Plant the whole stem so only the leaves are poking out of the soil. The stem will turn into a root stem.. The fertilizer in the soil is ok as long as it isn't over .25% nitrogen. Since you are using fertilized soil don't use the miracle gro. Water it with about a cup of water and if you can use bottled water because tapwater from city lines has chlorine which wont hurt a mature plant but could kill a baby seedling. Be carefull when separateing the roots the more soil you keep intect to the roots the better... good luck and in the future grow seedlings in seperate 1" containers like what little annual flowers come in.