Hello, Julia. Browning of hydrangea blooms suggests a moisture problem. Hydrangeas first abort flower buds when they lack enough moisture. Next to go are the blooms and leaves.
Because of the intense heat here in Texas, this is a common problem here. Extra water and some sun protection can help prevent browning of the sepals (that is what hydrangea petals are called) but once browning has begun, there is nothing you can do to reverse the damage. If it bothers too much, you could deadhead the spent blooms just like people deahead roses. If you deadhead a hydrangea that reblooms through the growing season, you may get new blooms in about 4-6 weeks, provided that the weather and the length of your growing season are just right.
If you need help with watering, you can use the finger method. This involves checking the soil moisture at the same time of day every day for about two weeks. You insert a finger to a depth of 4" near the base of the shrub. If the finger feels dry or almost dry then you can add 1 gallon of water and then write a note in a wall calendar indicating when you watered; if the finger feels moist, do not water; if the finger feels wet, take action -if necessary- to prevent root rot.
After two weeks, review the notes in your calendar and determine how often you had to water: say, about every 3 days or every 4 days, etc. Then set your sprinkler or drip irrigation to add 1 gallon of water on the same frequency (every 3 or 4 days). If the temperatures vary by 10-15 degrees either way and stay there, use the finger method again. Reduce watering in the fall. And continue watering once every 2 weeks during dry winters, provided the ground has not frozen.
Cottonseed meal is a great fertilizer for hydrangeas. You can also fertilize hydrangeas with compost, composted manure, cottonseed meal or a general purpose slow-release fertilizer. They are not heavy feeders though so a cup of cottonseed meal in June is fine for those who live in the northern half of the country; those in the sourthern half, because of the longer growing season, can fertilize in May and July. Because July and August are so hot over here, I tweak that suggestion and fertilize in early May and the mid June. Few plants would like to be growing and blooming during the month of August!
Hint: a newly planted hydrangea may not need any fertilizer on the first growing season. That is because the wholesale nurseries tend to add those round fertilizer pellets to the potting soil mix. So if you plant a hydrangea that came with fertilizer pellets last April -for example-, you can skip fertilizing until 2011.
You can read still more information on hydrangeas in this website: https://www.hydrangeashydrangeas.com/fertilize.html
Does this help you Julia?