There’s a lot of misinformation regarding coffee grounds and how they affect your houseplants (for better or worse).
This can cause a lot of confusion and concern for more finicky plants such as gardenias and roses.
Some sources claim that the grounds are a perfect substitute for fertilizer on acid-loving plants or kill/repel all sorts of pests.
Others claim that caffeine will kill your plants and destroy beneficial microbes, earthworms, and other soil-dwellers.
The reality is coffee beans consist of the bean and the grounds, the latter containing minerals like calcium, potassium, magnesium, and iron. Adding these grounds to soil enhances plant health and growth.
Thus, sharing coffee with your Gardenia bush is genuinely sharing love. Your gardenia will gladly accept the grainy gift of used coffee grounds.
The nutrients from cups of coffee ground can augment your plant’s fertilizer and encourage better plant growth and flowering.
There are even a few ways you can donate them:
- Tea treatments
- Composting
- Direct application
Plus, you can source leftover coffee grounds from your local coffee shops. It’s essentially free organic matter that many discard after a single use.
How To Use Coffee Grounds for Gardenia Plants
It’s easy to gain the benefits of coffee grounds, either as composting or tea.
Coffee grounds can be used as a fertilizer to improve drainage, water retention, and aeration of soil by adding organic matter to it.
However, before getting into the actual application, we should address the elephant in the room: coffee myths.
Common Myths About Coffee Grounds
There are several myths about coffee grounds, which can often lead to poor gardening choices.
In addition, the grounds provide an acidity that helps to keep the soil pH balanced, which is essential for healthy gardenia plants.
Here are some of the big ones and the facts.
Coffee Grounds are Highly Acidic
This is perhaps the biggest myth out there.
In reality, the acids in coffee grounds are water-soluble and will have minimal effect on acidic soil. It is important to take note that fresh coffee grounds are acidic and used coffee grounds are neutral.
Plants such as hydrangeas, rhododendrons, azaleas, lilies of the valley, blueberries, carrots, and radishes will benefit more on fresh coffee grounds. On the other hand, use used coffee grounds for plants like tomatoes, clovers, and alfalfa.
Research has determined that the grounds themselves have a pH of only 6.5 to 6.8, which is at the threshold of neutral (7.0).
There is also an independent report done by Washington-based Soil and Plant Laboratory, Inc. for the magazine Sunset, which tested a sample of used coffee grounds from Starbucks (we will ignore the oxymoron here) and determined it had a pH level of 6.2.
Your plants will enjoy the benefits of coffee in no time with only a teaspoon of grounds per gallon of water and one cup of grounds for every four to six cups of potting soil.
While the study itself has no real academic merit, Sunset published the results online, which is probably one of the main reasons people still believe this myth.
Coffee Grounds are an Excellent Source of Nitrogen.
While there is some truth to this when looking at coffee’s NPK, it’s pretty low in nitrogen and phosphorus compared to commercial fertilizers.
They are rated at 2% percent nitrogen by volume, and this nitrogen ends up feeding microorganisms instead of your plants (which can still be quite beneficial).
However, when used as a composting material, the grounds have a C/N ratio of 20:1 and help maintain higher composting temperatures.
This latter fact is perhaps where this myth gained its footing and why using grounds as part of a composting process does have a positive nitrogen-based benefit on the soil.
Coffee harms/kills plants.
This is another big one based on studies where high concentrations of caffeine were used.
The myth also weighs in on the fact that caffeine is a natural insecticide and can slow the spread of (or even kill) some bacteria and fungi.
In reality, there’s little evidence to prove that coffee grounds can harm or kill plants under normal gardening conditions and may benefit them.
A study published in the journal Urban Forestry & Urban Greening (Hardgrove & Livesley, 2016) tested the effects of coffee grounds on five different commercial crops (broccoli, leek, radish, sunflower, and viola).
They found the plants suffered from a period of stunted growth, followed by increased yields, and attributed this to a combination of harmful compounds and beneficial effects on the soil itself.
It should be noted that there is some evidence to suggest coffee grounds can kill seeds and seedlings but do not harm established plants.
Coffee grounds kill earthworms.
Most gardenias are grown by gardeners because their large beautiful blooms and distinctive aroma set them apart from other types of plants.
They prefer full sun, but shade should be set at midday, or the leaves will burn.
Spider mites, whiteflies, and mealybugs are some of the most common pests in gardens.
This is unsubstantiated outside of cases where grounds were the only available food source.
There has been no hard evidence of increased earthworm fatalities due to coffee grounds in cases where other composting materials were present.
Applying Coffee Grounds Directly
You can spoon the coffee grounds directly on top of your plant’s soil to serve as a form of mulch.
The grounds will bind together when they dry out and repel excess water.
Adding some compost, leaves, or mulch on top will help the grounds retain moisture and will speed up the composting process of your chosen organic matter.
It is likely that a disease or pest has damaged or killed your gardenia, so you can treat it by lowering humidity levels and Pruning it. The pH of acid soils with a pH of 5.0 to 6.0 is preferred by gardenias.
Coffee grounds can stunt weeds and help block them from sprouting, which can reduce the amount of manual weeding.
While not proven, there is some evidence suggesting coffee grounds on the surface may also repel slugs and snails.
Alternatively, you can mix the grounds into the soil, where they’ll provide not only organic matter but also improve aeration.
Remember, they’re not a substitute for fertilizer but can be used to augment the feedings and improve beneficial microbial populations that would otherwise feed off the fertilizer.
Applying as a Compost
Mix equal parts of coffee grounds, grass clippings, and yellow leaves to create a simple compost that can keep your gardenia’s roots warm while providing tasty organic material.
You can also put the coffee grounds in your actual compost pile (along with the paper coffee filters or an equal amount of leaf matter) to boost the potency and efficiency of the compost pile.
Coffee grounds provide a slow-release fertilizer with a high concentration of nitrogen, phosphorous, and potassium when planted in early spring.
Not only will this boost the compost, but it will generally benefit any worms, especially in worm boxes, as long as you continue adding an equal amount of carbon material along with the grounds.
You can then use this compost as usual around your gardenias.
Applying as a Tea Treatment “Liquid Fertilizer”
The third common method of using coffee is playing barista and serving your plants a bucketful of composted tea when it’s time to water them.
To do this, grab 2 cups of your used coffee grounds to 5 gallons of water.
Allow the bucket to steep overnight, then use the resulting weak coffee blend to water your plants.