hi - ive bought some red drum cabbage plug plants, they seem sturdy and healthy about six inches high. My question is this - Ive never grown cabbages before and they look very open , by this I mean they have a two inch stem and then four inches of leaves, if they are going to form a solid ball of cabbage do I need to plant them deep to cover the stems or will everything thicken up including the stem ? help much appreciated .