This is a late response to your post but usually the recommendation is to start seeds 6 - 8 weeks before your last frost date. So if you start during the first weeks of April, they should be good to go out in June. I am about to start some tender veggie seeds (tomatoes, etc) this weekend myself in order to have them out by around Memorial Day when the ground has hopefully warmed up enough.

If what you have are hardy hibiscus, they may be able to survive in your area if you mulch them well. They are late bloomers but once they get going in warm soil, they should grow pretty fast.

Hi Cheerpeople, This is another late answer but I also live in zone 5 and I have many hardy hibiscus that I leave out every winter, I just cut them down to the ground and they come back every year. It sounds like you have a "Lord Baltimore" I would leave it in the container for the first year and then plant it in the ground. They do grow very fast in warm weather and you will have to stake it because the stems get very heavy and will flop to the ground.