If you love coffee and hydrangeas, I’ve got the perfect hack for you. Instead of throwing out your coffee grounds after enjoying a fresh brew, save them to add to your soil.
It may seem like magic, but the coffee grounds intensify the blue color in hydrangeas, resulting in bold blue blooms.
Although I like to think that the coffee grounds have magical powers, the truth is that science casts its spell.
Coffee ground magic
Coffee grounds are a great organic kitchen waste product to recycle in the garden to benefit your plants and soil, and apart from boosting the health of hydrangeas, they can also change the color of the blooms. But be warned, fresh grounds are too acidic and may damage the plant.
It’s all to do with the soil's pH level
Since coffee grounds are acidic, they gradually lower soil pH, enhancing blue hydrangea blooms. In fact, the coffee grounds help hydrangeas absorb aluminum in the soil, which is responsible for the color change. So, both aluminum and acid have to be present for the magic to happen.
A higher acidity level will result in bluer blooms, while alkaline soils will produce pinkish flowers, and neutral soils will produce purple flowers.
However, for faster results, Ground to Ground suggests that gardeners add aluminum sulfate or elemental sulfur to the soil, which will intensify blue hues in Hydrangea macrophylla varieties.
The pH scale explained
The pH scale ranges from 0 to 14, with 7.0 being neutral. Levels that are 0-6.9 are acidic, and those in the range 7.1-14.0 are more alkaline. For blue flowers, you need an acidic soil between pH 5.2-5.5.
Getting the balance just right
Before adding your spent coffee grounds to your hydrangeas, it's worth checking your soil’s pH level, because Ground to Ground advises that adding too much acid to your soil could stress the plant and inhibit growth.
Apart from adjusting the color, getting the soil's balance just right benefits these beautiful plants. Acidic soil improves nutrient availability, enabling the plant to absorb iron, manganese and phosphorus efficiently, which encourages healthy foliage, strong roots and vibrant blooms.
However, if the soil becomes too alkaline, hydrangeas can struggle to absorb nutrients. And instead of a healthy plant, the leaves can become yellow and the blooms may weaken.
How to add coffee grounds to hydrangeas for bold blue blooms
Start off by adding coffee grounds to the soil around the base of your hydrangea in late fall or spring. Work them in around the plant, rather than leaving them sitting on top of the soil, and give the area some water. You don’t need to add coffee grounds to your hydrangeas on a weekly basis, but they will benefit from a few sprinkles during the growing season.
Be patient
Don’t expect an overnight color change. It will take a while for the coffee grounds to work their magic, but if you want to enjoy bold blue hydrangeas, it will be worth the wait.
Best hydrangea types for color change
Big leaf hydrangeas (Hydrangea macrophylla), such as mophead and lacecap varieties, are a good bet, as they will change color depending on the soil’s pH level. You can also try adding coffee grounds to Mountain hydrangeas (Hydrangea serrata), which also respond to a change in acidity.
