Keeping adequate light source and a fan to mimic wind will help create stronger plants with thicker stems. Be sure to water your seedlings as soon you’ve transplanted them and add more soil if it sinks down. If you have a very leggy plant you can gently bend the stem so that most of it is inside the pot and it’s not sticking out too much. Keep adding soil until the base of the cotyledons.
Gently press the soil down around the plant as you add the soil, otherwise after you water you’ll end up with it sinking down. Holding them by the base of the plant, place them in the center of the soil, then while carefully holding the seedling in the middle, add more soil around the plant. Indoor container rosemary plants can be pruned any time during the spring. Outdoor rosemary thrives in zones 7-11, so March-May are usually the best time to prune.
After they are clean get them ready and fill them half-full with seed starter soil, at this stage I also add a sprinkling or worm castings in the center. Plants that have become overgrown or leggy should be gradually pruned by cutting back about one-third of the plant every 3-4 months during the growing season. Make sure you disinfect your seeding trays and pots first. After sorting through your seedlings and checking which ones are the healthiest you’re ready to move them into larger pots.