Some home gardeners look for ways to utilize food waste in their gardens to help alleviate the problem of excess waste. This has led many to believe that coffee grounds can help your plants grow. While some anecdotal evidence claims the effectiveness of such treatment, several studies prove otherwise, making many people wonder if coffee grounds are good for flowering plants like hydrangeas.
Coffee grounds may not be good for hydrangeas when applied to the soil directly. They are not acidic enough, and their nitrogen content is not readily useful for hydrangeas. Some substances in coffee grounds can also inhibit plant growth. However, when composted, they can be beneficial.
I will explain the qualities of spent coffee grounds and why they can be bad for hydrangeas. I will also discuss how to properly use coffee grounds and maximize their benefits to your garden. So sit back, take a sip of your coffee, and enjoy reading!
There’s enough reason to believe that ground coffee can benefit plant growth because it contains three essential macronutrients: nitrogen, potassium, and phosphorus. However, coffee grounds may have some qualities that are not beneficial for flowering plants like hydrangeas.
Let’s check out some vital information about coffee grounds:
Contrary to popular belief, not all coffee grounds are acidic. The acidity can vary depending on the coffee plant species and the roasting method used for the coffee beans before they were ground.
Moreover, the acidic components are usually extracted from the grounds and dissolved in water after brewing because they are water-soluble. As a result, the remaining coffee grounds are almost neutral in pH (6.5 – 6.8).
Considering acid-loving plants like hydrangeas, the almost neutral coffee grounds might prove unnecessary and even counterproductive.
For instance, to get the sought-after blue flowers, you must grow your hydrangeas in acidic soil with a pH below 5.5. Such conditions will encourage the plant roots to absorb aluminum, resulting in beautiful blue flowers.
In contrast, a higher pH between 6.0 and 6.5 can produce pink blooms. While this isn’t entirely bad–and some people prefer pink hydrangea flowers–pH levels around 6.5 can limit the plant’s ability to take in iron, which is an essential micronutrient.
Adding spent coffee grounds directly into the soil around your hydrangeas won’t raise the soil’s pH immediately. However, they won’t lower the soil pH enough either, making it unnecessary to use coffee grounds for your hydrangeas.
Some studies on decomposed coffee grounds also revealed that their pH levels could range from below 5.0 to over 8.0. This large gap is due to the varying chemical reactions in the soil, making the result unpredictable.
Suppose the degradation of coffee grounds in your garden soil yields a highly alkaline pH level. In that case, it can prove detrimental to your hydrangeas. Therefore, using coffee grounds in your garden is a gamble.
Coffee grounds contain 2% nitrogen and traces of potassium and phosphorus. However, these forms are not readily available for plants. They need to be decomposed first before they can add value to the soil.
The tricky part here is that while soil microbes break down the coffee grounds, they consume the nitrogen in the soil. This is the same nitrogen that plants are supposed to use. In other words, the soil microbes will compete with your hydrangeas for nitrogen as they decompose the coffee grounds.
This mechanism can prove counterproductive for your plants. To overcome this problem, gardeners recommend using a nitrogen-rich fertilizer after adding coffee grounds to the soil. This can present another issue.
Although a nitrogen-rich fertilizer can promote lush foliage, it can inhibit blooming. Gardeners grow hydrangeas for their blooms, so this situation can be disadvantageous.
To make matters worse, the nitrogen from coffee grounds will soon become available for the hydrangea after decomposition. The minute amounts of phosphorus in the coffee grounds won’t suffice to promote blooming.
The key is to regulate the amount of fertilizer to add to your hydrangea. You can achieve this by using a phosphorus-rich or balanced fertilizer with an NPK ratio of 10-10-10. Remember that overfertilizing your hydrangeas can also be risky.
Considering these details, there isn’t much value in using coffee grounds for hydrangeas.
Once decomposed, coffee grounds can improve soil texture, drainage, and water retention. These may be some of the few reasons to believe coffee grounds are good for your hydrangeas.
Drought stress can inhibit hydrangea blooming. With the decomposed coffee grounds’ ability to hold moisture, your plant can gain more steady access to water. Moreover, coffee grounds provide a mulching effect to regulate soil temperature.
