Answer: The bloom color is related to soil pH, with the best blue when the soil is acidic, so to maintain the best blue you should use a fertilizer for acid loving plants. With a container plant, you might use a complete water soluble fertilizer (with minors) such as Miracid at the lower label rate each time you water. Do not exceed the dilution on the label as oversupplying it with nitrogen will encourage legginess. Fertilize spring through summer, stop in mid fall. You could also top dress occasionally with a good quality compost or use a compost tea once a month or so.
You need to water carefully so that the soil stays evenly moist, never sopping wet and not dried out. In summer heat especially it is important that the plant not be allowed to dry out. Raising the container up off the ground by a few inches can also help keep air circulating around the container and thus keep the roots cooler; this helps reduce heat stress as well.
A location in morning sun or bright dappled light all day would be best, since hot afternoon sun can scorch or stress them especially when grown in containers. Wind protection is also important, as wind is drying and causes stress. In winter, too, wind can cause excessive dieback.
Endless Summer blooms on both old and new wood, so pruning time is not as important as with some types of hydrangeas. I would suggest you reach inside the plant and remove some of the longest stems from time to time. This way you should always have blooming stems on the plant. If you prune it back hard all over all at once, you will cause a gap in the blooming cycle. You could however trim it back quite short all over in the early spring. This would delay blooming, however it would allow you to control the size as much as desired.
The blooms do change color as they age and eventually turn papery or tan. This is normal. You can deadhead or remove the flowers as they fade if you do not like the look.
Enjoy your hydrangea!
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