Quick answers:
If the ph in my new garden is around 5, will they turn blue/violet? Yes, sometimes; no, other times.
If the ph is higher is there any point buying a load of the appropriate soil and mixing it with the soil in the garden? Not much
If the soil is more alkaline is there any point, once in the ground adding the vinegar and chemical next Spring? Depends on how much do you want those blues.
Details:
The discussion on what to do is not always easy/clear. Hydrangea macrophyllas like the one you purchased will respond to the amount of acidity and availability of aluminum in the potting soil. Acidity is measured using a logarithmic scale where a reading of 7.0 means the soil is neutral, levels below 7.0 are considered acidic and levels above 7.0 are considered alkaline. If planted in the ground, the blooms will slowly change the colors they have when they initially open... as more and more roots get from the potting soil into your garden soil.
Aluminum is what you need to turn pink macrophyllas blue or purple. But it is not easily absorbed. For the plant roots to absorb aluminum, you also need acidic soil. Alkaline soil will tie up aluminum with other minerals and the roots will not be able to absorb whatever aluminum there is. Thus, potted hydrangeas need potting soil that has an acidic soil pH and also need potting soil that has aluminum. Amend the potting soil regularly for both things to keep the soil acidic and have some aluminum. You can do this by using fertilizer for ericaceous plants like azaleas, camellias, hydrangeas, blueberries, etc. (it has sulfur to help acidify the soil) and by using aluminum sulfate (it has both, aluminum and sulfur).
I do not recommend using vinegar to acidify the soil. Vinegar CAN acidify the soil but, it is also a herbicide used to kill weeds in organic weed killing programs. Hydrangeas have tiny, shallow, fibrous roots that do not react well sometimes when vinegar is diluted in water and applied to the soil. The leaves are affected by the root injury and they look like they are going to die. To remove the leftover vinegar still there, you would need to water a lot.
When blueing macrophylla pink blooms, add aluminum sulfate regularly, as all the frequent watering that is done in the summer will quickly send aluminum and sulfur through the pot's drainage holes. On the other hand, do not accidentally use too much as the sulfur can burn the hydrangea's tiny roots. Check the label instructions for guidance.
To see if things will work in your new garden, you may need to do a soil test or at least walk and see how other hydrangea blooms look like in your neighbor's gardens (or ask them if they get blues or pinks). There are soil pH kits sold at some plant nurseries that will also provide this info although some may not be that accurate. You can always transplant the shrub, leave the soil alone (without pH/aluminum amendments) and see how the colors appear in its first few years. After 1-2 years of being transplanted into the ground, the bloom color should be somewhat stable and reflect the soil pH and aluminum levels in your new garden soil. At that point, you can begin tweaking as needed.
Once you start amending the soil, plan to continue doing this forever per your product's label directions. If you change your soil from a pH of 7.6 down to 6.0 and then stop regularly amending the soil, the soil will return back to 7.6 in time. Note that you may need to also amend during the winter months. I would also recommend delaying a dosage of amendments if temperatures are unusually high (say, in the hot summer months).
Note too that some macrophylla varieties resist turning blue and others do not change much. Named cultivars can be researched to see if they will turn blue but potted plants sold at grocers and florists do not provide that information.
Adding aluminum and-or soil pH amendments? Many soils have naturally occurring levels of aluminum. You would think that the blooms would then be blue always. But if the soil is alkaline, you get pink blooms. These soils do not need more aluminum. You just need to acidify the soil so the roots can absorb the naturally occurring aluminum in alkaline soils. The opposite also happens. Some acidic soils have no aluminum or very little (a soil test is needed to find out) so you get only pink bloomss; for those, amend using aluminum sulfate. Remember to water the hydrangeas first if the soil is dry and you are going to fertilize or amend the soil.
Luis
