I am transplanting the plugs into 4" square pots. They will stay in the 4" pots for approximately 6 weeks before I transplant them again into 12" round waxed pulp baskets. Here is when to transplant out of 4" pots. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-nnEr_uaoiRE.htmlfeature=shared
On plugs, you are waiting for the roots to to be dense enough to hold the soil ball together. You can test that by wetting the soil and gently pulling on the stem of the plant. The soil should come up cleanly with the plant. If it seems solid and holds together, it is ready to transplant from plug tray to 4" pot.
Petunia seeds should be planted indoors ten weeks before your last frost of winter. The growing area must have artificial grow lights as sunny windows do not provide enough light for petunias to grow correctly. Petunia seed is very small, almost dust in consistency. Purchasing pelletized seed will make planting easier. Most seed is planted by sprinkling on the soil surface. Do not top dress the soil after planting. The plugs in the video were made by taking soft tissue cuttings.
We purchase our plugs from a Ball Seed. They are a supplier for commercial growers. They are a great supplier if you are looking for large quantities of plants.
Here is a link of one of my trips to see the new varieties that were available through Ball Seed: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-EvHBRjAvi6M.htmlfeature=shared