Coffee grounds are what’s left over after you’ve brewed a fresh cup of coffee. They are usually thrown away or added to a compost heap. But coffee grounds contain nitrogen, phosphorus, and potassium (NPK), which are the three primary nutrients in commercial fertilizers. So, why not add them to your garden soil?
It is generally agreed that coffee grounds are good for vegetable gardens. After all, the best fertilizers contain NPK in various ratios. But there is evidence that some vegetables don’t like coffee grounds. Researchers have also found that coffee grounds can reduce plant growth.
Are coffee grounds safe for your garden?
Coffee grounds are a natural fertilizer and, as such, are well-suited and safe for your garden. However, like everything in life, you need to use them in moderation.
For example, some gardeners report that using coffee grounds in thick layers around garden plants can have highly negative effects on plants and seeds. This might be because of the caffeine in coffee.
Additionally, the fine particles that form coffee grounds tend to lock together and form a barrier that resists water. This, in itself, can result in plants dying.
But if you mix your used coffee grounds with compost or some other organic matter before you use it as a mulch, you should avoid this problem. Otherwise, rake your coffee grounds into the soil so that they don’t clump together.
Dr. Linda Chalker-Scott from Washington State University Extension is sold on coffee grounds. She’s done loads of research and has good, solid advice.
For example, she warns that because they compact so easily, and create a barrier to air movement and moisture, coffee grounds should only be used in thin layers. She also advises they be covered with a much thicker layer of organic mulch like wood chips.
That would be a thin half-inch (max) layer covered with about 4 inches of thicker mulch.
A caveat to remember is that you should not use coffee grounds in the ground where you grow seedlings or very young plants. Also, tomatoes, lettuce, and several other veggies don’t seem to like coffee grounds.
More about that later. But if you do have coffee grounds, the good news is that you can keep them indefinitely. Don’t chuck them in the trash.
If you aren’t using them in your own garden or compost heap. Rather, keep them in sealed plastic bins or give them away to people who care.
What’s in coffee grounds?
We have mentioned that coffee grounds contain nitrogen, phosphorus, and potassium. But how do these nutrients help your plants?
Nitrogen encourages leaf growth and is good for the development of chlorophyll. This is, of course, what makes leaves and the other parts of plants green.
The Oregon State University (OSU) Extension Service points out that coffee grounds are not a nitrogen fertilizer, because they only contain about 2% nitrogen by volume. But they do provide an excellent nitrogen source for composting.
Phosphorus helps plants form new roots and make seeds, flowers, and fruit. They also use it to fight diseases.
Potassium is the nutrient that helps plants stay strong and keep growing. Like phosphorus, it helps them fight diseases.
An article on the University of Wyoming Extension’s website by Lisa Ogden confirms that there is approximately 2% nitrogen plus 0,06% phosphorus and 0,06% potassium by volume in coffee grounds. She maintains that they break down gradually and are released into the soil over time.
She also confirms that they make a great addition to compost.
But even though coffee grounds contain all three of these nutrients, it doesn’t necessarily mean you won’t ever have to buy a balanced NPK fertilizer for your vegetable garden ever again. You can read more in Risks of Coffee Grounds for Vegetable Gardens below.
Other nutrients this waste matter contains include calcium, iron, magnesium, boron, copper, iron, zinc, and chromium, depending on the references you source. Other compounds include fatty acids and essential oils.
Benefits of Coffee Grounds for Vegetable Gardens
We have already discussed the advantages of using coffee grounds to get beneficial NPK. Lisa Ogden, quoted above, maintains that coffee grounds improve the structure of the soil.
She also maintains that in addition to increasing soil water retention, they can moderate soil temperature. Additionally, they increase organic matter as they degrade and help to attract beneficial worms.
The OSU Extension Service quotes anecdotal evidence that suggests coffee grounds in your garden will repel snails and slugs. Others agree to say that they contain compounds that are toxic to many insects including fruit flies and beetles.
