I have an Annabelle. Mine flowers once but holds the flowers all season long until they dry on the stem.

Some stem browning out in the Fall is normal as the plants harden and prepare for winter but Lord, the stems should not be falling down if it is that. I would first check the soil moisture and inspect the length of the stem -including the area where it connects to the base/crown- looking for signs of "damage" by insects (not likely but always worth a shot). The alternatives are difficult to identify without technical help. Root rot can make the stems loose color as the roots decay and then the stem browns out albeit root rot tend to a-l-s-o cause leaf issues like wilting, which you do not report. Odd that root rot would not cause leaf symtoms. Note that if it is root rot, leaf wilt may happen once the stem browns out near/around the leaf's petiole. A non scientific test for root rot is to see if the soil is wet for long periods & to extract the plant and check the roots, even smell the roots. You can do that to one of the worst ones since pulling it out will affect the look of the hedge so think about whether you want to do that or take it for analysis instead. See the info on Phytophthora Root Rot below: http://www.aces.edu/pubs/docs/A/ANR-1212/index2.tmpl Another fungi called corniespora (Sp?) can cause stem rot in hydrangeas, albeit very rarely and it tends to make leaves wilt too. There are fungicides recommended but I am not sure if this is your issue since your notes did not mention leaf wilt. Bacterial pathogens like pseudonomas can cause either leaf spots and-or stem rots on hydrangeas. There is no easy way to diagnose psedunomas as Agric Extension Folks are the ones who have the equipment to detect/identify it. The usual clean sanitation practices are recommended as well as the destruction of infected plants and use of copper fungicides. So, I suggest a quick trip to the nearest Agric Extension Office with a sample in a tightly sealed transparent bag since this is spreading so fast.

As Luis said, a new plant shouldn't need fertilizer, and you can often do more harm than good with fertilizers, such as what Luis cited about excess nitrogen causing more leaves and fewer flowers, but also excess fertilizer can wash out of soil and into nearby waterways. So unless a soil test tells you something is missing I wouldn't add fertilizer now. If you want to add a slow release product like composted manure next spring that shouldn't be an issue, but I honestly never add fertilizer to mine and it grows very enthusiastically. What you are likely seeing is transplant shock. The plant has had its roots disturbed, the soil and sunlight levels are different, and the watering regimen has changed, so it is reacting to all those changes by looking a bit stressed. The best thing you can do for this plant is to keep the soil evenly moist (not soggy or dry). Stick your finger in a few inches to check, both in the original rootball and in the surrounding soil. Water with a long slow trickle with a hose or a jug with pinholes punched into it so the water soaks in deeply. I cover the soil under the plant (but not actually against the stems) with a couple of inches of mulch, either shredded wood or compost, which will over time break down and provide your plants the nutrients they need to grow. This will help keep soil moisture even and help reduce weeds. You don't say where you are, but in most of the country Annabelle appreciate shade in the hottest part of the day. Mine here in central NH gets sun from early morning until early afternoon and then is shaded by a building and it grows happily with little attention from me after its first couple of years. I do need to remove suckers to keep it reasonable sized but it will be a while before you have to worry abou that. If you are in a warmer, more southern region, like Luis in TX, it will need more shade to thrive. If you see leaves looking limp, give it some temporary additional shade for a few weeks while it settles in by placing something like a lawn chair where its shadow will give some shade while the plant adjusts to its new home.

Looks like a too sunny of a location for Annabelles to be happy. Have you considered Hydrangea paniculata? They are far more sun tolerant and come in a range of sizes as well.