Challenging Misconceptions: Why Coffee Grounds Won't Change The Color Of Your Hydrangeas

Gardeners love the big, colorful blooms of hydrangeas. Gardeners who also have an affection for a morning coffee will often save the spent grounds and mix them into the soil around their hydrangeas. This is usually due to the common idea that coffee grounds will change the color of hydrangea blooms. However, that is not the case. Unfortunately, coffee grounds won't do much to change the flower color — at least not immediately.

The reason this misconception has become such a common practice revolves around the natural acidity in coffee. Soil pH is one of the primary determinants when it comes to the color of blooms for certain types of hydrangeas. So, the logic goes that adding coffee grounds to your soil will make it more acidic, thus influencing the color of the blooms — neutral or basic soils render white or pink blossoms, while more acidic soil produces the more popular purple and blue tones.

However, this logic is fundamentally flawed. That's because while fresh coffee grounds are mildly acidic, with a pH of around 5.0 to 5.5, used grounds are neutral or slightly acidic at best at 6.5 to 6.8. As a result, spent grounds won't alter the soil pH enough to cause a change in the color of the blooms. Any acidification of the soil the does happen is generally very short-lived, or could take years of applications to work. It's also worth noting that the idea that all hydrangeas can change color is also a myth: only certain varieties of big leaf and mountain hydrangeas change color as a result of soil pH. Finally, it's not just the pH of the soil that determines bloom color, but the aluminum content as well, something coffee can't help with.

That is not to say that there aren't beneficial ways to use coffee grounds in your garden. They add vital nutrients, such as nitrogen, to compost. Tossing used grounds into your compost can also help expedite the decaying process. They improve soil structure and attract beneficial creatures such as earthworms, while driving away some harmful pests such as slugs. So, there are still plenty of good reasons for you to save your coffee grounds for your plants, including hydrangeas.

In fact, while used coffee grounds will not instantaneously change the color of your hydrangea blooms, they can help you grow bigger and better hydrangea blooms. This is simply because they help enrich the nutrients in the soil and can aid in water retention — both of which are crucial components when it comes to growing big, beautiful blooms. However, any impact they have on the color of those blooms is negligible.

Lest you decide to use fresh coffee grounds instead of used ones to create blue hydrangeas, since they are far higher in acidity, it is best not to attempt that. For one, the high acid and caffeine content can actually overwhelm or burn plants in some instances. Additionally, unused coffee grounds — when moistened by watering or rain — tend to form a caked, compact layer on top of the soil that can block water absorption.