Myth or Fact: Can Coffee Grounds Turn Hydrangeas Blue?

Have you heard that coffee grounds can turn the blooms of your hydrangeas blue? Or perhaps they can make your blue hydrangeas have even brighter blooms this season? In this article, gardening expert and hydrangea enthusiast Jill Drago gets to the bottom of this "garden hack" to help you determine if it's myth, fact, or somewhere in between.

Blue Hydrangeas After Coffee Grounds in Garden

Blue hydrangeas are the envy of gardeners everywhere. Even though there are other plants that have blue blossoms, nothing quite beats the large spherical blue blossoms of a hydrangea. But what about gardeners who want to change the color of their hydrangeas, or just get brighter blue blooms? Many hobby gardeners test different ideas and theories, which is what makes gardening fun!

One such theory is that that coffee grounds can give your garden the brightest blue hydrangea flowers you have ever seen. As we all know, some things we find on the internet can be great tricks, other times they can be major flops.

So, what about coffee grounds? Does they actually change your hydrangea’s color to blue, or help enhance their color? Or will adding them to your plant’s soil prove to be just another failed garden hack? The answer really isn’t cut and dry, but let’s take a deeper look, and help decide if it’s something you should try this season!

The Short Answer

The short answer is “maybe.” Unfortunately it’s not cut and dry. Most evidence is anecdotal, with some online publications recommending it. But changing the color of these popular shrubs really comes down to science.

Hydrangea have pH sensitive flowers. If the soil is more acidic and has a 5.5 or below pH, that will favor blue blooms. The lower pH levels allow the plant to absorb more aluminum, producing blue flowers. So, if you live in an area where your soil may react to the coffee grounds by reducing the pH levels of the soil, they can be used to in theory, help produce beautiful blue blooms.

The Long Answer

Shrubs With Blue Blooms
Unfortunately, the answer to this question will come down to the science of your soil.

As mentioned, this “gardening hack” can have roots based on science and testing. There’s a few conditions that would need to be met to get it right. It starts with the type of hydrangea you have. Some types will not change color, no matter what you do to the soil.

Others are pH sensitive, so it’s important to determine your hydrangea type and variety/cultivar first. Once that’s done, it’s on to soil composition, and altering the pH levels of your soil. So, let’s take a deeper look at what you can expect for each of these important factors.

Which Hydrangeas Change Color?

Shrub Changing Color
Only certain types of hydrangeas react to the pH sensitivity of the soil.

Hydrangea macrophylla, or bigleaf hydrangea, is the plant you are looking for here. Almost all blue hydrangeas are of this species. Hydrangea serrata, or mountain hydrangea, have been known to have some success when it comes to color controlling.

Within these species there are a few nuances. If your hydrangea blooms are pink, purple or blue you will likely have success changing the colors of your flowers. However, if your hydrangea flowers are white they will always stay white regardless of what you add to the soil. You may notice some pink on these white flowers as they age, but this happens independently of the soil pH.

What Makes Them Blue?

Shrub Changing Color to Purple
The acidity of the soil you plant in can change their color from pink, to purple, to blue.

Hydrangeas as a species are acid loving plants and their flowers are sensitive to the pH of your soil. What does this mean exactly? The lower the pH of your soil the more acidic your soil will be and the bluer their blooms.

The more alkaline your soil, the more pink or red their blooms will be. If your soil is neutral you will get purple flowers, and sometimes a mix of blue and pink flowers.

The lower pH of your soil the more acidic it is. Acidic soil allows for the uptake of any aluminum that may be in the soil. Aluminum is what will give your hydrangeas that beautiful blue hue.

Soil pH Explained

Soil pH is measured from 0-14. A rating of 7 is a neutral soil. Any ratings below 7 are an acidic soil, while ratings above a 7 are sweet or alkaline. The best soil pH for blue flowers is right around 5.5.

Turning Your Flowers Blue

Shrubs with Blue Flowers
If you’ve decided to turn your blooms blue, there are other more scientific ways to approach it.

If you have planted bigleaf hydrangeas and your flowers are pink but you were hoping for blue you have a few options. First is adding organic material, such as compost, to your soil. This will slowly lower the pH of your soil.

Secondly, you can purchase a soil acidifier from a garden center and follow the label instructions. When using anything chemical, less is more to avoid burn.

Before you add anything to your soil, it is always recommended to perform a soil test to find out if you need to acidify your soil at all. Through this soil test you will also want to find out how much nitrogen you have in your soil. Soil tests are available at your local garden centers, or online.

So, What About Coffee Grounds?

Adding Spent Grounds to Garden Soil
There are anecdotal reports of some gardeners getting great results with coffee grounds in the garden.

There have been reports of gardeners getting wonderful results using coffee grounds in their soil. As you likely know, especially if you are an avid coffee drinker like myself, coffee is acidic. So, it seems promising, now that I think about it, that this could turn your soil acidic as well. 

Coffee grounds are an organic material, and organic materials are always better for the longevity of our plants as well as our soil. When you’re working with organic materials you do not have to worry so much about over-applying and subsequently burning your plants root systems.

For Those That Want to Test

If you are ready to give this experiment a go, I would suggest using fresh grounds, or some diluted leftover black coffee from your morning pot of Joe. Using brewed or used grounds alone likely won’t do much good because they lose a lot of their acidity during the brewing process.

Brewed Coffee

Fertilizing your hydrangeas with some leftover black coffee straight from the pot could work quite well once properly diluted. It would act like a drench, and give your soil plenty of the acid that it needs to produce beautiful blue flowers.

Dilute your leftover black coffee and water your outdoor plants. Keep in mind that while this will add acid to your soil, it may not be enough to actually turn your flowers blue.

Fresh Coffee Grounds

Start by adding your fresh grounds to your soil in the fall. Sprinkle them around your hydrangeas and work them into the dirt. Reapply your the grounds as you would apply your fertilizer, two or three times a year is all you need.

This method will not work overnight, and you will need to reapply multiple times. Since bigleaf hydrangea (Hydrangea macrophylla) flower buds form on old wood it could take an entire growing season before you see any actual results from this method.

When it’s Not Worth Testing

Blue Blooms on Shrub in Garden
There are plenty of tried, and tested methods that can help support blue blooms.

Sadly, I think the cons outweigh the pros here.

Studies have shown that the addition of used coffee grounds can increase the amount of water retention in your soil. This is great, to a point. The water retention has been seen to be too great, to the point where growth of the plant becomes inhibited. This will not only impact the growth of the plant, but the growth of the beautiful blooms they are known for.

These studies report that the growth of weeds in the soil decreases greatly with the use of coffee grounds in your soil. That sounds great, but what does that mean for your hydrangeas? These shrubs are larger and will not be affected as quickly as small herbaceous weeds would be. The caffeine content can also naturally stunt the growth of your plant.

Many gardeners have reported that using coffee grounds can deter pests of all sorts fom furry critters like deer and rabbits, down to slugs and other creepy crawlers. This might be true, however, adding coffee to your garden can also kill off earthworms and other beneficial microbes in your soil.

Final Thoughts

There are countless articles on the internet that go into detail about how coffee did in fact help turn their hydrangeas blue. I don’t doubt that these gardeners may have had this experience. There is a lot of information missing though.

Most importantly, the long term effects of adding used coffee grounds to your soil. For me, I would consider this garden experiment as a myth, or at least a “partial hack” that you’ll need to test. But in my opinion, there’s too much at risk without enough proof to back it up. I’ll continue adding my coffee grounds to my compost bin, and using other methods to keep my hydrangeas blue.

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