Planting them in containers is often the recommended method when the soil conditions lean towards alkaline, as in your area. So yes, the azalea soil (or any other acid planting mix) would be advised. And since your tap/irrigation water is also alkaline, a dilute vinegar solution would help. And all container plants will need regular, periodic fertilizing and I'd lean towards MiracleGro for Acid Lovers (aka Miracid) for these as it contains acidifiers.

Put all these together and your blue mopheads should remain blue!

Great advice gardenrgal! You just confirmed everything I had thought about doing from reading, thanks again! I just can't wait for spring so I can go out and triple the amount of hydrangeas I already have; I really finally want to buy some of the endless summer collections!

Brian,
I vacation on Cape Cod also and absolutely love those Hydrangeas we see! Ihave added about 10 to my yard. If you get curious, check out Limelight Hydrangeas - they are stunning! (Although limegreen/white in color)
Happy gardening

I don't advise vinegar but I do recommend azalea fertilizer and agricultural sulfur. This will definitely make them blue.

Even if one provides acidic soil conditions, if the irrigation water is alkaline -- as it is in much of SoCal - it will negate that effect. Watering with a dilute vinegar solution IS recommended for a lot of acid loving plants in these areas, including Japanese maples.

Tulipsmiles: They are beautiful, I already can't wait to go back this summer! Thank you for letting me know about the limelights, after doing some research now I'm thinking about adding one of those as well!

Gardengal: Thanks for the reassurance! I do already do everything to the soil to turn them blue, and it's still not strong enough. So hopefully the vinegar will help out. Thanks!

I mat have overdone my watering with vinegar solution and my heathy hydrangea leaves and branches are now starting to wilt. What should I do? Are they going to die?

Hydrangeas have a shallow, tiny and fibrous root system so diluted vinegar in water might injure the roots. Because of that, diluted vinegar is best left for other uses. Remember that diluted vinegar is considered a part of organic weed killer program so, use other products to acidify instead, like garden sulfur, aluminum sulfate or greensand. Add more water if you need to dilute it when you realize you used too much vinegar.

This time of the year though, there may be other causes for wilting of leaves, such as heat stress: windy conditions, not enough water, too much sunlight (past 10-11am), transplanting -which disturbs the roots- or temps above 85F. As soon as the causes of the wilting are addressed, they look better.

But the effect of vinegar on the roots would be more long lasting on foliage and would display injury on the leaves after a while. The leaves may eventually drop or parts may brown out. So I would dilute the solution by adding yet more water directly to the soil.

The vinegar-water solution is - or should be - so dilute, it will have no adverse effect on the roots. All you want to do is make the water slightly acidic....somewhere around 6.0 to 6.5. And unless your tap water is highly alkaline, all that means is 1-2 tablespoons of table vinegar per gallon of water

And water the soil at the root zone, not the foliage. And do not use horticultural vinegar - only 5-7% table vinegar.

Using vinegar to help acidify irrigation water and therefore help to maintain a low soil pH is a very widely recognized practice in areas where ericaceous plants are not well suited to growing conditions. It is a completely different application than using vinegar (and not diluted vinegar) as an herbicide, so don't confuse the two :-)

Also if one considers applying highly diluted vinegar risky, it is mild compared to the reaction of sulfur when used for soil acidification. Once in the soil, sulfur is converted to sulfuric acid and that lowers the pH. btw, sulfuric acid is what is in car batteries. And folks are known to use that in dilution for irrigation purposes as well.

The reason you get blue flowers in acid soil (low pH) is that it makes aluminum ions available to the plant. Most potting soils contain no soil and no aluminum, so you will need to adjust the pH and add aluminum. You will also want to avoid fertilizers with phosphorus, because P is antagonistic to Al and will tie it up, rendering it useless. When growing hydrangeas in container mixes I would suggest using a slow release bluing fertilizer. Growers typically treat containers in the autumn and in the spring, because the uptake process is slow. Use these according to the directions, because too much Al can be toxic to plants. Clay and slate are natural, safe, sources of Al and should also do the trick.