You can use coffee grounds for plants in a variety of ways. Use coffee grounds as a top dressing, alongside plants that are mulched to help attract earth worms, and to break down for nitrogen. Use as a compost addition, to help the brown elements in compost break down into rich soil, and use them in many other ways as well.
There are many options to use coffee grounds for plants, either in your garden, container garden, or just as a nutritive addition to shrubbery and landscape plants. The small amount of coffee waste from daily use is great to add directly to plants, either indoors or out. If you want to bring in coffee waste from an outside source, like a cafe or restaurant, then a bit more care to get the most out of the grounds is a great idea.
There are a few traditional ways to use coffee grounds for your garden and outdoor plants. One recommendation is to only add coffee as a direct addition to established plants. Avoid adding it to seed starting mixes or areas, and avoid adding it close to young seedlings.
Coffee contains a decent, but variable, level of nitrogen which is a valuable resource for leaf based plants. Adding it to flowering plants, without compensating with potassium for flowers, may cause excessive leaf growth and reduce blossoming. If you don’t like gardening by the seat of your pants, get a soil analysis done before adding amendments to your soil.
Coffee is, by definition, plant material, and can help improve the water retention value of sand or clay soils as well as adding nutrients. It can also help improve the soil structure, the same as adding other organic matter. However, this addition works best alongside mulching, compost, and other soil amending techniques.
Coffee Grounds for Acid-Loving Plants:
Coffee used to brew espresso has a higher residual pH than coffee used to brew drip coffee, or mocha pot coffee. A top dressing of coffee grounds, mixed with your other mulch, can help plants that like an acidic environment. Always mix coffee with a secondary mulch like straw, shredded paper, shredded leaves, or grass clippings, to help spread out the coffee’s affect, and to encourage fungal growth as fungi can break down the coffee grounds better than just soil microbes and worms.
Plants that like some soil acidity include blueberries, azaleas, hydrangeas, some conifers, beach, willow, oak, mountain ash, dogwood, and even magnolias. Daffodils, nasturtiums, and even rhododendrons like a bit of acidity. However, always do a soil pH check before adding any amendments to your garden, even something as common as coffee grounds.
Coffee as a General Amendment:
As long as it’s worked into the top layer of soil, or very lightly spread, coffee can work as a general additive for many plant types. It can be lightly sprinkled over a lawn, for a boost, as long as the lawn is well established and older grasses. Newly seeded lawn will have trouble with germination, and could have failure for the grass to thrive from having coffee grounds applied due to the caffeine being a natural germination and growth inhibitor to some plant species.
Coffee can also be added near the root zone of brassica plants, such as kale, and other leafy greens. It can be added to potato hills, alongside compost and mulch. Avoid adding too much coffee near flowering or fruiting plants, as too much nitrogen can encourage leaf growth to the detriment of flowers and fruit. Some recommend using coffee for tomatoes, while others say the tomatoes don’t like it. You can experiment, mature plants may like it while seedlings may have a negative reaction to the caffeine.
Coffee may also do well in the root zone of low acid fruits, like elderberries. However, too heavy a layer of coffee can be detrimental to rose family plants, like apples, roses, strawberries, or hawthorn. For these plants, including coffee in the compost pile will be the most reliable.
Coffee in the Worm Bin or Compost Bin:
Some earthworms like more acidic environments, like red wiggler worms that like breaking down manure. Other worms may not have the tolerance for high acidity. If you’re working with primarily red wriggler worms in your compost, they can break down a decent amount of coffee grounds per month. I would only start with small amounts, to make sure the worms tolerate it well. The paper coffee filters, if you use them, can be added to the worm bin or compost as well.
When using vermicomposters (worm composting), make sure to balance any addition of coffee with plenty of food scraps, some egg shells for calcium, and maybe even some dandelion or wilted nettle leaves for minerals for the worms. Well rinsed coffee grounds, like those through a drip coffee machine, will have a nearly neutral ph, despite the coffee beans themselves being acidic. Coffee acids are water soluble, and can also leach out of soils over time.
For other varieties of worms, like your normal large garden worms, mixing coffee grounds into courser mulch is a good way to add them to the soil without overwhelming either the worms, or other soil microbes. Remember that coffee is a green material for compost and needs to be balanced out with brown compost material, like dried grasses and leaves.
Coffee is decently high in nitrogen, and counts as a green compost material when adding to your compost bin. If you have a lot of “brown” materials, adding coffee can help your compost heat up faster, and kill weed seeds better. Finished compost will work better than commercial slow-release fertilizer, while adding direct coffee grounds to the soil will also provide some slow release nitrogen and micronutrients.
