The story can be used anywhere you like. Put the story on a sign, trees to be planted nearby. Or actually have a wizard there who actually wants the trees planted. Maybe it's just someone the PC's need to help before they can get something from the other person. Make the story whatever you would like to fit your game.

A man seeking an apprenticeship wished to prove his worth and mettle to the local wizard. The wizard said to him: 'I have ten trees I wish to have planted, but I want them to be in five rows of four. Accomplish this task, and I shall train you.â'

So the man set about planting the trees. When he was done, the Wizard took him under his wing as his apprentice, and imparted his first bit of sagely advice: 'Never skip meals.'

The trees must be planted on the edges of a five-pointed star(draw a star with a single line on a piece of paper, at every intersection, make a big dot to represent a tree, there will be ten dots).

When the trees are planted in this pattern, they sprout up to full-grown height, complete with fruit. The fruit will allow for memories to return, allowing previously forgotten information to return as the fruit is eaten. This is extremely helpful to spellcasters, who can return the memory of a single spell by eating the fruit of the tree. The fruit doesn't last long, mind you, so you can't take it with you. But, if you're in the area, and you need a quick pick-me-up, head to the grove you planted. That is, if you can remember it's there. ;)

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This post brings up the idea of arrangements of trees (or other objects) bearing arcane significance.

What if you have a dungeon with arrangements of trees that through manipulation (cutting down trees, planting new ones) you can nudge into different magical patterns, creating different effects?