Is that indoors? You photo appears more like a screened porch.

Some thoughts ... that pot looks small for the size of the plant. Hydrangeas are rarely successful indoors, household humidity is too low for them and they need a period of dormancy (as in chilly fall and winter). Direct afternoon sun isn't appropriate for hydrangea, it is usually too bright - morning sun would be more typical in caring for them.

I don't know what amount water you used when you fertilized. Miracle Grow is not my favorite product, but reading the online label says 1/2 teaspoon mixed into one gallon of water for indoor potted plants. I cut the strength suggested on the packaging by half when using a fertilizer, applying a solution diluted from the manufacturers directions.

I too would switch to a bigger pot and use new potting mix. There may not be much in terms of minerals needed by the plant in that potting mix. Try testing the soil with a soil kit sold by local nurseries in order to tell if you have too little (or too much) of the basic minerals nitrogen, phosphorus and potassium.

Potting mix - unless it is reinforced with added fertilizers - just doesn't carry any nutrient load to speak of. Any nutrients must be supplied by the grower via supplemental fertilization, therefore any testing is pointless.

A larger container with better potting mix as was previously suggested is the way to go. Then fertilize weekly with a half strength formulation of the fertilizer of your choice.

Keep it watered and when the blooms fade do as everyone else says or plant it in the ground. You may have overdosed it since you didn't say how much water you used for the scoop. You could flush out the soil if that is the case but I doubt you harmed it in any way.

I have to water my two florist hydrangeas every other day and it isn't super hot yet. Once the blooms fade they are going in the ground or a much larger pot. The second pot (Amethyst) often wilts so much that the blooms look like they are done for but after a good watering they are plumped up once again.

RE: original topic... 3-5 cups per day is a bit too much water. Otherwise, that's simply the plant's behavior. The leaves get like that due to sun exposure. As long as they aren't wilted or drooping or turning white or black, they're fine. As for the blooms, they can turn brown or limp around the edges of the petals, especially if in direct sunlight. Also, hydrangea blooms tend to turn brown as they age.

I theorize you got this as a gift or otherwise from a florist source. It will behave like this when subjected to natural conditions 100% of the time, especially when raised in unnatural conditions (a.k.a. Greenhouses). Growth and blooms that occur in natural conditions will be more apt to handle them.

Also, fair warning, if you plant a "topiary" hydrangea "tree", it will come back as a regular hydrangea with a dead stick in the middle next spring.

This forum really needs sticky threads on Florist Hydrangeas and winter hardiness. Who's with me?!