With so many cafes offering using coffee grounds for home gardens they have to be good, right? Or are they??? Personally, I’m a fan of using the spent grounds from our home kitchen, but there are pros and cons to using coffee grounds in the garden. Today is world coffee day, so let’s explore some of the potential perks (hehehe), issues, and myths of coffee grounds for gardening. Spoiler: We compost our coffee grounds before using them in the garden. Jump to the end of the post if you’d like to cut to the details on using coffee grounds in a compost pile or feeding them to a worm bin.
Potential Benefits of Using Coffee Grounds in the Garden
Used coffee grounds contain nitrogen, potassium, phosphorus, and micronutrients much like many of the other plant materials that we’d normally compost for the garden. Coffee also has other properties, including its natural caffeine content and potential acidity to take into consideration. In some circumstances, these could be beneficial and in others potentially detrimental. Then there are the factors of scent, texture, and more that may play a role. So can spent coffee grounds be safely used as a booster for garden health? And if so, how? Let’s look at the potential issues first, and then how these play into different coffee ground options for use (or not) in the home garden.
Considerations and Potential Issues
Caffeine and Not So Perky Plants
Caffeine might be what we humans like about coffee, but it tops my list of potential problems with using coffee grounds in the garden. Even spent coffee grounds still contain plenty of caffeine that will leach out into wet soil and further release as the grounds break down over time. Caffeine can have a detrimental impact on plant growth and reduce seed germination, so consider suitability for use and if so where with care. You may want to carefully moderate quantities or use coffee grounds with intent in areas where you want to restrict or control growth.
Coffee is far from the only plant that has developed natural allelopathic properties. As noted in our companion planting post, some plants benefit their neighbours and others have negative allelopathic prosperities that inhibit competing plants from growing around them. The list of allelopathic plants might surprise you – even common garden favourites like sunflowers have their secret dark side. For an interesting read, check our this article on the evolution of caffeine and plant survival. Curious about allelopathy? Check out this article from Permaculture news for a more in-depth look at allelopathic plant properties and their potential advantages and disadvantages.
Acidic Soil Additives
Coffee is often purported to be a good addition for acid loving plants; however, it turns out that spent grounds are often not as acidic as expected. When I first started adding coffee to our garden years ago, I used it with my acid loving plants. They didn’t seem to mind, but they didn’t seem to benefit greatly either. More intriguingly, the pH of the soil didn’t seem to vary either. Hmmm. The acid in coffee is water-soluble, so brewing alters the balance. Coffee is acidic, but the spent grounds much less so. Still, qualities can vary. Not sure? Acid-loving plants like blueberries, cranberries, and rhododendrons are less likely to complain if you want to use it as a side-dressing or mulching additive.
Studies indicate that coffee can have antimicrobial properties. As anyone who has left the grounds container a little too long before emptying or found a forgotten cup in the work break room knowns, it has it’s limits, though! On the flip side, the antimicrobial effects can also help with inhibiting undesirable disease and fungal issues in the garden (or in the compost bin during breakdown). Many plants have a degrees of natural antimicrobial properties, including a variety of popular herbs grown and used for natural medicines, cleaners, and more. This doesn’t mean that they’re incompatible with healthy living soil, nor does it preclude them from being composted; however, awareness and moderation is beneficial.
Does your soil need the boost? Coffee grounds may not be needed or suitable for already rich soil or near plants with other preferences. Depending on the application, it might not give you the desired boost either. Although coffee may have some antimicrobial properties as noted above, its not impervious. Others indicate that it’s effects on microorganism growth can actually reduce available nitrogen during its breakdown into the soil, negating the value of mixing it into the soil instead of breaking it down into compost elsewhere first. It also isn’t a balanced additive on its own, even if the full nutrient profile was available. Its better used as part of a combination of additives or as one of the ingredients in a well varied compost.
Texture and Structure
Although adding quality organic material can help with soil structure over time, moderation, and mixing are key. Finely ground coffee grounds can lump and clump together. This can create problems, especially for direct application, uses unless you take the time to distribute with care. As for whether their texture and other properties can help with pests and other problems, we’ll talk about that debate in potential uses below.
