We place plants like tomatoes, eggplant, peppers and cabbage family varieties in peat pots, which contain regular soil, after they are about 3 inches tall. The reason for this is that these types of plants dont like their roots to be disturbed when they mature. The peat pots are larger than the plastic tray compartments and allow the root structures to grow with less restriction. Notice the wooden stick containing the name of the vegetable.
When a plant goes into a peat pot you must monitor its soil moisture, because the peat pot loves to absorb water. They will normally require more moisture than when they were in the plastic trays, so check them daily.
Some people have complained that when they put the peat pots in their outside gardens that the pot doesnt break down and the plant becomes root bound. If you have dry soil and dont use any underground watering system with black plastic as we do, this is possible but not common. We have never seen even the remains of a peat pot at the end of the growing season in our garden and have not experienced a root bound plant. If you are concerned with your peat pot decomposing, be sure to soak it in water before planting in the outside garden, and cut the sides just as you are placing it into the soil.
Some gardeners use Jiffy Peat Pellets but we find them to be more trouble than they are worth, and they cost a pretty penny too. We have found that the best time to buy peat pots is in the fall when the merchandisers want to get rid of them.