Hi and welcome
Frankly, most of us here in Florida have learned the hard way not to try to push the zone, and we enjoy growing all the other wonderful shrubs and flowers that you can grow here but not up North. Many of our native and exotic things also bloom for more than just a few weeks at a time as well, which makes them a lot more worthwhile to give garden space to. (not to mention time and effort, water and fert etc.) Why have a struggling lilac bush bloom for a total of 4 weeks out of a 9 month season, when you can have 9 months of steady bloom from a lovely plumbago?
There's no reason you couldn't create an artificial dormancy for your lilacs. Probably an old fridge (try the Re-Store) out in the garage would keep them cold enough through the winter for a couple of months. It will be easy while they're small, but I'd think in a couple of years you're going to have lilac bushes that are in big pots and are getting too large to be moving in and out of a fridge. The use of electricity to keep them cold would bother me, not only the cost (it's not cheap to run a fridge) but the waste of energy.
Why do you have your new plants indoors now? Where lilacs grow well, they like full sun all day and grow in much hotter daytime temps than they'd be getting here in Florida. I was just in Salt Lake City where the lilacs were finishing up their bloom, and it's sunny and hot there now.
Just a thought, wouldn't it be easier and cheaper for you to just take a trip up North when the lilacs are in bloom and enjoy them that way?
