Many of us start the day with a steaming hot cup of coffee. Most even have a second one later in the day. As a result, we produce quite a lot of coffee grounds every time. Some people then throw the used coffee straight into the trash. Others, however, are exploring the amazing potential of these remains. They use them in homemade peeling creams and masks, as an odor neutralizer or to repel mosquitoes and snails. Many hobby and professional gardeners also put the coffee grounds in their composter and worm box or directly in the flower pots and beds. However, despite the popularity of this gardening trend, not all types of plants really benefit from coffee grounds. In fact, in some cases, this homemade fertilizer can do more harm than good. In today’s post, we want to dive deeper into the topic and give you the answer to the question “which plants don’t like coffee grounds?”.
Coffee grounds – fertilizer or rather not?
What does coffee grounds contain?
It’s safe to say that everyone knows how mankind’s favorite morning drink is made. The whole, roasted coffee beans are first ground into a powder and then placed in a fine metal or paper filter through which hot water is poured. This process pulls out most of the ingredients contained in the coffee.
However, surprisingly large parts remain in the pleasantly fragrant but unappetizing coffee grounds. These include caffeine, hydrophobic compounds (oils and essential oils, lipids, triglycerides and fatty acids), insoluble carbohydrates (cellulose and sugars), structural lignin and phenols.
Coffee grounds also contain large amounts of potassium, nitrogen, magnesium and phosphorus. That doesn’t sound so bad, does it? After all, plants need all of these minerals and nutrients to grow and thrive. The problem, however, arises from one of the components that makes coffee such a popular beverage – caffeine.
Spent coffee grounds contain about half the nutrients of freshly ground coffee
Did you know that coffee beans are actually the seeds of the coffee cherry?
Why is caffeine bad for plants?
Caffeine is a natural compound found in a variety of plants – tea plants, cocoa beans and of course coffee beans. In nature, it serves the purpose of a pesticide and herbicide. The soil in a caffeine-producing area is almost always barren because the compound inhibits seed germination and suppresses the growth of other plants. This effect is called allelopathy in biology.
Few other plant species grow around coffee plants
Although most of the caffeine is pulled out by the brewing process, 1 gram of used coffee grounds still contains up to 8.09 mg of caffeine. The average caffeine content of unused, fresh ground coffee is around 13.5 mg per gram.
Ironically, fresh coffee grounds work better as a weed killer than as a fertilizer. In order to be able to use it as the latter, it must first be completely decomposed. The easiest way is using a composter or worm bin. Apparently the coffee worms like it just as much as we do. However, coffee grounds should not make up more than 20 percent of the total contents of your compost or worm bin.
Coffee salt from strong coffees serves as a real plant killer
Compost instead of using directly, but only in moderation
The next best and easiest alternative is to use coffee grounds from the already decaffeinated coffee. The technologies used remove around 95 percent of the caffeine from the coffee beans. A decaffeinated espresso then only contains about 2 mg of caffeine. The caffeine left in the coffee grounds is therefore absolutely harmless to your plants.
You can fertilize with decaffeinated coffee grounds without much concern
Do coffee grounds affect soil acidity?
Many gardeners claim that used coffee grounds lower soil pH and increase soil acidity. This should make it the ideal fertilizer for acid-loving plants such as hydrangeas, rhododendrons, azaleas, lily of the valley, blueberries and carrots.
However, this assumption is based on a study that used freshly ground coffee powder. Used coffee grounds actually have a neutral pH of around 6.5 and do not affect soil acidity. Even if you sprinkle expensive, freshly ground coffee in your garden beds or flower pots, the watering will neutralize the pH value again within a very short time.
Spent coffee is actually not sour at all
Which plants don’t like coffee grounds?
There are certain ornamental and crop plants that will not benefit from coffee grounds in any form, be it caffeinated or non-caffeinated, as compost or worm castings. Studies have shown that it can stunt growth and slow down the germination process in these types of plants. Crops include primarily tomatoes, potatoes, peppers, radishes, broccoli, leeks, alfalfa and clover sprouts, and lawn grass.
As for ornamentals, violets, bluebells, crocuses, lavender, sunflowers, geraniums, feather asparagus, zebra hangingweed, orchids, periwinkles, ivy, tulips, daffodils, and carnations have all experienced adverse effects after using coffee grounds in their soil. Yellowing and drooping leaves and stunted growth are some of the first signs of an adverse reaction to coffee grounds.
Your plants will let you know immediately when they are uncomfortable
So we have already answered the question “Which plants don’t like coffee grounds?”. Unfortunately, recycling used coffee is not always as easy as it first appears. Not all plants benefit from the remaining nutrients.
Tea, cocoa and, of course, coffee plants can be fed with coffee grounds