Coffee as Mulch and Pest Repellent:
Coffee works well as a top dressing that’s mixed into the surface of the soil. Mulching with it, in a heavy layer, can cause the same issues as clay soil. The water sheets off as the coffee particles compress. This can cause a lack of water to the root zone of the plants, and plant stress. If you want to include coffee in a mulch layer, sprinkle it on top of a straw mulch. Or, use it as a top dressing with grass clippings, and rake it into the soil surface to prevent particle compression.
Coffee is said to repel some pests, and the sometimes strong odor of coffee can help reduce flying pests around certain plants. I’ve read that it can help reduce the number of squash bugs that locate your squash plants. As well as semi protecting seedlings from slugs and snails. Now, it’s not 100% a repellent. It’s more the scent of the coffee that helps protect plants, the same as companion planting with strongly scented herbs.
For a pest repellent, I would place 1/4 inch of coffee grounds around the seedlings, at least 3 inches away from the stems. Once the seedlings are established, about one to two weeks, I would then rake the surface of the soil to make sure the coffee doesn’t form a solid, water-repellent layer. This is also a good use for flavored or spiced coffees, as the additional scents will help confuse pests more.
Amending Indoor Plants with Coffee:
I have amended some of my indoor plants with coffee grounds, however, it is recommended to only add coffee to indoor plants periodically, once or twice per year. I use about 2 tablespoons of coffee twice a year, for a 1.5 gallon potted plant in addition to a regular indoor fertilizing schedule. I wait until the plant is at least one year old before adding coffee grounds to their soil. Usually, I blend the coffee into fresh potting soil either when changing soil, or potting the plant up into a larger container.
If used on too young a plant, coffee can reduce growth until it breaks down adequately for the plant to use it. I would not mix coffee into seed starting mix, or soil mix for rooting plant cuttings.
Coffee grounds will break down slower in an indoor pot, compared to an outdoor garden. This is because the indoor pot relies on microbes and bacteria, and doesn’t have the fungi and worms that the outdoor garden does. This is why the recommendation for adding coffee to indoor plants has such a low volume of coffee grounds being added.
Concerns with Using Coffee Grounds for Plants:
There are some studies that demonstrate an anti-growth factor from the caffeine in coffee grounds, that affects most plant types. It can reduce seed germination rates, and slow general plant growth even with the addition of other fertilizers. Coffee from brewing espresso remains higher in acidity, and in caffeine, than grounds from a drip coffee maker, or mocha pot, due to less water passing through the grounds. Most of coffee’s anti-growth factor can be easily removed by using home brew coffee grounds on the garden, and not bringing in the coffee shop “coffee pucks” from the espresso machines. Caffeine is a growth inhibitor for many plants, and also part of the coffee plant’s natural insect defense. Using coffee that’s been well leached or composted is the best bet for avoiding nearly all negative effects of residual caffeine on garden plants.
There are also some recommendations to use coffee as a plant mulch. However, the fine particle size, especially of the espresso grind, can compact and prevent water from penetrating the soils. They can also be too strong in caffeine, and acidity, if it’s the fine grind, and thus negatively affect desired plant growth. However, using coffee grounds in paths in connection with wood chip, straw, cardboard, or other mulches would help reduce weed growth and could be beneficial in the long run to your garden.
Finally, there are concerns that coffee grounds in compost could negatively affect worms, while other sources say worms love coffee. If coffee grounds are spread out, so that water can run through them, before adding to the compost or worm bin, the acidity and caffeine that can negatively affect the earth worms can be removed. Again, grounds from a drip coffee maker, or mocha pot, will have less acidity and caffeine left in them than espresso grounds from a coffee shop.
Remedying Concerns when Using Coffee Grounds for Plants:
If you’re worried about the caffeine, anti-bacterial properties, and acidity of the coffee, as well as the residual caffeine’s potential to harm worms, soil microbiota, and plant growth, using mushrooms as a first stage break-down will remove all of those issue. Some mushrooms, like oyster mushrooms, will grow on coffee and help it break down.
Other mushrooms like King Stratopharia will grow on straw and wood mulch. Adding coffee to mulched paths, or a lasagna garden bed seeded with these mushrooms, will help the coffee not affect plants. It will also give you tasty mushrooms throughout the growing season. The mushrooms themselves can also help boost plant growth, as they breakdown the organic matter near the plants and encourage worms and other beneficial microbes to thrive near them.
So if you don’t trust the acidity of coffee grounds as a garden addition, then using mushrooms as a primary decomposer, enjoying fresh mushrooms, and then composting the coffee may be a great method for you to use. Bonus, worms love mushrooms and spent mushroom mycelia makes great worm food, compost additives, and even mulch.
Used wisely, coffee is a great addition to garden nutrition, and will help boost plant growth and productivity. Paired with something like oyster mushrooms, or wine cap mushrooms, coffee can even help you add an entire new layer of harvesting and produce to your gardening endeavors.