If you prune it, you will likely not get blooms for the season. They should begin to produce sweet potato-like tubers before long, and it will be a perennial in your area (as it is in my zone).

I grow it and it doesn't bloom for me until late November. If you have that many, try pruning a couple and see what happens.

Several decades ago I collected seeds on an upland hillside in Guatemala that was completely covered with a profusion of pink dahlia blossoms. I still have one aged clump growing in my backyard as a result. Annually, it grows to about twelve feet before it self prunes back to human size. I'm seldom around in the winter months, but when I have been, the November/December bloooms are a real show (until the winds hit--hence, self-pruning). When I have purposely pruned the plant during the growth season for one reason or another, I have noticed no slowing in growth, only forced branching. Flower production has only been impacted when bud bearing branches were topped.

Just as a note of interest, my surviving plant's siblings and progeny can be found in several locations throughout Ventura County. Two in particular are doing quite well, one at Seaside Park (The Ventura County Fairgrounds) and another in the workyard of one of our local stonemason coops. The last time I saw the second of these, it was over twenty feet tall and in full bloom. It had the ideal location in full sun next to a two story construction.

I like what Kayjones says about experimenting. It's worth a try.

Happy growing, RTH

Here is a link that might be useful: How to Grow the Tree Dahlia--Dahlia imperialis