The University of Arizona Cooperative Extension agent, Jepp Schalau, states that coffee grounds appear to suppress some of the dreaded fungal wilts and rots. They also seem to have an effect on bacterial pathogens.
These nasties include Fusarium, Pythium, and Sclerotinia rot as well as E. coli and Staphylococcus.
There is also a suggestion that coffee grounds dug into the ground will help to absorb heavy metals that may contaminate the soil.
Risks of Coffee Grounds for Vegetable Gardens
The greatest risk of coffee grounds is shown by research to be related to growth suppression. This is the main reason we don’t recommend using coffee grounds for seedings and younger garden plants.
Sarah J. Hargrove and Stephen J. Livesley experimented by growing five different plants in sandy, sandy clay loam, and loam soils with and without used coffee grounds and fertilizers.
The plants they chose were broccoli, radishes, leeks, sunflowers, and violas and all of them grew poorly in soil that contained coffee grounds. On the plus side, weeds didn’t grow much either, and the water-holding capacity of the soil increased.
Growth suppression couldn’t be explained by a change in the pH of the soil or the availability of nitrogen. So, the researchers figured it was probably due to phytotoxic effects. These plant-toxic substances were thought to have been in the coffee grounds.
The OSU Extension Service did a germination test. They mixed potting soil with coffee grounds at a ratio of 25% by volume. Then they planted lettuce seeds in the mix and in potting soil without coffee grounds.
They also found that the coffee grounds plants had stunted growth when compared to those planted in ordinary potting mix. But maybe it wasn’t the coffee because other academics say that lettuce loves coffee!
Another possible risk is that because people know coffee grounds contain nitrogen, they think it will be a good nitrogen fertilizer. As we’ve already said, it isn’t.
Which vegetable plants benefit from coffee grounds?
Several university extensions have written articles about using coffee grounds. Drawing from these, we can say that there are quite a lot of vegetable plants that benefit.
For example, according to the University of Vermont Extension: Lettuce, especially, seems to benefit (from coffee grounds) and the grounds may benefit acid-loving plants since the grounds are slightly acidic.” They also say asparagus, potatoes, and cole crops like broccoli, cauliflower, and cabbage-like coffee grounds in the soil.
Many people believe that coffee adds acid to the soil, and certainly, acid-loving plants seem to agree. Blueberries, cabbages, carrots, and radishes are other vegetables that appear to benefit from adding used coffee grounds to the soil.
Even though plants that prefer acidic soil do seem to like coffee grounds, plants that like alkaline soil like it too! The simple explanation may be that despite what many people claim, coffee grounds have a pH between 6.5-6.8, which means they are close to neutral.
Additionally, the acid coffee contains is water-soluble, so most of it is in the coffee we drink.
What plants do not like coffee grounds?
We can safely say that all the plants coffee grounds research covered caused stunted growth. So, they don’t like this additive. We have mentioned several vegetables including lettuce, broccoli, radishes, and leeks.
Others include Chinese mustard, alfalfa, rosemary, lavender, and clover. Many consider non-edible plants like geraniums, asparagus ferns, violas, and sunflowers to be anti-coffee grounds garden style.
People frequently pinpoint tomatoes as vegetable plants that don’t like coffee grounds. But it’s not exactly clear why, especially since many people encourage gardeners to use coffee grounds around tomatoes.
So, are coffee grounds good for vegetables? Many people believe they are, and they base their judgment on personal experience.
But many different factors come into play, including the type of soil you are adding your coffee grounds.
It’s much the same with using eggshells in the garden as a mulch or to deter slugs and snails. Some people believe in them, others don’t. But there’s certainly a lot less scientific evidence for eggshell benefits than for coffee grounds.
We’ve decided to give Dr. Chalker-Scott, who confesses to being a non-coffee drinker, the final say.
While she likes the idea of using compacted coffee grounds to create barriers to air movement and moisture, she doesn’t like the idea of using them as a thick mulch.
Apart from this, we are all still waiting for more experimental research.