Excessive acidic soil can kill or hamper the growth of plants like asparagus fern, Chinese mustard, Italian ryegrass, lavender, orchids, rosemary, tomatoes, and geranium. The roots of these plants are also not potent to absorb the nutrients added by the coffee grounds in the soil.
In most cases, the grounds are too acidic to be used directly on soil, even for acid-loving plants like blueberries, azaleas and hollies. Coffee grounds inhibit the growth of some plants, including geranium, asparagus fern, Chinese mustard and Italian ryegrass.
Compost coffee grounds for a vegetable garden instead of using them directly. Remember, coffee grounds are not good for tomatoes or seedlings. Get a soil test, especially if your plants are not thriving or you want to change the acidity of the soil.
Most plants will grow well in a slightly acidic to neutral pH (between 6.0 to 7.0) but others prefer acidic or alkaline soil. Therefore, scattering coffee grounds indiscriminately round all your plants is not a good idea.
How Often Should You Add Coffee Grounds to Plants? You can fertilize houseplants an average of 7 to 10 days in a row, but no more, as there is a risk of over-acidification of the soil. A cup of ground coffee should be added to the compost once a week.
Some gardeners report success in turning their hydrangeas blue by applying coffee grounds to the soil. The coffee grounds make the soil more acidic, allowing the hydrangea to more easily absorb aluminum. In addition, fruit peels, lawn clippings, peat moss and pine needles, are thought to have a similar effect.
Coffee grounds are rich in nutrients, especially nitrogen. They also have some amount of other nutrients like potassium and phosphorous. Overall, this means that adding coffee grounds to your garden can work fairly well as a fertiliser. Coffee should be spread in a thin layer, rather than being clumped in one place.
Since coffee grounds have such a pungent and intense smell, mosquitoes do not like to come near it. Mosquitoes are repulsed by all types of coffee, whether it is fresh, used, or burnt. The most effective way to use coffee grounds for mosquito control is by burning them, as it creates a stronger aroma.
There are a select few plants that can benefit from fresh grounds—including acidity-loving Hydrangeas, Rhododendrons, Azaleas, Lily of the Valley, blueberries, carrots, and radishes—but generally, most common houseplants will prefer low to no acidity.
Coffee grounds are highly acidic, they note, so they should be reserved for acid-loving plants like azaleas and blueberries. And if your soil is already high in nitrogen, the extra boost from coffee grounds could stunt the growth of fruits and flowers.
One study published in the Journal of Agricultural and Food Chemistry found that coffee grounds were more effective at repelling rodents than commercially available products. Coffee's bitter compounds are responsible for its repelling effects. These compounds make the coffee unpalatable to rats and mice.
Sarah says the key is composting them first. "Give it six months to a year. The compost will break down the phytotoxins, and you'll still get the great benefit to the soil from using them."
Why coffee grounds? Since the bears have a strong sense of smell, such a pungent treat is extremely attractive and stimulating to them. The bears roll around in coffee grounds like a cat in catnip!
Coffee Grounds to Deter Pests
Coffee grounds are commonly used to keep animals including slugs, snails, rabbits and fire ants from eating plants in the garden, and to keep cats from treating garden beds like litter boxes.
Which plants shouldn't you try this with? Don't add eggshell fertilizer to plants that prefer acidic soil, like blueberries. Ericaceous plants such as mountain laurel, pieris and azaleas also fall into that category. In extreme cases, low acidity for plants could impact their overall health and/or the resulting crop.
Coffee beans, grounds and brewed coffee contain caffeine, a very dangerous and toxic chemical to cats and dogs. Ingestion can be life threatening. According to the Pet Poison Helpline, a moderate amount of coffee can easily cause death in small dogs or cats.
Mosquitoes hate the smell of lavender, citronella, clove, peppermint, basil, cedarwood, eucalyptus, peppermint, lemongrass and rosemary. They also hate smells such as smoke, for further insight, see our exploration on, does smoke keep mosquitoes away?
These things are great for nutrient poor lawns and lawns on clay or sandy soil. However, coffee grounds still contain some residual caffeine, which can cause harm to the micro fauna in your soil, and that's why it is not recommended to apply coffee waste directly to your lawn.
Coffee grounds are an inexpensive and easy way to give your roses the extra boost they need to thrive. Just remember to use them in moderation and always test your soil's pH levels before adding coffee grinds. With a little coffee and a lot of love, your roses will be blooming all season long. Thank you!
The shells also contain other minerals that help plants grow, including potassium, phosphorus, and magnesium. Eggshells are, therefore, an effective and inexpensive fertilizer for outdoor garden soil and houseplants.
Weed Control
Coffee grounds have natural allelopathic properties, meaning that they inhibit the growth of some plants and weeds.
It's best to start adding coffee grounds to the soil months before the blooming season begins, ideally in the late fall. You can repeat the process with your typical fertilizing schedule. With a little caffeine and a lot of patience, your hydrangeas should reward your efforts come spring with the boldest blue globes.
This is a great way to make those flowering plants that refuse to bloom start blooming again as well. Geraniums in particular just love coffee, and so do Peace Lily plants!