Coffee grounds are a staple in the gardening world, touted as a cure-all for various gardening woes. From fertilizing to mulching to pest control, it seems like there’s nothing a spoonful of used coffee grounds can’t do. But do these claims hold any scientific weight?
Gardening myths abound, handed down through generations without much scrutiny. And when it comes to gardening hacks like eggshells, Epsom salt, and especially coffee grounds, some claims have a hint of truth while others are completely false.
So, let’s find out when coffee grounds can help your tomato plants and when they might do more harm than good.
Should You Use Coffee Grounds for Tomatoes?
…To Fertilize Plants?
The most common claim is that coffee grounds can be used as a nitrogen-rich fertilizer or to jumpstart the growth of tomato plants. There’s some truth to this claim, but it’s not as miraculous as it sounds. Coffee grounds contain around 2% nitrogen, along with variable amounts of phosphorus and potassium, which are essential nutrients for tomato plant growth. As the grounds decompose, they release these nutrients into the soil, making them available to the plant.
In theory, coffee grounds should work as a slow-release fertilizer when added to the soil. However, their nutrient concentration is much lower than that of regular slow-release fertilizers specifically designed to improve plant growth. While coffee grounds may provide a small boost of nutrients over time, they are unlikely to have a significant visible effect compared to other organic fertilizers. So, if you’re experiencing growth or nutrient deficiencies, coffee grounds may not be the solution.
…To Acidify Soil?
The second claim is that coffee grounds can lower the soil’s pH, making it more acidic and beneficial for tomato plants. Although tomatoes prefer slightly acidic soil, coffee grounds may not be the best way to achieve that. The average pH of coffee grounds is around 6.5, slightly below neutral. However, this pH level varies, and some studies suggest that coffee grounds can actually be alkaline.
Moreover, the effects of coffee grounds on soil pH are unclear and inconsistent. Even if the grounds are acidic, there’s no strong evidence that adding them to soil significantly impacts the pH in the desired manner. To effectively lower soil pH, it’s best to use gardening products specifically designed for that purpose and conduct a soil test to determine the cause before making any unnecessary amendments.
…As Mulch?
While many organic waste products, such as dried leaves or straw, work wonderfully as mulch for tomato plants, coffee grounds don’t make the list. On paper, they sound like an excellent choice for mulch—fine-textured, organic, slow to decompose, and nutrient-rich. However, they have some unintended negative side effects.
To achieve the full benefits, you need a thick layer of mulch, at least an inch or two. But saving enough coffee grounds to create such a layer is challenging. Additionally, when applied around tomato plants, coffee grounds compact and form a hard layer that prevents the soil from receiving moisture and air. This lack of air can deprive the roots of oxygen, essentially suffocating them.
Coffee grounds are still a great soil amendment, but they are not suitable for creating an effective mulch layer. Instead, sprinkle a light layer of coffee grounds on the soil before adding another organic mulch, or mix the coffee grounds with the mulch before application. This way, you can enjoy the benefits of coffee grounds without the negative side effects.
…To Stop Weeds?
Similar to the mulch argument, coffee grounds are often suggested as a weed prevention tool. Unfortunately, this claim is not entirely true either. To effectively suppress weeds, you would need a thick layer of mulch, which, as discussed earlier, is not ideal for your plants.
The argument also stems from the belief that coffee grounds can inhibit the germination of some weed seeds. While this is true for certain weeds like clovers, the effects on a range of weed seeds are unknown. The amount of coffee grounds needed to suppress weeds is unclear as well, whether for inhibiting germination or as mulch. It’s better to rely on approved weed-suppressing methods and manually remove weeds as they emerge.
…To Kill Slugs?
Slugs are a common problem in tomato gardens, and various remedies are suggested to combat them. We’ve already debunked the claim that crushed eggshells can kill slugs. Now, let’s address the myth surrounding slugs and coffee grounds.
This claim originates from the caffeine content in coffee. It is true that high concentrations of caffeine can kill slugs, as research has shown. However, coffee grounds contain such minimal amounts of caffeine that it’s unlikely to have any effect on slugs or snails. The concentration of caffeine used in studies that had no effect on snails (0.01%) is still greater than the amount found in coffee grounds.
In other words, if you’re dealing with a snail problem, you’ll need to find a remedy outside of your coffee cup.
…To Prevent Fungal Diseases?
Fungal diseases pose a significant threat to tomato plants, and coffee grounds are often suggested as a preventive measure. Some studies have examined the effects of coffee on fungal activity in soil and linked caffeine to the suppression of various fungal strains. However, the concentration of caffeine needed to have an impact is much higher than what coffee grounds offer. For example, one study used a concentration of 0.1% caffeine.
Other studies indicate that soil amendments, including coffee grounds, can increase microbial activity, which may help suppress fungal diseases. However, these studies mainly focus on specific fungal strains and the impact on other fungal diseases remains uncertain.
While adding coffee grounds to your soil mix before planting is unlikely to cause harm, don’t solely rely on it to prevent or kill fungal diseases.
…In Compost?
Out of all the claims, the benefits of adding coffee grounds to compost are 100% true. A healthy compost pile requires a balanced ratio of carbon-to-nitrogen materials for efficient breakdown and nutrient enrichment. Coffee grounds are considered a “green material,” rich in nitrogen, making them an ideal addition to compost.
Adding around 20% coffee grounds to your compost mix is optimal. However, it’s essential not to exceed 30% as higher concentrations can inhibit decomposition. So, instead of wasting your coffee grounds on gardening myths that don’t deliver, save them for your compost pile. This way, you’ll ensure successful results without worrying about any negative side effects.
To learn more gardening tips and tricks, visit Marmalade Cafe.
Remember, while coffee grounds may have their benefits, they aren’t a magical solution for all your tomato plant needs. It’s always best to rely on scientifically proven methods and seek professional advice when faced with specific gardening challenges. Happy gardening!