Using Coffee Grounds in Your Garden

Your garden will thank you!

Coffee grounds are a great addition to the garden or compost pile and an easy way to make use of something that would otherwise end up in a landfill. Most coffee filters are also compostable. 

They add organic material to the soil which improves drainage, aeration in the soil and water retention. Used coffee grounds will also attract microorganisms such as earthworms which help plants to thrive and grow. 

Whether you’re spending your afternoons working in a greenhouse or taking care of a few plants on your balcony, used grounds can benefit your plants. 

So, when is it a good idea to use coffee grounds in your garden? 

There is limited research when it comes to using coffee grounds in the garden. Most of what’s available is:

  • tests to determine if grounds are acidic

  • what happens as grounds break down (they start to shift from acid to a more neutral pH)

  • testing the effects of grounds on various agricultural crops (depending on the plant it can enhances or deter growth)

They have a significant nitrogen content, which means they can help improve soil fertility. However, this doesn’t mean you want to rely entirely on them to feed your plants.

Coffee grounds are especially good for acid-loving plants such as azaleas, hydrangeas, lilies and blueberries. If you’re the proud owner of a vegetable garden, root crops such as radishes and carrots respond well. Especially when mixed with the soil at the time of planting. However, tomatoes are not particularly fond of coffee grounds.

Compost Them!

The safest way to use grounds is to add them to compost. It’s recommended that you use only 10 to 20 per cent for your total compost. 

“Any higher, and they might inhibit good microbes from breaking down organic matter, said gardener and writer Julie Martens Forney, “Another way to approach this volume is to add 4 parts shredded leaves to 1 part coffee grounds (by weight). Some folks still suggest adding lime or wood ash to the compost to offset the initial acidity of the grounds. You can do that, but it's not really necessary. If you want to do it, aim for a ratio of 1 cup of lime or ash to 10 pounds of grounds.”


Other Uses for Coffee Grounds

  • Many gardeners use leftover grounds as a mulch for their plants.

  •  If you have an outdoor garden it can keep pests such as slugs and snails away from your precious plants. The caffeine negatively affects these pests so they tend to avoid the soil where you’ve added grounds. 

    As of now, this is purely anecdotal. While many gardeners swear that slugs will steer clear of coffee grounds one gardeners experiments suggest it’s a myth. That same researcher did a similar test to see if ants are repelled by coffee grounds. Ants weren’t exactly fond of coffee grounds, but they didn’t disappear entirely.

  • Coffee grounds also make excellent worm food!

  • Some gardeners also claim that cats don’t like the smell of coffee. So, if neighborhood cats are finding their way into your garden this might help them stay away.

A Few Things to Keep in Mind

Many people have tried using coffee grounds and the results haven’t been great. It’s possible that the caffeine suppressed the growth of other plants to reduce competition for space, nutrients, water or sunlight. We can’t be sure how much caffeine is left after the coffee is filtered and some plants will be more sensitive to caffeine than others. 

According to Ann Marie Hendry at GrowVeg, coffee grounds shouldn’t be used alone for mulching. Like clay soil, they consist of very fine particles. This means they are prone to locking together and can create a barrier which prevents your plants from getting enough water. 

“The solution is to mix coffee grounds with other organic matter such as compost or leafmold before using it as a mulch”, she said. “Alternatively, rake [them] into the top layer of soil so that they can’t clump together. Variable particle sizes is key to good soil structure.” 

One study found that using spent coffee grounds resulted in poorer growth for broccoli, leeks, radishes, violas, and sunflowers in all soil types. On the other hand, they did improve the soil’s capacity to hold water and decreased weed growth. 

Their guess is that the poorer growth was due to some of the plant-toxic compounds which naturally occur in the grounds. So, if you aren’t getting the results you want, experiment with your plants and see what works best for them!

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