Spent coffee grounds have long been used, with varying degrees of success, as soil amendments for plants. While some plants don’t take to it, hydrangeas do benefit from this soil amendment.
Coffee grounds contain nitrogen, and they can increase the acid content of the soil. Both are of interest to hydrangea plants.
Adding used coffee grounds to the soil of hydrangea plants can also improve the quality of the soil. I will discuss the benefits you can expect to see, how often to add coffee grounds and whether they’re suitable for potted hydrangea plants.
Benefits of Coffee Grounds
Hydrangeas enjoy slightly acidic soil and coffee grounds can help alter the pH of the soil so that it’s less alkaline and more acidic.
This in turn may influence the color of the blooms your hydrangea produces. Pink colored blooms can be turned to blue simply by altering the soil’s acidity.
It works the other way around too — blue hydrangea blooms can be turned to pink by dialing down on the acidity of the soil.
If you’re looking to alter bloom colors in your hydrangeas, coffee grounds can be a way to experiment with soil acidity and find that sweet spot that will help you alter bloom colors.
A pH meter or soil testing kit can help you assess the pH of the soil and make necessary amendments based on the desired outcome.
Not all hydrangea blooms change color. This trick doesn’t work for hydrangeas that produce white flowers.
But changes in bloom color aren’t the only expected benefit of coffee grounds. Other benefits include:
- Improving the quality of the soil by adding organic matter to it
- Ensuring better moisture retention, which hydrangeas thrive in
- Rich in nitrogen, coffee grounds help stem, leaf and root development
- When composted, coffee grounds can be used to also improve the drainage of the soil
- Coffee grounds can be beneficial to earthworms which will in turn help improve soil quality
You can use coffee grounds after they’re brewed but you can also throw them on a compost pile and use them in a composted form. I prefer going this route simply to maximize its benefits.
The way you brew your coffee grounds can also influence on the acidity of your spent coffee grounds. Most brewing methods will wash away a significant amount of acidity, and coffee grounds will only retain a residual amount of acidity.
Even so, they can exert an effect on soil acidity, but it will not be immediate as in the case of commercially available soil acidifiers or soil amendments. Therefore, effects will be seen only with repeated application over time.
If you want a faster way to turn pink blooms blue, acidic soil amendments can be a faster way to achieve that. These too have completely organic formulations, but they use other natural ingredients such as fish meal, kelp meal, sulfur and others.
How Often to Use Coffee Grounds?
If you compost coffee grounds, simply use them whenever you’d otherwise add compost to the soil of your hydrangeas.
I add compost in spring, at the beginning of the growing season, and sometimes even in mid-summer. In my experience, adding compost 2-3 times a year is enough to keep your hydrangea plants happy.
If you’re going to use coffee grounds without composting them first, you can add a few cups a week, depending on the size of the bush. I wouldn’t go overboard with it, however.
Excess coffee grounds may lead to excess nitrogen in the soil, which can cause plant development issues and may even attract pests and insects.
Adding too much coffee can also flip the pH balance of the soil and cause other undesirable effects like retaining too much moisture and adding too much residual caffeine to the ground, which young hydrangeas may not enjoy.
If you have a pH meter, you can periodically check the pH of the soil to see if you’re overdoing it or not. Generally, hydrangeas prefer soil with a pH of around 6-6.5.
This should give you an idea of how much coffee grounds should you use on your hydrangea bushes.
Much also depends on the size of the bush too. A mature hydrangea plant will not likely be influenced adversely by a few cups of coffee grounds that you may be throwing over the soil every week.
Can You Use Coffee Grounds on Potted Hydrangeas?
I try to steer away from using coffee grounds on potted plants. There are at least two reasons why. First, coffee grounds can stink, especially when moist. And hydrangeas enjoy moist soil.
Second, moist coffee grounds can be a breeding ground for bacteria and fungi, both of which I want to avoid.
That said, you can use coffee grounds on potted hydrangeas by being mindful of a few things.
Composted coffee grounds are better for potted plants as they reduce many of the concerns associated with plain coffee grounds like smells, residual caffeine, too much acidity and too much moisture retention.
Mixing potting soil with coffee grounds will improve aeration and drainage of the soil.
That said, if you scatter coffee grounds over the potting medium of your hydrangeas here and there, you may not notice any downsides, just make sure not to go overboard with it. Keep an eye on how your hydrangeas react and draw your conclusions from there.
My main concerns are fungal growth and smells, so I simply avoid using coffee grounds on potted hydrangeas unless they’re composted first.
As you can see, coffee grounds are a versatile addition to the soil of hydrangeas. They add nitrogen, organic matter, improve moisture retention, can improve aeration and drainage, and, because of residual acidity, they can alter the color of hydrangea blooms.
If you’re looking to harness the benefits of coffee grounds related to improving the quality of the soil, you can use them in composted form 2-3 times a year.
If you want to alter the pH of the soil, you may need to use them more often, scattering them over the soil of your hydrangea.