If you place a hydrangea in an area where it can achieve its mature size, kimcoco, you will never need to prune it. Hydrangeas should not be pruned for the sake of pruning; only when they outgrow their bounds, when you have safety issues or when you have some aesthetic problems should you prune. It should not need winter protection in your zone; just make sure it gets 3-4' of any type of acidic mulch.

Here is a link that might be useful: Annabelle Care

Interesting, though I remember reading that they should be pruned back early spring for some reason, so I googled and found this info:

It is recommended to prune this hydrangea back to 2 - 3 feet each year to help it support the heavy flower stems. Deciduous.
Zones 3-9

I also found this info at Bordines.com (it's a PDF file so I can't cut and paste):

Annabelle Hydrangeas may be treated like woody perennials and cut down to 3" each spring to encourage a compact growth habit.

I also found this info with regard to fertilizing from colorchoiceplants.com:

Fertilizing: Fertilize in early spring by applying a slow release fertilizer specialized for trees & shrubs. Follow the label for recommended rate of application.

Note: Once established do not fertilize the cultivar Annabelle - fertilization causes lush growth and the plant will not be able to hold the blooms.

Interesting schools of thought.

Good observation, kimcoco. One thing I would love to see in literature like that is a note that describes for what growing conditions does the item applies or on what zone/location that was tested. I still remember how my first lilac bush was scorched and died even though the mail order nursery insisted until near the end that full sun would be fine for the bush. Well, the plant label did say that but in practice, that does not work in some areas of the South like Dallas/Fort Worth. Now I grow one in afternoon shade and ignore labels that say full sun.

Returning back to the original question, the literature that I have read and the logic from some of the gardeners is that light pruning is recommended to let the stems thicken from one year into another. That sounds ok in my zone but, since your zone 5 is colder, I assumed Annabelle stems would be new from the ground each year. I realize this is your first season so you may not know if they only come back new from the ground. If that is not the case where you live then prune lightly to help them thicken.

How has this shrub performed for you in WI so far, kimcoco?

Luis

I planted them in early February, so I'd hardly consider them "shrubs" this early on...LOL

They're small, MAYBE 2 feet in height, healthy, but I'm having a difficult time visualizing how they will expand into the beautiful masses I've coveted in others' landscaping. What a stunning plant.

How many years until they "fill in", typically? I have two of them spaced about 24-36" apart (whatever the recommended spacing was), and a plastic border in front of that to be sure they don't move onto my neighbors property.

With regard to the lilac, I had the same luck with my Pieris I purchased this year. I was told full sun was ok, but my Pieris did not agree, and after doing more research on the internet, not only should it not have full sun, but it can usually handle only early morning sun. And depending on where you look, they are only hardy to zone 6, not 5. Shame because it would have looked stunning in my back yard. I planted another in a shadier area of my yard, and so far so good. Only time will tell.

Luis, do you fertilize your Annabelles? Do you recommend fertilizing after they're established, or just for the first growing season?

-Kim

Most potted plants contain fertilizer pellets so I start fertilizing on year two; hydrangeas do not need much fertilizers (when planted on the ground) anyway so this is fine for them. In non-sandy soil that contains average amounts of minerals, you could skip fertilizing for a year and not notice.

I follow these suggestions from www.hydrangeashydrangeas.com: fertilize in May and July on the South or fertilize in June in the North. They have a section for Fertilzing and one for Supporting Annabelle Blooms).

For a small bush, I add 1/2 to 1 cup of cottonseed meal or manure (more for bigger plants). Throughout the growing season, I add weak fertilizers like coffee grounds or liquid seaweed. Then I stop all fertilizers in late August or in September to make sure the plant goes dormant. Here in Texas, hydrangeas go dormant either in November or December. You may want to stop fertilizing earlier since your shrubs will do dormant before mine.