Mine is planted under and near the trunk of a deciduous Magnolia stellata, which has roots near the surface. It required frequent irrigation during the first 2 years, until the roots became well established. It is now 4 years old and is about 3' X 3'. I grow many different cultivars of H. serrata and the largest one (Woodlanders') is about 4' tall and 7-8' wide. It has never been pruned in the 8 years that I have had it. It also receives several hours of direct sunlight each day, morning & afternoon. The roots of a Dogwood, Cornus floridus(yours?), are also near the surface and the Hydrangea will require supplemental moisture until established. I would not recommend any application of fertilizer in that circumstance, unless applied equally to the entire root system of the tree, same applies to irrigation. I do not, as a rule, fertilize any of the plants under my trees.
Rb

Hi Razorback , thanks for your quick response. You wouldn't happen to be Michael Dirr would you? LOL Anyway, the dogwood is the hybrid one from Rutger's 'Stellar Pink' and is about 8 -10 feet away from where I want to plant this shrub. The underground sprinkler system is on every other day at around 4:30 AM and our soil is decent because everything I put in the ground does pretty well. I have about 8 perennials under this dogwood and a birdbath as well. The dogwood was originally 4' tall about 9 years ago and is now 20' by about 15'. The arborist told me that it must grow about a foot per year due to the new growth at the terminals.It is one happy tree. I'm glad to hear that the shrub doesn't need pruning. I have H. quercifolia 'Pee Wee' in another area of the yard and I only pruned it this spring due to a die back of a central stem from the cold snap we had. I really like that I won't have to prune this one. I have about 4-5 feet away from the dogwood, 2 camellias and a weigela along with some assorted perennials among these shrubs. All these were planted this spring and are establishing well.What color are the flowers on your 'Purple Tiers'? Blue or pink? In the pictures I've seen on the net, I prefer the intense violet blue over the pink color.

H. serrata, like H. macrophylla, will normally have blue flowers in acidic soil, pink in alkaline. There are exceptions, of course, in cultivars of both species. My soil is clay based and the only ammendment I regularly use is ground composted pine bark(pine bark fines), so the flowers are blue on my 'Purple Tiers'.
Rb

Yes, I'm not Michael Dirr, but became aquainted with him years ago through a mutual friend. Attend many of his lectures locally and have twice visited the CANR facility in eastern GA where he and his associates and staff hybridize, propagate, grow and evaluate hundreds of Hydrangeas. The effort is primarily directed toward H. macrophylla's, but some work is being done with H. arborescens. That is the location where 'Lady in Red' was developed.