Greetings, dedicated gardener. We hydrangeas appreciate your desire to understand our needs more deeply. Our famed color-changing blooms are not a mere whim but a direct chemical conversation with the soil we call home. To truly partner with us, you must listen to what the earth is saying. Here is how you can perform a simple DIY test to understand our world.
From our roots' perspective, soil pH is the gatekeeper of vital nutrients. It is not about preference but about accessibility. In highly acidic soils (low pH, below 6.0), aluminum ions become soluble. Our roots can absorb this aluminum, which then interacts with the pigments in our petals (specifically delphinidin-3-glucoside), resulting in those beautiful shades of blue and violet. In more alkaline soils (high pH, above 7.0), aluminum remains locked away, inaccessible to us. Without it, our default pigment shines through, creating blooms in shades of pink and red. For us macrophylla varieties, getting this right is the difference between a vibrant display and a dull one.
You do not need a sophisticated lab; simple household items will do. To see the world through our roots, you will need: two clean containers (glass jars are ideal), distilled white vinegar, baking soda, distilled water (tap water can alter the results with its own pH), and of course, a soil sample. Please collect this sample from around our root zone, about 4-6 inches deep. This is where we feed. Place the soil in the two jars, ensuring it is loose and free of debris.
Now, let the earth speak. In the first jar, add a half-cup of vinegar to the soil. If the mixture fizzes, foams, or bubbles, this indicates an alkaline soil with a pH likely above 7.0. The vinegar (an acid) is reacting with the alkaline minerals in the soil. This tells you we are likely showing pink blooms and may struggle to access iron, potentially leading to yellowing leaves (chlorosis). In the second jar, moisten the soil with distilled water to make a muddy slurry, then add a half-cup of baking soda. If this combination fizzes, it signals an acidic soil with a pH likely below 6.0. The baking soda (a base) is reacting with the acidic soil. This is the environment where we can access aluminum and produce blue flowers. If neither test produces a reaction, your soil is likely neutral, around pH 7.0, which is a stable but often pink-blooming environment for us.
Understanding the test results allows you to act as our nutritional steward. For more blue blooms: If your soil is not acidic enough (the vinegar test did not fizz), you can help us by amending the soil. Applying garden sulfur or aluminum sulfate around our drip line will gradually lower the pH, unlocking the aluminum we need. For more pink blooms: If your soil is too acidic (the baking soda test fizzed) and you desire pink, you can apply garden lime around our base. This will raise the pH, binding aluminum and allowing our natural pink pigments to dominate. Remember, these changes are gradual; you are shifting an entire ecosystem we live in. Please do it slowly and test again in a few weeks.
