>>8110635Providing you're working with a species of Hydrangea that's affected by soil Ph value, the closer to Ph14 (extremely alkaline) the soil becomes the flowers should increasingly turn deeper/darker pink/red in colour.
There are a small number of common methods used by gardeners to artificially treat soil to change its Ph value when working with Hydrangeas, either to make the soil more acidic or alkaline as desired; but keep in mind that in so doing you might restrict your options in terms of what other plant species you'll be able to grow in the garden.
Beyond that it might be possible to find certain 'fixed color' species of the plant that retain a darker red flower regardless of the soil conditions, though for that you might need to find a more specialist garden centre rather than the typical gardening section of your average large DIY store.
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Also, well done to everyone who contributed to this, the best thread on /wg/ for a good long time.
