This is quite a remarkable picture considering that the colour of Hydrangeas are determined by two factors: the amount of light the plant receives and the pH of the soil. Though they will tolerate full sun, which they appear to be receiving in this photo, they much prefer and will thrive in partially shady conditions. The blue flowers are produced by a highly acidic soil, while the pink are produced by a neutral or only slightly acidic soil. Considering these plants are bunched so closely together, there must be quite an amazing blend of fertilizers in the soil to grow such a variety of colours in such a small area.
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