If you’re a regular coffee consumer, you might feel guilty about dumping several scoops of used grounds into the trash every morning. Gardening websites have cited used coffee grounds as good fertilizer, especially when mixed with compost, and you might be wondering if coffee grounds would make good plant food for peonies.
However, acidic coffee grounds aren’t a good match for this flower, which prefers a relatively neutral soil of 6.5-7 on the pH scale.1 While you shouldn’t scatter them directly on your peonies, the acidity may not cause harm when used in compost if the other components in your compost balance the pH to a neutral level.
Even so, coffee grounds retain residual caffeine, which might eventually kill your plant. Keep reading for alternative caffeine-free compost ideas to enrich your peony’s soil, as well as some reasons you might want to still use coffee grounds in your compost, albeit in limited amounts.
How Much Caffeine Is in Used Coffee Grounds?
Even after you’ve made a fresh brew, the coffee grounds stubbornly keep a moderate amount of caffeine. It’s estimated that half a tablespoon of coffee grounds contains 41 mg of caffeine. That’s a pretty high amount, especially considering you probably use more than 1 ½ tsp. of coffee grounds to make your morning brew.
Coffee grounds are controversial in the gardening community. While most people agree that coffee accelerates plant growth, some studies concluded that its effects are only temporary due to the nature of the caffeine. Several studies support the idea that caffeine boosts plant growth for a time, but often the plant quickly turns brown and dies after its brief growth spurt. Other studies have offered caffeine’s natural insecticide properties as reasons for its continued use, and yet others raise the question of whether some plants may tolerate coffee grounds better than others.
What Type of Fertilizer to Use for Peonies
Regardless of caffeine’s effects on plants, coffee grounds aren’t good for peonies—at least not directly—because coffee grounds have a fairly acidic pH level of around 5. Peonies require only a slightly acidic to relatively neutral soil pH hovering between 6.7-7.
If you’re researching coffee grounds as fertilizer, chances are you want to invest in a natural, cheap fertilizer that doesn’t use sketchy pesticides. Here are some excellent additions to mix in your compost that will benefit your peonies (no coffee grounds required):
Even though your goal is for the compost to have a neutral pH, you still want to add acidic components such as citrus and vegetable peelings for a well-balanced, nutritious mix.
Should I Ever Add Coffee Ground to My Compost?
Given the mixed results on caffeine’s effects on plant growth, it’s difficult to say whether coffee grounds are more helpful or hurtful. They contain an excellent source of nitrogen and antioxidants, which plants need, but the caffeine is questionable. You also shouldn’t directly add coffee grounds (or compost containing an excessive amount of coffee grounds) directly to plants requiring a neutral or basic soil pH such as peonies.
Blueberry bushes actually need a fairly acidic soil, preferring a pH between 4.3-5.5. You can try to mix a little coffee grounds into the compost for your blueberry bush, but don’t go overboard. Decaf coffee grounds would probably be a better choice since it still contains the acidity and antioxidants without as much caffeine. If you don’t drink decaf, ask your local coffee shop if they’ll save their grounds for you.
Peonies require a soil pH of 6.5-7, so you shouldn’t add coffee grounds to their soil directly since coffee is around 5 on the pH scale. However, there’s some debate as to whether coffee grounds are an appropriate additive to compost at all because some studies hint that caffeine may eventually kill plants. In moderation, coffee—especially decaf—may be beneficial to your compost because it lends many nutrients. You’ll just need to watch out for the caffeine levels and not feed it to your plants that thrive on neutral-basic soil.
You may also be interested in: Are Coffee Grounds Good For Crepe Myrtles? What to Know!
Featured Image Credit: Christiane, Pixabay