You won’t get far on a gardening blog without finding an article about coffee grounds. This DIY additive is one of the most common gardening hacks out there, and also one of the most hotly debated. Our experience is that using coffee grounds in garden spaces is a fantastic way to boost plant growth. Plus, you don’t need a compost bin to add small amounts of coffee grounds to your garden and container plants. As long as they’re brewed, this additive can be used just as it is.
There’s a wealth of misinformation online about coffee grounds. There are videos and articles claiming uncomposted coffee is too acidic, nitrogen-heavy, and dangerous for your plants. That just makes us laugh because coffee grounds can be a valuable and safe resource in the garden. If you don’t believe us, try them for yourself!
Myths about Coffee Grounds
Before we jump into all the reasons we love coffee grounds, let’s dispel some myths. Here are the most common misconceptions about spent coffee grounds.
Coffee grounds don’t make the soil pH more acidic. Sorry, but they aren’t a cure-all for your azaleas and blueberries, or even for the light acidity that tomato plants enjoy. While unused coffee grounds are a little acidic, used coffee grounds have a neutral pH. The acidity originally in the grounds is water-soluble, so it ends up in your coffee instead of the spent grounds.
The caffeine in coffee grounds will not damage your plants. When you brew coffee, most of the caffeine goes into the cup. The used coffee grounds are left with about 5 milligrams of caffeine per gram (for comparison, an 8 oz cup of coffee has 95 milligrams of caffeine). So, as long as you properly brew the coffee grounds beforehand and only apply a light dusting, the caffeine content will be too minimal to hurt the plants.
Coffee grounds don’t repel slugs and snails. We can see where this myth is coming from. Snails and slugs are often deterred by coarse textures, like diatomaceous earth, that cut into their soft bodies and cause them to essentially dehydrate and die. Even though coffee grounds have a coarse texture, they aren’t rough enough to discourage these pests. Slugs and snails are also deterred by caffeine, but as we mentioned, spent grounds don’t contain enough to have much of an impact.
Fresh coffee grounds are not a nitrogen fertilizer. They may be 2% nitrogen, but coffee grounds don’t supply significant amounts of nitrogen as a fertilizer to the soil. If you’re adding coffee grounds directly to nitrogen-loving plants, supplement the soil with a nitrogen slow-release fertilizer.
Benefits of Coffee Grounds in the Garden
Like any fertilizer, coffee grounds provide nutrients and other trace minerals to the soil that can help boost plant growth. Just some of the nutrients spent grounds provide are phosphorus, calcium, magnesium, and potassium. Along with providing nutrients, a coffee ground application may also remove toxic metals from the soil. In fact, this organic material is so good at that, that studies have been done on using fresh grounds to treat contaminated water!
In order for soil to retain moisture without becoming waterlogged, it needs to be made of variable particle sizes. Gardeners often build up this good soil structure with compost and other green materials. Coffee grounds will also get the job done, and just as well as most other organic matter! To really boost the soil texture, though, the used grounds should be mixed in instead of just laid on the soil surface… but more on that later.
We may love the smell of morning coffee, but garden pests sure don’t. According to anecdotal evidence, the strong smell repels some pests like mosquitoes, fruit flies, and fleas. Caffeine is also a natural pesticide, so unbrewed grounds may deter other pests if used sparingly.
Using coffee grounds may also discourage most cats from using your flower beds as a litter box. It should be noted that cats shouldn’t consume caffeine, even the small amounts in spent beans, so be wary of kitties in your yard with an unconventional appetite.
Using Coffee Grounds in Your Garden
Start by collecting spent coffee grounds. I like to keep a small bin by my coffee pot so I don’t forget to keep them. If you’re not much of a coffee drinker, you can ask a local coffee house for some used grounds. They go through plenty and are usually happy to share.
The type of roast you use doesn’t matter. Whether you drink blonde or espresso, those coffee beans all come from the same coffee plant. However, you don’t want to mix coffee grounds with used tea leaves or other food scraps. Those are best left to use in the compost bin.
How Much to Use
Probably the reason for all the confusion about whether coffee grounds are good for plants or not is because the danger is in the dose.
Too much of any fertilizer or additive in the soil will have negative effects on vegetable gardens, and coffee is particularly fickle. Large quantities of used coffee grounds may build up too much of the same nutrients and overload the plants. Just as dangerously, thick layers of coffee grounds will compact and become hydrophobic, shedding the water needed for plant growth. Also, if this organic matter dries out, it’s very difficult to re-hydrate.
So, the amount of coffee grounds varies depending on the current soil structure and water needs of the garden plants. However, it also means that large amounts of coffee grounds are a good way to kill off all weed growth in an area. You just won’t get beneficial plant growth either!
How to Apply Coffee Grounds
Add coffee grounds strategically to your vegetable garden. While you drink coffee in the morning, sprinkle coffee grounds directly on the soil. For larger amounts, work them into the soil and layer compost, grass clippings, dried leaves, leaf mold, or other organic materials on top. Even though coffee grounds are a good green material in compost, they shouldn’t be used on their own as mulch.
Like any fertilizer, you’ll need to reapply the grounds to the garden soil every few months. Always assess the soil first though so you don’t mess up the texture.
Composting Coffee Grounds
If you want to add your fresh coffee grounds into a full-on compost pile, we have a complete guide here. Coffee grounds are considered a green compost material and so must be countered with brown compost material. You also have the option to compost unbrewed coffee grounds for the green category, though you’ll need to be aware of the extra caffeine. It’s beneficial for some acid-loving plants, but some ornamental and vegetable plants don’t appreciate the stimulant in their soil.
Caffeine or not, worms love coffee grounds, so they’re a great addition to a garden worm bin. Lastly, when you use coffee grounds as green compost material, you can throw in the paper coffee filters with it! A paper filter is typically a brown compost material, so it’ll complement the grounds perfectly in the compost pile.
Frequently Asked Questions
Q: Which plants like coffee grounds?
A: Using coffee grounds that haven’t been brewed works well with the most extreme, acid-loving plants. Used coffee grounds, on the other hand, are pH neutral and won’t have the same effect on the soil. Because of this, they can be used on other plants that prefer a neutral, balanced fertilizer.
Q: Which plants do not like coffee grounds?
A: Spent coffee grounds are pros at increasing soil moisture, so garden plants growing in drier conditions aren’t going to tolerate them well. This includes orchids, succulents, and drought-tolerant species.
Q: Is it good to put coffee grounds in your garden?
A: Yes, as long as you use brewed grounds and only apply a thin layer or mix them into the topsoil. Or, you can use coffee grounds as a green material in your compost or worm bin. Your garden plants will appreciate the essential nutrients!
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