Some hydrangea species are more cold-hardy than others, allowing them to grow in USDA Zone 4. However, many less winter hardy cultivars that grow in northern Idaho and New York may need adequate mulch to protect their roots from the cold.
Coffee grounds make good mulch but avoid applying a thick layer on the soil. Spent grounds can be too fine and inhibit aeration in the soil when newly applied. Around an inch (2.54 cm) of coffee grounds should be enough.
You can improve the mulch by adding a few inches of other materials, such as bark and straw.
Plants with caffeine benefit from the substance’s ability to inhibit the growth of weeds and other nearby competitors. Spent coffee grounds still contain caffeine and several other compounds with inhibitory effects on other plants.
There are plenty of studies with conflicting results. The differences in the results could be due to the following reasons:
- Varying methods of extraction.
- Different plant species.
- Different concentrations of caffeine.
- Other external factors, such as existing soil nutrients and microbes.
Therefore, there is no guarantee that coffee grounds will help boost the growth of your hydrangeas.
Since there aren’t enough studies on coffee grounds and hydrangeas, the insights in this article are mainly based on related research and the science behind the chemical components of coffee grounds and their reactions.
If you want to try using your spent coffee grounds for hydrangeas, you may need to do a few things:
A soil test will help you determine whether or not your soil needs amendments to better accommodate and support the growth of your hydrangeas. If the soil doesn’t appear to lack essential macronutrients and micronutrients, it may be better not to use coffee grounds.
On the other hand, you may consider using coffee grounds for the following purposes:
- To boost the soil’s drainage.
- To improve water retention.
- To regulate soil temperature.
Suppose the soil appears to be significantly lacking in phosphorus. In that case, you may use a phosphorus-rich fertilizer like Jack’s Classic Blossom Booster Fertilizer (available on Amazon.com). It has an NPK value of 10-30-20, which is suitable for flowering plants. It is easy to use and has excellent reviews.
It can help avoid the issue of too much nitrogen preventing the development of blooms. However, be careful when using fertilizers in addition to your coffee grounds. Over-fertilization can sometimes be worse than under-fertilization.
Read and follow the instructions when using commercial fertilizers. When in doubt, apply the fertilizer at half the recommended strength.
Several studies revealed that adding coffee grounds to compost or vermicompost resulted in higher nutrient values in the finished products. However, coffee grounds should only make up at most 20% of the pile.
When using coffee grounds for your compost pile, you must add enough carbon-rich materials. Ideally, the pile must contain 25-30 parts carbon-rich source to 1 part nitrogen-rich source.
Coffee grounds can inhibit the growth of weeds and other plants. Some components of coffee plants work together to destroy competition to ensure the survival of the species. While there’s not enough evidence it will do the same for your hydrangeas, there’s no guarantee it won’t.
You can conduct your own experiment to see whether coffee grounds can benefit your hydrangea cuttings. A word of advice: Use coffee grounds sparingly unless you’re willing to gamble on your plant’s life.
When growing cuttings in pots, you can apply coffee grounds in one or a few pots and grow the other hydrangea cuttings as you usually would. See which setup works best for you.
Note that several factors can come into play and affect how well your hydrangeas respond to coffee grounds. These factors include:
- Soil quality (nutrient, texture, drainage, etc.)
- The climate in your region
- Microclimate in your house if you’re growing the hydrangeas indoors
- Type of coffee
- Brewing technique
Coffee grounds can be bad for hydrangeas when used incorrectly and in excess. The resulting pH of coffee grounds after degradation can be too acidic or too alkaline for your hydrangeas. Moreover, excess nitrogen can inhibit the blooms.
Check your soil’s pH and nutrient content before using coffee grounds. If possible, you might as well add your coffee grounds to the compost or vermicompost pile, as it’s safer that way. Just be sure your pile doesn’t contain over 20% coffee grounds.
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Hi! I’m Sophia, and I love plants – especially an expert in growing house plants. I stay in Chicago, United States of America, and through my blog and social media platforms, provide tips and tricks on how to grow healthy, vibrant plants indoors. Check out more here.