It was a silly tradition. It certainly fit with how he was perceived. Arthur Weasley the joke. Arthur Weasley, the muggle lover. So when Bill was young and Arthur came home with a set of muggle gardening tools, he had smiled and offered up an acorn from the old tree.
The two had planted it together, and Bill had forgotten about it until it sprouted. When that happened, he had declared it "Mine!"
When Charlie was older and realized that Bill had a tree (One he was not allowed near) he decided that he wanted his own. Preferably far away from Bill's tree. So Charlie got his own tree, playing in the dirt with his father while they planted the acorn a few meters away from Bill's. In the clearing were two saplings, happily absorbing the little sunlight they received.
Percy had been much less happy about the prospect of dirt, but he soon realized that if he was not allowed near his brother's trees, the reverse would happen if he had his own. Soon enough, the boy had his knees on the ground and dirt smudged on his nose, helping his father plant the acorn.
George received his own tree, something he was thrilled about. He had planted it emphatically, shovelfuls of dirt flying into the air as he dug. The boy had been all laughter when he hit his father "accidentally" with most of it.
Fred had found no less then eleven acorns, and was helping his father to figure out which one would be best. The way he did this was by explaining each personality of the acorns, and then asking which one would be the best for his tree. Ultimately, they decided on the smart, fast, and funny one. Arthur didn't ask how he had decided which was which.
Ron had spent more time trying to eat the acorns then trying to plant them. Pulling them from his hands and putting one of the saliva covered acorns in the ground, Arthur had explained (again!) why Ron could not eat everything that he saw. Ron had nodded happily before wandering off... and sticking a rock in his mouth.
Ginny spent more time yelling at the squirrels for not leaving her any acorns then she spent trying to find the right one. When accidental magic sparked and lit one of the squirrel's tails on fire, Arthur helped her find one quickly, putting the fire out with a flick of his wand. She dug into the earth with a vigor, putting her tree far away from those nasty boy trees.