Used Coffee Grounds as a Direct Soil Additive
Straight onto Soil as a Degradable Mulch
Sprinkling coffee grounds onto the soil is one way to go, but it can look untidy and takes much longer to break down into the soil beneath. Finely ground coffee applied directly to soil can lump and clump, especially if applied thickly or in bulk. If they dry and cake on the surface, the ground can contribute to hydrophobic soil, repelling water instead of allowing it to move naturally into the soil beneath. Coffee can, however, be mixed with other mulching materials for better behaviour. You can take advantage of the allelopathic effect if used in a mulch for supressing unwanted growth, but this is less helpful if you are mulching close to desirable plants.
Mixing into Soil as a Degradable Additive
The fine grounds are easily mixed into soil to degrade over time, but moderation is important. The combination of the allelopathic and antimicrobial effects may not be worth the small potential gains in added nutrients or organic matter, as noted above. Composting prior to use may be a better option.
Steeping Used Coffee Grounds as a Liquid Soil Additive
As as alternative to direct dry application, the grounds could be steeped in water and then applied as a liquid soil additive, similar to making a compost tea. But is there actually a real benefit, noting the potential issues above? Why not just make compost tea instead?
Used Coffee Grounds as a Pest Deterrence
Some gardeners swear by coffee grounds as a line of deterrence against crawling pests. Some say it’s the texture and, unlike diatomaceous earth which is commonly used as a sharp deterrent, it tends to stand up to weather for a little longer. Others say its the caffeine. Personally, I’ve not noticed any improvements in slug activity. Unfortunately!
The strong smell of coffee may also help to keep unwanted pests and critters away. This is one area where I actually think it has helped in our gardens. Personally, I think it helped quite a bit with deterring the neighbours cats from using parts of our garden as a litterbox, but who can say for certain? All I know is that there were fewer nasty gifts after I started sprinkling coffee in their favourite haunts. Note: If you have a family pet that’s prone to eating dirt, be careful. They’re unlikely to find the scent of coffee grounds appealing, but better safe than sorry.
Used Coffee Grounds as an Addition to the Compost or Worm Farm
This is how we prefer to reuse coffee from our kitchen in our home garden. The first priority recipient is the worm farm and, if we have excess, then it goes to other compost. Ours if their capacity in balance and into the green curbside collection bin if not.
Composting Coffee Grounds
Although some experts may disagree, coffee grounds are generally considered to be a great addition to the compost pile. As a food scrap, it’s a green addition even though its brown, so keep that in mind for balance. Coffee may not be a great option for the direct addition of nitrogen to the soil, but it is an excellent high nitrogen source for composting.
Into the bin it goes! Give it a good sprinkle to ensure there are no big lumps or clumps and let nature do its thing. The added time and breakdown of the coffee grounds in a mixed composting environment will also counter the concerns about allelopathic, acidic, or antimicrobial qualities when the finished compost is eventually used in the garden.
Coffee Grounds and Worm Farms
When we first got our worm farm, I wasn’t sure about coffee. If it can be used to deter (debatably) bugs and slugs, would it also be detrimental to the worms? It turns out, most worm farmers think that their worms are keen coffee. Well, at least we have that in common!
In addition to their nutrient profile for the finished vermicompost, the gritty grounds are beneficial for worm digestion. Just like us though, too much coffee can cause problems, so introduce with care and don’t go too crazy on the grounds. Like conventional composting, worm farms need variety and balance.
The grounds should never be fresh (unbrewed coffee grounds are significantly more acidic, as noted above) or applied in bulk to avoid creating an acidic environment or accidently harming your worms. Used coffee grounds should be cool and either slightly moist or wet, depending on the conditions in your bin. If you wish, you can pre-compost the grounds or cold rainwater brew them to increase the microbial content of used grounds to make them more appealing. I keep it simple. Our worms seem keen enough on them straight-up. And if the worms are happy, I’m happy, and the garden is happy.
When adding them to the bin, I apply them moist-ish and give them a good dispersed sprinkle to reduce lumps and clumps in the food layer. They’re used as a small part (our own household only, not café grounds) of a very varied food supply for our worms, so the light top-dressing of coffee grounds into our worm farm blends with the different other green and brown additions without pre-composting or blending. Easy peasy.
If we have excess grounds for some reason (coffee binge, guests, etc.) then the compost gets the extras instead of overloading the worm farm. And if the home compost is teetering on the edge of green/brown balance, we also have a pre-paid curbside green waste bin for extra waste. Industrial or commercial composting can be a good option if you’re stuck for space or have green waste that’s either ill-suited or excessive for your home composting options. It keeps the materials out of landfill and, depending on your service, might accept materials that you can’t (or don’t want) to compost and reuse at home. I find ours very useful.