Plug Inserts 12 Cell Seedling Trays are joined cells with drainage holes in a flexible plastic sheet. The whole sheet fits inside the Seedling Trays. Each 9 x 13cm (3_ x 5″) cell can be detached on its own. This size is good for large seeded plants like cucumbers, melons, and squash or for seedling clumps like Lobelia, Alyssum, or microgreens. We recommend using sterilized seed starting soil when starting seeds indoors. Placing your seedling tray on a seedling warmer mat will generally speed up germination and give you more even germination, with all the seeds sprouting at once. If using the clear seedling dome that fits over the seedling tray, you should remove this once the seedlings sprout.