Is Watering Your Plants With Coffee A Good Idea Or A Recipe For Disaster?

Coffee isn't just delicious, it's also regularly considered a garden staple. Considering all the unexpected uses for leftover coffee grounds, it may seem harmless or even beneficial to water your plants with coffee if you have some left over in the carafe or pot at the end of the day. Unfortunately, regularly watering your plants with coffee can cause more harm than good, as research has found that it can stunt or even kill young plants.

Caffeine, part of the reason coffee is so adored by humans, is also the major reason it can cause serious problems for your plants. Coffee, tea, and cacao plants likely produce caffeine not just because it is an insecticide, but also because it can stunt the growth of other plants around them, decreasing competition. With the average cup of coffee containing around 140 mg of caffeine, this can quickly spell bad news for any plants you share your drink with. While there is caffeine in spent coffee grounds as well, it is in far lower quantities. 

While regularly watering your plants with coffee is a recipe for disaster, small and occasional amounts of the drink are unlikely to cause serious harm, so don't panic if you've already given your plant a drop of your leftover beverage. Don't forget, though, that a cup of coffee you've made for yourself may not only contain more caffeine than your plants can handle, but also sugar and milk. This could encourage fungal growth and possibly attract insects.

If you've given your houseplants or container plants a large amount of coffee and they appear to be suffering as a result, you can try flushing the unwanted drink from the plant's growing medium. This strategy is often successful for saving plants that have been overfertilized, so it may allow you to salvage your over-caffeinated plants as well. To flush the growing medium, slowly and repeatedly pour pure water around the base of the plant until water runs freely through the drainage holes at the bottom. This should leach many unwanted chemicals out and give your plant a fresh start. 

There are serious downsides to regularly watering your plants with coffee, and even adding too many spent coffee grounds directly to your plant's growing medium is something you should never do. But that doesn't mean that coffee can't be useful in the garden. The best way to make coffee a garden friend instead of foe is by adding it to your compost bin instead of giving it directly to plants. By adding the important step of composting when using coffee in the garden, you can help ensure the nitrogen is accessible to your plants and that they aren't overwhelmed and harmed by the coffee.

Coffee can also be an effective slug killer. Oregon State University recommends using 2 cups of coffee to 1 cup of water to create a soil dredge and adding it to gardens where slugs have been an ongoing problem. Be careful not to overuse this mix, though, and don't put it too close to seedlings or other plants that could be vulnerable to damage from the caffeine.