Many gardeners add spent coffee grounds to their soil. These are easy to come by in most homes and are said to benefit plants such as hostas—but there is no scientific proof of them doing so.
Spent coffee grounds are beneficial to soils when used in moderation after being composted. Though there have been no studies done on coffee grounds and hostas, we can say that they will benefit plant growth if mixed into the soil properly.
Otherwise, studies show large amounts of coffee grounds can reduce plant growth, as can fresh grounds.
Below, we’ll look at the studies done to see how coffee grounds may be a benefit or detriment to your hostas.
Are Coffee Grounds Good for Hostas?
The simple answer is yes, coffee grounds are good for hostas when used correctly. However, that doesn’t mean you should rush to dump all your spent grounds into your hosta garden.
When used incorrectly, coffee grounds can have adverse effects and even potentially kill your hostas.
Below we’ll look into the benefits and drawbacks of using spent coffee grounds in hosta soil, and how to best disperse them into your garden.
- Coffee grounds are a good source of nutrients, such as iron, zinc, and nitrogen. When used correctly, in moderation, they benefit the soil and will help your hostas grow.
- Coffee grounds can compact and form clumps that hold too much moisture and prevent air flow. They need to be mixed into either soil or mulch, never layered on top of the soil alone. Thick layers of coffee grounds can prevent evaporation from your soil and lead to root rot for your hostas.
- Limited amounts of coffee grounds in the soil, especially when mixed with other compost, can help to retain moisture and control soil temperature.
- Fresh grounds can hurt plant roots, so composting your coffee grounds before adding them to your garden is best.
- Hot coffee grounds can harm your compost or soil. To prevent this, wait for them to cool before adding them to your mix.
- Just like any fertilizer, coffee grounds can harm your plants if used in excess or in already nutrient-rich soils.
- Coffee grounds in soil have been shown to decrease seed germination rates. If you’re starting hostas from seed, wait to add coffee grounds until the plants are bigger.
- Studies have shown that use coffee grounds do not make soils more acidic. If you would like to add them for this reason, choose another soil amendment.
- There is no scientific evidence that coffee grounds deter snails and slugs. Keep this in mind if you choose to use them for pest management.
Limited studies have been done about coffee grounds and plants, and there are no studies that observe the effects on hostas in specific.
What do Coffee Grounds do for Soil?
Although we don’t have enough studies to say for sure what coffee grounds do to hostas specifically, we can look at what coffee grounds do for garden soil.
By connecting the dots between good hosta soil and what coffee grounds do for soil, we can get closer to the correct answer to the question: do hostas like coffee grounds?
Fresh coffee grounds have can have phytotoxic effects on soil, and aren’t recommended for use in your hosta garden.
However, composted coffee grounds can be good for soil in moderation. You want your coffee grounds to make up less than 20% of your compost and to limit the amount of compost put in or on your soil.
This is because large clumps or layers of coffee grounds will stick together, preventing adequate airflow and water evaporation in your soil.
Proper amounts of composted coffee grounds add nutrients to the soil, retain water similar to peat moss, help to control soil temperature, and more.
Like any other soil additive, it’s important to know what you’re using coffee grounds for and how much of them to add to benefit your hostas without harming or destroying them.
For example, adding coffee grounds to an already soggy soil mixture is likely to kill your hosta garden, as will adding too many. But if your soil is very sandy and doesn’t retain moisture well, some coffee grounds will help your hostas tremendously.
The same goes for nutrients. Just like it can be bad for humans to take vitamins we don’t need, hostas shouldn’t be given nutrients or fertilizers for the sake of it, but because we know their soil is lacking.
Do Coffee Grounds Kill Slugs?
It’s commonly believed that coffee grounds will prevent slugs and snails from snacking on your hostas, but is there truth to this?
It’s only partially true. While caffeine does kill and deter slugs, spent grounds may not have enough caffeine to deter them from eating your plants. Coffee grounds may kill slugs over time rather than right away.
Since there are no formal studies on the matter, we can only rely on anecdotal evidence.
Is Decaffeinated Coffee Good for Hostas?
When it comes to the nutritional merits, decaf works just as well for your hostas as caffeinated coffee. The only difference is the lack of caffeine, which isn’t beneficial to hostas, anyway.
If you’re an organic gardener, check into how your coffee is decaffeinated, as chemicals are often used during this process. These evaporate when brewed, but it’s still good to know what you’re putting in your garden—and your coffee cup!
Should You Use Fresh Coffee Grounds on Hostas?
It is not recommended that you use fresh coffee grounds on hostas, as they may harm your plants.
Instead, brew your coffee, allowing the grounds to cool, then compost the coffee grounds. Once the grounds have composted, mix the compost into your hosta garden or mulch.
Can you use coffee grounds as mulch?
Coffee grounds can be mixed into your mulch, but shouldn’t be the only mulch used in your garden. Avoid thick layers of coffee grounds as well, as this will harm your hostas as the coffee compacts.
What kind of fertilizer do hostas like?
Hostas love organic fertilizers that are rich in nitrogen, potassium, and phosphorus. Choose a balanced fertilizer or perform a soil test to see what your garden needs in specific.
If you want to know more about how to fertilize your hostas, check out our hosta fertilizer article.
Do hostas like acidic soil?
Hostas can tolerate many soil types, including slightly acidic soil. Hostas prefer a pH between 6.5 and 7.5. Want to know more about hosta soils than just pH. Check out our article that includes tips on improving your hosta’s soil.
Like many gardening tools and tricks, the efficiency and effects of coffee grounds vary based on how you use them. They can be detrimental or helpful depending on the gardener.
Use a moderate amount of composted coffee grounds in your hosta garden for the best results.