The most simplistic answer is to restrict the amount of aluminum the plant has access to - it is the aluminum in the soil that influences the flower color. Acidic soils allow greater availability of aluminum; neutral to more alklaline soils bind the aluminum, making it less accessible to the plants. Having said that, let me go further to develop all the issues involved :-) 1) Not all bigleaf hydrangeas (H. macrophylla and closely related H. serrata) will respond to soil chemistry the same way. Some are much more stable in their coloring and are less influenced by the availability of aluminum. IOW, some pink varieties will hold their color very well regardless of soil pH or at most, turn a mauvey-purple color in more acidic soil. And whites will always be white :-) 2) Changing soil pH is not always an easy process. And it tends to be rather temporary, as most soils have a buffering capacity that will resist change and bring the soil back to its normal pH level. Your hydrangeas bloom blue now because you have acidic soil (as does much of the eastern portion of the US). You will need to add lime to effect a change to a more neutral pH and this will need to be repeated periodically, at least several times a year. Typically the recommendations are to add 2.5-5 pounds of dolomitic lime per 100 square feet or 3-4 tablespoons per plant. The exact amount can vary depending on soil conditions. For existing plants, work it into the soil well around the root zone. It takes time for this work, so it is suggested to do this in fall, again in early spring as the plants resume growth and you may need to apply again during the growing season. Be careful not to overapply - too alkaline a soil will result in iron deficiencies or chlorosis. You want to hit right around 7.0. Irrigation and rain water can also influence coloring. Generally, areas that have naturally acidic soils tend also to "acid" rain water and this will often carry through to well or tap water as well. You may need to adjust this too. As the hydrangeashydrangeas site suggests, it may be easier for you to grow your hydrangeas in containers to achieve truly "pink" flowers, as container soils are much easier to control with regards to pH and aluminum concentrations. FWIW, some cultivars that are known to maintain a very pink to purple coloring (and that's always subjective) regardless of pH are 'Alpenglow' (aka Glowing Embers), 'Pia' (aka Pink Elf), 'Kardinal', 'Fasan', 'Mowe' (aka Geoffrey Chadbund), 'Harlequin' (aka Buttons & Bows) and 'Hornli'. You may have better success starting with one of these rather than trying, often unsuccessfully, to turn a true blue pink in your not very suitable soils. Good luck!

Thanks this was a helpful thread. I have not looked on Houzz for this kind of content before. I too find these dwarf plants are not very dwarf and need pruning. Here in Pacific NW pruning these kinds of hydrangeas (Oakleaf) is easy and its harder to screw up the timing and lose the flowers unlike the mop head hydrangea. Oakleaf also need less water so that is ideal. I was just at a garden this morning where their mature Oakleaf had simply eaten the entire front foundation planting. It was hacked back to a 4' high clump but we are going to replace it since it is crowding out a native ribes. I am going to recommend Munchkin and assume that with pruning it can be maintained at 4 x 4. I do love the smaller flowers and the bark of Sikes Dwarf however. Z6-8 Photo is probably Snow Queen. It layers so beautifully. Any thoughts between choosing Sikes Dwarf and Munchkin anyone?

Rather than 'atypical', I'd say the plant you bought as 'Pink Elf' (syn. 'Pia') was mislabeled. This is not uncommon with hydrangeas, specially those purchased from mass marketers like the big home improvement stores. It will be interesting to see how this plant progresses. There are many different forms of lacecap hydrangeas, but few of them are very dwarf and most will grow quite a bit larger (in warmer zones) than the 3'x3' standard for Pink Elf. That plant is definitely NOT Pink Elf, atypical or otherwise.

Pink Elf is just a trade name for Hydrangea macrophylla 'Pia'. You can search under either of those names for more info. This is a dwarf form, rarely exceeding 3'x3', and is an excellent choice for containers. In zone 9, holding in a container overwinter will be fine. Make sure they have afternoon shade and even moisture. 'Pia' is one of the few pink flowered mopheads that retains its very pink coloring regardless of soil conditions.