Turns out we have two hibiscus trees, probably Rose of Sharon variety, in the front garden. They finally started blooming over the past few days.
Now that I know what to look for, I've seen these plants all over the neighborhood. Turns out they turn into rather tall, leggy shrubs (or small trees), so we may have to find some new homes for them. Three large plants in one small front garden is probably two too many. But, for now, we'll enjoy the pretty purple flowers.