About 8-10 weeks before the last frost is the perfect time to start baby plants from seeds. In our area historically May 1st is the last day frost is possible. We are right at the foothills of Mary's Peak and the old timers say: "It's time to transplant baby plants into the garden when there is no more snow at the top of Mary's Peak". Because the weather is changing there is hardly any time that the Peak is covered with snow. Therefore, I will stick to May 1st to transplant the very tender annuals into the garden.
We have a small greenhouse with heated benches that are perfect to start the seeds. One of the benches has a bottom heated sand box with automatic mister which we use to start plants from cuttings. My favorite time to work in the greenhouse is on a rainy day. The sound of the raindrops are fantastic, like through a loud speaker.
Some of the starts will be planted in our large greenhouses. It is mostly to speed up the season, but also to protect tender plants, such us Lisianthus, Tuberose, and Tweedia. Those are not suited for our growing zone. Before we plant the baby plants in the garden, we will let them acclimatize to the new environment so they do not get sun scorched. We will do that by gradually moving them from the greenhouse into the shade, then slowly into less shade, and finally into the sun. It will take several days. Best is to plant them on a cloudy, misty day.