Hey that's a DanG good idea Jeff!
If it is only 4 inches then all I have to do is put 4.5 inches of fill instead of 4 feet! Not near as much wheelbarrow work that way.

My next door neighboor has a walnut tree about 4 inch DKH (diameter knee high - its not very tall). I was thinking about rigging up a big mirror to give it a little more light. I can reach it with my hose to give it water this summer and this winter I could put an electric blanket around it to keep it warm. Maybe all this extra attention will help it grow a little faster so that maybe in 20 years or so I can saw a couple of 2x4's when he isn't looking
