A pot full of marigold seedlings can look successful at first, then become awkward to separate. Decide whether you want a few plants to stay in the container or whether you are raising individuals for later planting.
For plants remaining together, remove surplus seedlings to provide space appropriate to the variety. For plants being moved on, prepare the new pots before lifting them so the roots spend less time exposed.
Handle the root area gently
Moisten dry compost before separating seedlings, and use a small tool to ease them out rather than pulling hard on the stem. Support the root mass as you move each plant.
Keep the identity of different varieties attached to their new pots. Similar seedlings can be difficult to tell apart once their labels are lost.
After moving, settle the compost around the roots and check its moisture. Do not compensate for the disturbance with a strong fertiliser dose. Watch whether the seedlings remain upright and resume growth. Their next stage depends on light, root space and suitable conditions, not simply on how full the original seed tray looked.
Related reading: Growing Marigolds: Light, Soil, Watering and Pruning.