Home & Garden Gardening

How Safe are Coffee Grounds, in the Garden?

Question: Are Coffee Grounds Useful and Safe to Use in the Garden?


There are many claims about the affects of using coffee grounds in the garden, from disease control and to enhanced growth, to improving soil tilth. Most are still being studied and outcomes tend to vary widely.


A common concern about using coffee ground in the garden is that they will lower the soil's pH. Research suggests that no significant pH change takes place, because as the grounds decompose, their pH neutralizes.

Coffee grounds do release nitrogen into the soil, an element that tends to be very dynamic. They are certainly easy to make use of. Is it worth it? Well, it's a free source of nitrogen and if you don't use them, you will just have to throw them away. As long as you don't overuse them, coffee grounds can be a good supplement, for many types of. Let your plants be your guide.

5 Ways to Use Coffee Grounds in the Garden

  1. Add to Compost - Coffee grounds are a nitrogen source, so even though they are brown, they would be considered a green composting material, like plant debris and grass clippings. To actively compost, most experts recommend about 1 part green material to 2-3 parts brown, like leaves.
    Even if, like me, you don't adhere to active composting ratios, your coffee grounds should not comprise more than 20% of your compost pile. More than that could slow the composting process and otherwise negatively impact it.

    I often get asked whether you can also compost coffee filters. I haven't found a lot of research on the topic, but the one caveat seems to be whether the filters are bleached. The white filters could potentially have been treated with bleach or synthetic chemicals. If you are a strict organic gardener, you may want to avoid these. According to Colorado State University Extension "...coffee filters and paper towels may have been treated with synthetic chemicals and bleach. Those attempting to maintain an organic garden will need to consider these possibilities before tossing items into the pile."

    However other sources I've read think the bleach will probably have dissipated by the time it makes it to your compost and won't harm your soil or you. If you are concerned about the bleach, there are unbleached filters available. Of course, if you used a reusable filter, this conversation would be moot.

    On the plus side, coffee filters compost very quickly and the worms seem to love them. They would be considered a dry ingredient, which can be hard to come by in the lush peak of summer.

  1. Add to Worm Bin - It seems even worms like a bit of coffee, now and then. Just don't give them more than 1 cup per week and not all at once. The worms can't handle excessive acidity, so don't be tempted to add more.
  2. Mulching - As the organisms in the ground slowly break down the coffee grounds, it adds nitrogen to the soil and improve the overall soil structure. Earthworms also help work the grounds into the soil, further improving its texture.
  3. A thin layer of coffee grounds not only improves the soil, the abrasive, sharp edges and coffee's natural acidity combine to make a good slug barrier. In fact, research suggests that caffeine is toxic to slugs, making it a double whammy. I use them around my Hosta plants.

    Don't add a think layer of coffee grounds, because they will compact and form a solid crust that won't allow air or water through. One inch of grounds will do the trick.
  4. Side Dressing Plants - Side dressing is kind of like supplemental mulching or feeding. You add just a little material at a time, to boost what is already there. As with mulching, only add about one inch of coffee grounds at a time. You might even want to mix the grounds with your compost and side dress with that.
  5. Liquid Fertilizer - Steep 2 cups of grounds in a 5 gallon bucket, for 6 - 12 hours. Use to water and foliar feed your plants. The benefits of foliar feeding are the subject of much debate, but as always, let your plants be your guide. If they do not appear to be doing well after feeding, stop feeding.
    Making a liquid fertilizer from steeping the used grounds is not quite the same as using left over coffee to water your plants. Left over coffee is more acidic and has other compounds in it that are removed from the grounds. Some acid loving plants enjoy the occasional watering with coffee, but they take their coffee black. Skip the sugar and cream.

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