Pro gardening tip: Planting trees in holes two holes deep allows only megatrees to grow.

Mind you, this can take a much longer time for trees to sprout.

Edit:

I hate how the megatrees make lumber much harder to collect without using scaffolding. This makes them be effectively shorter, so you can reach the lumber without climbing.

The problem with holes two units deep is that you can't climb out of them easily (you need to hop down there to get the two bottom pieces of lumber.) So I actually remove three ground pieces, one hole two units deep, one hole one deep immediately next to it.