Some while ago I noticed a walnut shell in one of my planters, but didn't bother removing it as I was always either doing something else or I forgot. Two days ago I saw that it was growing and was about 4" tall! As walnut trees can grow to 30m or more it is obviously not suitable for a planter OR my small garden. Any advice welcome, such as, - can I restrict it's growth to keep it in the planter or what? hopefully photo to follow.