The best time to transplant these little flowering plants is after the last occurring frost in your area. As seedlings, the plant will not be able to tolerate the harsh conditions of the winter, especially in regions where the winter environment is colder. Transplanting the flower soon after the area's last frost will ensure that the plant blooms during spring. It will also help the plant stay on a regular flowering schedule the following year.
Sweet William flowers can be started outdoors with proper fertilizer and water drainage, but growing them indoors first will ensure the best blooming schedule and most durable plants. As seedlings, the plants will likely succumb to the harsh winter environment outside. Growing the plants inside during winter will help them become strong since indoor environmental factors can be controlled. When you have a healthy seedling, or a rooted plant that has not yet flowered, you can transplant it outdoors with proper fertilization and shading.
In addition to the timing of transplantation, the plant's growth stage is important as well. If the seed is grown inside and is going to be planted after the last frost, the seed's roots must be firmly rooted in the soil. This is a sign that the flower is growing. It is also a sign that the roots are doing an effective job of keeping the plant strong because the roots hold the plant in place. Roots are the most important indicator of effective growth, so waiting for a stem to grow is not necessary prior to transplantation.
When Sweet William is transplanted, it must have adequate lighting and soil to continue to grow as well as root itself in the ground. Since it is a small flower, it should be placed along flowerbed borders to get enough light for proper growth. There is no need to worry about a Sweet William affecting other plants negatively. Additionally, soil with good drainage is important.