffee grounds into the soil instead will yield better results. Liam Lapping, gardening expert from Flowercard, advises 'using coffee grounds as plant feed instead to supply your plants with additional nutrients rather than watering daily with coffee.' The potential benefit of coffee grounds is that they 'add to the organic matter of the soil, which can improve drainage, soil aeration and retention of water which helps keep your plants happy and healthy. A good rule of thumb for plant feeds is to feed them with these solutions once a week.' 

Again, there are no proven benefits to using coffee grounds as fertilizer, with not enough research done into the benefits or potential risks to some plants. If you're learning how to grow tomatoes, then tomato seedlings, for example, have been reported to react badly to coffee grounds. 

If you are keen to try watering plants with coffee, always try a little bit first rather than mixing in a lot straight away, and keep expectations low. If you need an effective fertilizer for plants growing in poor soil, buy one from a garden center. It will have the correct concentrations of all the nutrients your plants need during the growing season.