QUESTION: Are coffee grounds good for herbs? I was thinking that maybe I could add them to my herb containers. – Lisa A
ANSWER: To put it bluntly, no—coffee grounds are not good for herbs, and they should be used with care around the plants that do benefit from them. It’s true that generations of gardeners have amended their soil with coffee grounds or sprinkled them around plants because they believe the nitrogen in coffee grounds helps plants to flourish. Because coffee has been used this way so long, you’ll see it recommended as a fertilizer, mulch, and composting ingredient all over the web. However, coffee grounds are extremely acidic, so they’re best used around plants that thrive in acidic soil, like blueberries, rhododendrons and azaleas — if you use them at all. Take care if you choose to use them this way, though, and use a light hand, because coffee is so acidic that the pH level of some grounds have tested as even more acidic than acid-loving plants can stand.
If you want to use coffee grounds to nourish acid-loving plants, experts recommend using it as an ingredient in compost and balancing it out with a cup of agricultural lime or hardwood ashes for every 10 pounds of grounds. If you don’t have a compost pile going already, you can make a quick mix of coffee grounds and lime or ashes. Use shredded leaves mixed with 10 to 25 percent coffee grounds to make a balanced compost. If you are already making compost and just want to add a small amount of coffee grounds here and there, that’s perfectly fine. They count as a green (nitrogen rich) ingredient.
That’s not the only reason to be careful with coffee grounds, either. If your soil already has enough nitrogen to fulfill your plants’ needs, adding more with coffee can actually stunt their growth. Additionally, nitrogen does help plants to grow larger, but too much nitrogen can inhibit production of fruit or flowers. And it’s not only that—like coffee, coffee grounds are full of caffeine. Even after the liquid is squeezed out of the grounds, they still contain as much caffeine as a cup of tea does. Although some people may think caffeine would help plants grow more quickly, plants that produce caffeine actually suppress the growth of competing plants in their vicinity.
Coffee grounds also have antibacterial properties, which kill off beneficial organisms that help break down organic material in soil so plants can access the nutrients. One study showed that when they were used in compost, coffee grounds killed earthworms, too. They can also cause a mold bloom where they’re spread due to their acidity and high level of nitrogen. For all these reasons, you’re better off nourishing your herbs with another type of fertilizer entirely.