Q&A for How to Keep Blue Hydrangeas Blue

My best guess is that the pebbles and brick contain limestone, which is easily dissolved in water and will raise the pH of the soil. You'll have to water with a slightly acidic water.

No, you can use baking soda, but you will need to be patient. Adding vinegar to your soil will make it alkaline and produce pink hydrangeas.

Garden lime, sprinkled and worked into the soil below. Just do not allow it to touch the plant.

This is a natural progression for hydrangeas as the blooms age. The typical change will be, for example, from pink to green and then to a mottled white and brown. At the final stage, the colors will stay constant, often through the winter if not pruned in the fall.

If you want a true purple, you may have to give up the deep green look. Purple hydrangea flowers are usually lighter on the green spectrum than those of red-purple hydrangeas. The best way to change the color of your hydrangea is to buy a new cultivar of hydrangea with purple-red flowers that have been selected for this color. You may also want to look for deep purple hydrangeas in catalogs as they are usually more expensive than the hybrids.

Your hydrangeas are probably in an "F2" or "m2" generation. It will not have the same vigor, as the plants should be older. You can apply iron (III) sulfate once per year in early spring. For the most vigorous varieties, it may be more effective to add the iron to the soilless mix in the first fall after planting. With some varieties, you can apply a blood meal at a much lower rate than recommended on the label.