Are coffee grounds good for plants? Here's what gardeners and scientists say.
Should you use coffee grounds in the garden? The short answer is—YES, but do your research. Some plants and soils love used coffee grounds. Others…not so much. Use your spent grounds in the right ways to keep your green friends safe and thriving. We’ve got the dos and don’ts for you.
You’ve heard it before—plants love your used coffee grounds. We’ve even said it before. And it’s true! Some plants in some environments will love a sprinkling of used coffee grounds.
The key is to know which plants will thrive with a coffee treatment and which will not. In this article, we’ll walk you through the dos and don’t of using coffee grounds on plants.
What makes coffee grounds good for plants?
First off, we’re talking about used coffee grounds—the wet leftovers after you’ve made your coffee.
Unused ground coffee is not plant food! The wet, used grounds may still contain a little caffeine, but most of that magic lands in your coffee cup. As we’ll see, caffeine can stunt the growth of seedlings and cause other mischief in your garden.
What makes used coffee grounds good for some plants?
Gardeners have handed down their knowledge from generation to generation, and much of what we know about gardening is anecdotal. Recently, we’ve started to get scientific studies to validate—or invalidate—some of this evidence.
In gardening lore, used coffee grounds famously contain nitrogen and acid, which plants love and need. This is a key reason so many gardeners recommend the grounds for use in the garden with acid-loving plants, or in compost. The grounds are also said to contain phosphorus, potassium, and antimicrobial properties that may deter pests and weeds.
In gardening science, it seems that grounds are fairly close to pH neutral and only mildly acidic, with the acid decreasing over time. In terms of nitrogen, that too changes over time and circumstance, but science validates that used coffee grounds can increase the availability of nitrogen in the soil (1).
Also—worms love spent coffee grounds, so adding a thin layer to the soil can attract local worms. Even better? If you have a worm bin, add a small amount of spent coffee grounds each week.
What types of plants like used coffee grounds?
Gardening lore says that acid-loving plants enjoy the coffee ground treatment. Science tells us that spent grounds are less acidic than we think. There may be other factors that determine whether grounds will work well, including whether or not a plant likes dry or moist soil.
Many gardeners and garden scientists agree that you can test your grounds on these types of plants. The key word here is test. So much of gardening is specific to the type of soil you have and the conditions in your area.
And as we’ll see below, use the grounds on established plants and not seedlings.
Fruits and veggies. Try a thin layer of spent grounds on established blueberries, carrots, and radishes.
Flowers and plants, like hydrangeas, rhododendrons, and azaleas.
Water-loving plants, like Lily of the Valley.
Test other plants with weaker “coffee ground tea.” A weaker mixture of spent grounds soaked overnight in water can work well on a larger group of plants. You can even add this mix to a spray bottle and apply it directly to leaves to deter pests. Again, always test first!
What plants do not like coffee grounds?
At the risk of repeating ourselves, avoid using fresh coffee grounds on any plants. And you probably don’t want to use the wet, spent grounds on seedlings.
Other than that, the science has been mixed about which plants are definitely not coffee drinkers. According to The Spruce:
One 2016 research study found that using spent coffee grounds in growing broccoli, leek, radish, viola, and sunflower resulted in poorer growth in all soil types, with or without additional fertilizer. The good news is that the coffee grounds improved the water-holding capacity of the soil and decreased weed growth. (2)
For these plants, you could run your own tests, using the weaker coffee ground tea described below in the areas surrounding the plants—to improve pest control and encourage worms. But you may want to avoid using it directly on the plants or in the soil where they are planted.
How to use coffee ground in gardening—dos and don'ts
If you’ve done a little research on the topic, you probably came away confused. Should you use grounds in gardening or not!? And how should you do it?
Here’s the bottom line: Gardeners have been touting this trick for ages, so there’s definitely some truth to the method. Gardeners also live in very different climates with very different soil, wind, and rain conditions and grow different plants. What works in one circumstance may not work in another. So think of this as trial and error based on research, and check out these dos and don’ts.
Do THIS WHEN YOU ADD COFFEE GROUNDS TO SOIL
This list of dos consistently shows up in both anecdotal accounts and scientifically-validated studies.
Test your soil. Your soil is uniquely composed of a set of compounds that may already be excellent for growing a garden or may need a boost. It’s best to find out what your garden wants if you’re not committed to the error part of trial and error. You can pick up a soil testing kit at most garden supply stores.
Allow grounds to cool before using. Heat can kill the good stuff in your compost or soil.
Use grounds in compost, but only about 20-30 percent of the compost. Make sure you’re adding a carbon source, like dried leaves or paper, and other organic material.
If applying directly to the soil, apply sparingly and mix in. No compost bin? You can apply the grounds directly to the soil in a thin layer. Avoid areas where you want seedlings to grow and be sure to mix in with the soil and other fertilizer. This will keep your grounds from drying out and repelling water.
Use in mulch. Similar to adding directly to your garden, you can add a layer and cover it with other mulch material. The grounds will mix with the mulch providing extra nutrients and pest control properties.
Use as a coffee ground tea. Put a few tablespoons of spent grounds in a spray bottle and fill it with water. Let the mixture sit overnight, then you can use the liquid to spray the soil and leaves of indoor and outdoor plants. We recommend you test it first. Your plants will let you know if they like it! (3)
Do Not DO THIS WITH YOUR COFFEE TREATMENT
Don’t use it with seedlings. Used grounds may still contain a little caffeine, which may stunt the growth of seedlings. This isn’t true for all seedlings, and small amounts of grounds could even boost some growth, but this is still an area of debate, so be ready for error if you want to experiment.
Don’t use too much. If you have a coffee habit like ours, you have a lot of grounds! You may be tempted to find uses for them in your garden. But most studies say it’s best to keep the grounds other material at about 20 percent and no more than 30. (4) Never fear though, you can store them separately to add to your compost or mulch as needed. (5)
Have other tips for using grounds in the garden?
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