Increase the soil acidity closer to a soil pH of 6.0 and add a little of aluminum look for aluminum sulfate) if your soil has none; purchase a soil pH kit for better control at some of your local plant nurseries. Since soil amendment changes do not happen quickly, you may have to wait until the plant has a new flush of flower buds to see the impact. An already open bloom will not change much if you amend while the bloom has opened. If the shrub blooms also on new wood, wait until the summer flush of blooms to see if any amendment changes affected suffic the color. re-apply the anenments per product amount and application frequency or else the soil will slowly revert back to its original state. If you miss the mark and get too acidic, amend with some garden lime. Purples are difficult to realize and maintain as they tend to occur in a narrow band of soil pH.

luis - thank you so much for helping me with this. Okay, first I'll get a good soil tester before I do anything so that will help me realize if I need to add the aluminum sulphate. I will also find out if my hydrangea blooms on old or new wood. Your explanation is very helpful to me. Thanks so much!! ps. What do you mean by 'a narrow band of soil' please?

What luis wrote was "a narrow band of soil pH". Purple is not an entirely natural hydrangea flower color. It only exists with specific cultivars and within a very narrow band of soil pH.....between 6.0 and 7.0. Below 6.0 and the flowers will be blue. Above 7.0, they will be pink or red.

As to making the color deeper, the intensity of the flower color has little to do with soil pH. Instead, it is related to the cultivar, the sun exposure and to soil fertility. And even that's no guarantee......more a case of gardening happenstance.

Hi Garden Gal, thanks I missed the "ph' - now it makes sense to me. lol

I planted three of them in my garden last year, they're looking good but I remember last year, they weren't as dark as it showed on the tag. Planted on the north side of my house they get a little morning sun. They're shade by a large tree in the hot afternoon sun.

Thank you Luis and Gardengal. I understand now. I'll just leave it for now and see if the blooms come out darker this year. Thanks so much for all your advice. So much appreciated. I have learned so much from the both of you over the years.