Lately, social media has been extolling the benefits of using coffee grounds for gardening. This makes total sense if you're a javaholic—coffee perks you up, so why wouldn't it do the same for your plant babies?
We've looked into this reinvigorated trend to see whether you really can get a second wind from your morning cup of joe. What are the horticultural pros and cons of using coffee as a fertilizer, and is it worth the hype?
Coffee grounds are nitrogen-rich and contain other important minerals like phosphorus, potassium, calcium, iron, magnesium, and chromium. They work well as a slow-release fertilizer, and they fall under the wet "green" category for composting as opposed to dry "brown" items like pine needles and dead plant clippings.
If you're not a big fan of coffee but want to treat your plants, ask your local coffee shop for some of their waste. You'll be recycling and spending less on synthetic fertilizers. In addition, compost containing coffee grounds may release fewer greenhouse gases.
Some vermiculturists believe earthworms, a beneficial creepy crawly that aerates the soil and promotes growth, enjoy small amounts of coffee grounds.
But don't overdo it. A cup should be more than enough for a week if you have a small worm bin. And if you incorporate the grounds into the soil directly, your coffee might bring all these boys to your yard. (They're actually hermaphrodites, but IYKYK.)
The caffeine and diterpenes in coffee are toxic to insects. You may be able to foil the efforts of beetles and other pests like rats and mice by putting a bowl of coffee grounds near your plants. Slugs might dislike the rough feel of the grounds, and ants may not like the smell, but they won't necessarily turn and march away if they encounter a barrier.
Cats aren't keen on coffee either, so if strays have turned your accessible garden into a litter box, try using the leftovers from your morning brew to keep them at bay.
The trick to using coffee grounds as mulch is to mix them with organic matter such as compost and not layer them thickly—half an inch should do. If you do lay it on thick, you'll create an impenetrable clay-like covering that will dehydrate your plants.
Instead, rake your coffee grounds into the soil, so you get a mix of particle sizes in your top layer, and so the coffee can't form a mass and starve your plants of oxygen.
Coffee ground mulch can also suppress weeds.
Coffee grounds can help with drainage as well as water retention. It sounds contradictory, but both functions are essential to the health of your garden.
Specifically, if you compost coffee grounds before applying them, they help your soil hold water better. They can also absorb heavy metals and contaminants.
Plants that make caffeine do so because it decreases competition by inhibiting the growth of their rivals; caffeine hinders the germination of other seeds and the growth of young plants. But some plants aren't so sensitive to caffeine (hey there, cabbage), and you can avoid using coffee grounds around seedlings.
Another potential downside is that large amounts of coffee can harm overzealous dogs if you have any. This is one more good reason to toss the grounds in your compost heap for later: you can reap benefits without this drawback by composting your coffee for at least six months before using it.
If you plan on adding coffee grounds directly to the soil, sprinkle a thin layer and cover with compost, leaves, or bark mulch, or mix into the soil with some nitrogen fertilizer. When adding to compost, ensure the grounds don't exceed 20% of the volume to balance microbial activity.
Coffee grounds are close to pH neutral, so don't worry about them being too acidic. Fresh unbrewed grounds are more acidic and might help plants that prefer a lower pH, such as blueberries and hydrangeas.
Mix 1/4 cup of coffee grounds with just over half a gallon of water and set aside for about four hours. You can use this weak coffee as a liquid fertilizer for indoor container plants or as a leaf spray. Alternatively, dilute a quarter cup of leftover coffee from your mug with 3/4 cup of water. The solution can produce strong stems.
You can dry a three-inch layer in the sun on a newspaper-lined baking tray—the process takes about three days. Replace the newspaper daily and bring the bottom layer to the top for dehydrating.
However, if your layer of coffee grounds is too thick, it may grow mold. Alternatively, you can toast the coffee grounds in your preheated oven at 200 °F for 20 minutes and toss them at the halfway mark. Allow to cool and store compacted in a clean, dry, and airtight container in a cool spot. Be sure to use them up within two years.
This organic material has some benefits for your garden, but apply it with reckless abandon and you could do some damage.
For this reason, we don't recommend newbie plant parents use coffee as a fertilizer. Once you gain confidence, experiment and compost your coffee grounds to break down phytotoxins. Try decaf coffee, too!