Coffee: grounds for good gardening.

Coffee: grounds for good gardening.

Nikki Milligan

Spring is here and the gardening season is starting, so what can you do with all those coffee grinds you’ve been saving up…. Well we have some options!

There are often a lot of questions about using fresh coffee grinds or used grounds in the garden. If you ask us, fresh coffee grounds are best used for making coffee. The garden will be quite happy to get what’s left over. And, for the most part, used coffee grounds are a great additive and have many benefits. 

There’s a general perception that coffee grounds are acidic, but this is not always the case. In fact, coffee grounds can vary from very acidic to slightly alkaline. So if you sprinkle used coffee grounds around, don’t expect them to acidify higher pH soils for those acid-loving flowering plants like azaleas, hydrangeas, and lilies. Blueberries like slightly acidic soil acidic soil, as do some vegetables, especially root crops, like radishes and carrots. They’ll respond particularly favourably if you add your grounds in with the soil at planting time. Tomatoes, on the other hand, typically don’t respond as well.

Composting

Used coffee grounds make great compost, and can be highly beneficial for your plants. The grounds enrich your compost pile, releasing the essential nutrient nitrogen, as well as some potassium and phosphorus, plus other micronutrients. Just throw any used grounds onto your compost pile and mix them in. And while you’re at it, you can throw your paper filter on there as well.

Remember, though, that despite their colour, coffee grounds are a ‘green’, nutrient-rich organic material. So make sure to mix them in with enough ‘browns’ – carbon-rich materials such as dried leaves or woody cuttings. Your compost heap’s tiny munchers will process them happily, breaking them down and helping to release the nutrients. 

Fertilizer

Some choose to put coffee grounds directly into the garden, using it as a fertilizer to enrich the soil. But that’s not exactly how they work. While coffee grounds will add nitrogen to a compost heap, they don’t contribute nitrogen to your soil immediately. Instead, they act more like a slow-release fertilizer--delivering small quantities of nitrogen, as well as potassium, phosphorus and other micronutrients over time. 

The real benefit of adding coffee grounds to your garden lies in their addition as an organic material, improving soil drainage, water retention and aeration. They encourage micro-organisms that are beneficial to plant growth. And they also attract earthworms, which further aerate the soil.

Mulch

You can also use coffee grounds in your garden as a mulch. Some gardeners are hesitant to do so because they worry coffee beans contain caffeine, which they fear suppresses the growth of some plants. However, the amount of caffeine actually remaining in used coffee grounds is minimal, and there is no verifiable proof that plants suffer from exposure to caffeine. That said, it’s probably better not to spread coffee grounds around seeds or immature seedlings, as they could be more sensitive than adult plants, and even trace amounts of caffeine could have a detrimental effect on germination and growth. 

One thing to watch for, when you mulch, is clumping. Since coffee grounds consist of fine particles, they are prone to locking together, and can turn into a barrier that resists water penetration. In the soil, this can block plant root systems and cause them to dry out, and in extreme cases, causing them to die of thirst. The solution is easy. Mix coffee grounds with other organic matter (like much of what’s already in your compost) before using it as a mulch. Alternatively, you can rake coffee grounds into the topsoil, being sure to scatter them well enough that they don’t clump together

Pesticide

Coffee grounds can act as a natural pesticide. Many gardeners like to spread used coffee grounds around plants that are vulnerable to slug damage. No one is sure why the sprinklings work. One theory is that the texture of the grounds is abrasive, and soft-bodied slugs don’t like to go through them. Another idea suggests the minimal amount of caffeine left in the used grounds is something slugs don’t like. Ants are another potential pest, and though they don’t particularly like coffee grounds, they won’t run out of your garden to get away from them.

Some people also claim that coffee grounds on the soil work as a kind of cat repellent, keeping felines from using your flower and veggie beds as a litter box. 

On the other hand, you can use coffee grounds as worm food if you do vermicomposting with a worm bin. Worms are very fond of coffee grounds. 

Happy spring everyone!