The more you can simulate the environment from where they came from, the better the chances are that they will make it so try this:
* Choose a location that provides shade starting in the afternoon, say starting at 11am and that is not too windy (causes wilting; water evaporates faster). Sun after these times causes no problem during the winter months or during the start of Spring but will scorch the leaves during the Summer months. So go outside at 11am and see where you see shady areas about 5' in diameter. Those would be worth considering provided they stay shaded for the rest of the afternoon.
One word of caution: the sun at 11am in April is not as strong as the sun at 11am in July so monitor the leaves during the worst of the summer and consider transplanting if the leaves in direct contact with the sun turn all yellowish (includes the leaf veins) or white-ish.
* Choose well draining soil
* Maintain evenly moist soil (as best as you can)
* Mulch with 3-4" up to the drip line; past the drip line if the area is windy
* Choose soil that is not too alkaline or too acidic (soil pH below 5 and on or above 8 starts to cause problems with the absorption of some minerals)
* Hydrangeas planted on the ground do not require that much fertilizer. You can feed them twice a year (April-May and July-ish) by adding Ã‚Â½-1 cup of compost, composted manure, cottonseed meal or a general-purpose flow-release chemical fertilizer.
If there is a way to determine why the previous plants did not make it, that would help greatly because you could specifically concentrate on that problem.
For example, if it was due to soil moisture issues, you could check the soil moisture every other morning by inserting a finger into the soil to a depth of 4" and water (1 gallon) when the soil feels almost dry or dry. Every time you water by hand, make a note on a wall calendar and after 2-3 weeks of doing this, review the notes and see how often you watered on average (every 3 days, every 5 days, etc). Then set your sprinkler to water 1 gallon of water on that same frequency. Repeat the finger method test again if your weather changes considerably, say the temperatures go up or down 10-15 degrees and stay there. Since the ground does not freeze where you live, you should also continue watering during the winter methods although, once the shrub goes dormant, you can water every other week or so if your winter is dry. An inch of rain weekly should be more or less enough for these new plants. Note: if your soil is sandy, you will need to increase the amount of water by 50% (water 1.5 gallons per watering).
If you see advanced warnings of breezy/windy days by the Weather Service, consider watering the night before. Windy days can cause the leaves to wilt, a defense mechanism used by plants when they loose moisture thru the leaves faster than they can absorb moisture thru the roots. But if the soil is moist, the leaves should recover on their own by nightfall or by the next morning.
During the first summer, you will need to keep an eye on soil moisture, especially as temperatures get to the 90s or higher. This should not be as much a pain in future years. Watch for signs of wilting.
Does that help you?