Looks like it got a case of frost damage, fertilizer burn due to too much fertilizer or root rot. In any case, the plant is stressed so determine which problem is the cause and take appropriate action.

If you have had frost in your area, weather could have caused the problem and the plant should recover in time. The symptoms should last until leaf out reoccurs. This problem however, should not reoccur to the new growth unless you are lucky enough to have frost again. If this were to happen a second time without evidence of frost then I would not blame the weather.

If you applied too much fertilizer (too much aluminum sulphate can also cause burn) on top of the small root system in potted hydrangeas, you could have caused root burn... especially in the tiny roots near the top of the soil. To correct this, you can try to remove the excess by hand and switch to an organic fertilizer (they usually do not cause fertilizer burn). Organic choices include compost, composted manure and cottonseed meal.

Then again, it could also have a case of root rot because you are watering too much ("almost every day"). There is no way to "fix" plants that get root rot but if it is just starting, you might try controlling your watering regimen by using the finger method: water the soil early in the mornings when a finger inserted into the soil to a depth of 4" (10cm) feels either almost dry or dry. One gallon of water (3.8 liters) is a good amount to start with when watering a newly planted small hydrangea. If the soil is moist, you do not need to water. If the soil is wet, do not water either but you may also want to research why the soil is wet because hydrangeas like well draining soil. Each time you water, make a note on a wall calendar. After 1-2 weeks of using the finger method daily, go to the calendar and review how often you had to water. Then set the sprinkler system to water 1 gallon of water (3.8 liters) on the same frequency (for example: every two, three days).

Apply 3-4" (7.5-10cm) of mulch up to the drip line to help reduce the number of waterings and maintain the soil moist when the weather is windy.

See the link below for more information.

Luis

Here is a link that might be useful: Diseases of Hydrangea

Thanks, Luis. I didn't prune my hydrangea beyond deadheading, as there are only a handful of stems. There is no movement on the old stems, though, and all growth is occurring at the very bottom. Should I prune the stems down, or just leave them alone? Will new shoots emerge on these stems eventually?